Do green algae form obligate symbioses with fungi?

Do green algae form obligate symbioses with fungi?

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I understand that obligate symbioses means that the two organisms cannot live without each other and are in a symbiotic relationship, but do green algae from this with fungi?

Yes, Lichen are composite organisms of green algae (or cyanobacteria) living togther with fungi.

In most cases the fungi-part is completely dependent on that symbioses and is completely dependent on the symbiosis. For the algae it seems that some species can survive on their own, but not necessarily in the same enviroment as the lichen, while other species may again be completely depedent on the symbiosis (see also the comments).


From Trebouxia , the most common phycobiont, the following six isolates have been studied: Trebouxia albulescens from Buellia punctata, T. anticipata from Parmelia rudecta, T. decolorans from Xanthoria parietina, T. erici from Cladonia cristatella, T. gelatinosa from Parmelia caperata, and T. impressa from Physcia stellaris ( Laudi et al., 1969 Jacobs and Ahmadjian, 1971a Fisher and Lang, 1971b ). These algae were cultured either in organic or inorganic media and either under illumination or in the dark. All the Trebouxia cells showed a pyrenoid in the center of a large chloroplast. The arrangement of the thylakoids showed the same variation as already described for lichenized cells. Trebouxia erici grown under 1075 and 3600 lux instead of 215 lux showed a decrease in stacked thylakoids ( Fisher and Lang, 1971b ). Trebouxia decolorans and T. albulescens were more sensitive to strong light (3000 lux) in their cultured state compared with their lichenized form.

While the lichenized Trebouxia usually have a single pyrenoid, several may occur in the cultured cells ( Fisher and Lang, 1971b Jacobs and Ahmadjian, 1971a ). The pyrenoids are traversed by thylakoids whose appearance can be different even in one species, a fact that was noted also in lichenized Trebouxia ( Peveling, 1969a ). The pyrenoglobuli, which are aligned along the intrapyrenoid thylakoids, measured 100 nm in diameter in cells of T. erici that were grown in inorganic culture. Cells grown in organic media had pyrenoglobuli that were only 30–50 nm in diameter and they were fewer in number ( Fisher and Lang, 1971b Jacobs and Ahmadjian, 1971a ). Moreover, T. erici grown in organic media at 215 lux showed more pyrenoglobuli than cells cultured at 1075 or 3600 lux. After transfer of T. erici from organic to inorganic culture there was a marked increase of pyrenoglobuli ( Jacobs and Ahmadjian, 1971a ). Starch accumulation varies in lichenized and isolated phycobionts according to differences in the culture conditions. Fisher and Lang (1971b) found that young cells grown at 215 lux contained larger amounts of starch than cells grown at 1075 or 3600 lux.

The cytoplasm forms a thin rim around the chloroplast and reveals endoplasmic reticulum, mitochondria, and ribosomes. In the isolated T. erici more dictyosomes were observed than in the lichenized form ( Fisher and Lang, 1971b ). This is probably due to the higher division rate in culture. Two types of storage products are apparent in the cytoplasm of the cultured cells. One product is an electron-dense spherical body, alveolate or mottled in appearance, which is commonly present in vacuoles. Such bodies are absent in cells cultured in low phosphate medium. According to Fisher accumulations of polyphosphate were demonstrated in these bodies. The second type of storage body are electron-transparent storage droplets similar to those in lichenized phycobionts.

In addition to allowing for observations of the vegetative cells, cultures present a chance to analyze the structure in dividing cells.

The building of aplanospores and zoospores by T. erici is described as very similar ( Jacobs and Ahmadjian, 1971a ). The division starts with an expansion of the pyrenoid and its fragmentation into smaller parts. Pyrenoglobuli and the osmiophilic globuli outside the pyrenoid fuse. At this stage, at least one dictyosome is evident in each cell. Two daughter chloroplasts move to a parietal position against the cell wall and several mitochondria become aligned along the edge of each chloroplast. These mitochondria then divide into many smaller ones. At this stage the nucleus has assumed a central position in the cell and becomes surrounded by electron-transparent storage droplets. Simultaneously, many dictyosomes are present and appear to be producing large numbers of electron-transparent vesicles, some with electron-dense cores. The daughter chloroplasts then divide successively many times. Nuclear division occurs before cytokinesis. The daughter cells as aplanospores or zoospores are released through an exit pore in the wall of the zoosporangium. A very typical structure of the zoospores is the eye spot. But this differentiation is found only in zoospores which originated from cells in inorganic medium. The eye spot when viewed in longitudinal section consists of one row of ten electron-dense droplets.

The cell walls of isolated Trebouxia cells grown in organic or inorganic media possess a fibrillar surface which becomes obvious by carbon replicas or after freeze-etching. This fibrillar component of the cell wall was not detected in lichenized Trebouxia ( Jacobs and Ahmadjian, 1971a ).

What are lichens?

Have you ever seen a lichen and knew that it was a lichen? Not many people know what lichens are, and who would? They seem as though they are from another planet! Lichens are bizarre organisms and no two are alike.

Lichens are a complex life form that is a symbiotic partnership of two separate organisms, a fungus and an alga. The dominant partner is the fungus, which gives the lichen the majority of its characteristics, from its thallus shape to its fruiting bodies. The alga can be either a green alga or a blue-green alga, otherwise known as cyanobacteria. Many lichens will have both types of algae.

What are fungi?

Fungi are a diverse group of organisms that are in their own kingdom (Fungi), separate from plants. Fungi do not contain chlorophyll or any other means of producing their own food so they rely on other organisms for nutrition. Fungi are widely known for their role in the decomposition of organic matter. They are also necessary for the survival of the ecosystem around them, such as partnering with plants and trees for nutrients and survival.

Lichens are another such partnership for fungi to gain nutrients from another organism. The algal partner photosynthesizes and provides food for the fungus, so it can grow and spread.

Sclerotia veratri, a cup fungus. These types of fungi are the most common fungal partner in lichen biology. Photo by Chris Wagner, U.S. Forest Service.

What are algae?

Algae are in another kingdom (Protista) separate from plants and fungi. There are several types of algae: green, brown, red, gold. They can survive in salt water and in freshwater on their own, and in any environment when part of a lichen relationship.

Although cyanobacteria are called blue-green algae, they are actually bacteria, and are part of the bacteria kingdom, Monera. The "blue" in the common name refers to the fact that they need to live in water, and "green algae" refers to their photosynthetic abilities, like green algae.

Peltigera britannica, dog-pelt lichen. Notice the bright green surface that is green algae showing through. Look closely and you will see dark spots. Those spots are pockets of cyanobacteria. Photo by Karen Dillman, U.S. Forest Service.

Respiration and nutrition

At the cellular level, the metabolic pathways known for protists are essentially no different from those found among cells and tissues of other eukaryotes. Thus, the plastids of algal protists function like the chloroplasts of plants with respect to photosynthesis, and, when present, the mitochondria function as the site where molecules are broken down to release chemical energy, carbon dioxide, and water. The basic difference between the unicellular protists and the tissue- and organ-dependent cells of other eukaryotes lies in the fact that the former are simultaneously cells and complete organisms. Such microorganisms, then, must carry out the life-sustaining functions that are generally served by organ systems within the complex multicellular or multitissued bodies of the other eukaryotes. Many such functions in the protists are dependent on relatively elaborate architectural adaptations in the cell. Phagotrophic feeding, for example, requires more complicated processes at the protist’s cellular level, where no combination of tissues and cells is available to carry out the ingestion, digestion, and egestion of particulate food matter. On the other hand, obtaining oxygen in the case of free-living, free-swimming protozoan protists is simpler than for multicellular eukaryotes because the process requires only the direct diffusion of oxygen from the surrounding medium.

Although most protists require oxygen (obligate aerobes), there are some that may or must rely on anaerobic metabolism—for example, parasitic forms inhabiting sites without free oxygen and some bottom-dwelling (benthic) ciliates that live in the sulfide zone of certain marine and freshwater sediments. Mitochondria typically are not found in the cytoplasm of these anaerobes rather, microbodies called hydrogenosomes or specialized symbiotic bacteria act as respiratory organelles.

The major modes of nutrition among protists are autotrophy (involving plastids, photosynthesis, and the organism’s manufacture of its own nutrients from the milieu) and heterotrophy (the taking in of nutrients). Obligate autotrophy, which requires only a few inorganic materials and light energy for survival and growth, is characteristic of algal protists (e.g., Chlamydomonas). Heterotrophy may occur as one of at least two types: phagotrophy, which is essentially the engulfment of particulate food, and osmotrophy, the taking in of dissolved nutrients from the medium, often by the method of pinocytosis. Phagotrophic heterotrophy is seen in many ciliates that seem to require live prey as organic sources of energy, carbon, nitrogen, vitamins, and growth factors. The food of free-living phagotrophic protists ranges from other protists to bacteria to plant and animal material, living or dead. Scavengers are numerous, especially among the ciliated protozoans indeed, species of some groups prefer moribund prey. Organisms that can utilize either or both autotrophy and heterotrophy are said to exhibit mixotrophy. Many dinoflagellates, for example, exhibit mixotrophy.

Feeding mechanisms and their use are diverse among protists. They include the capture of living prey by the use of encircling pseudopodial extensions (in certain amoeboids), the trapping of particles of food in water currents by filters formed of specialized compound buccal organelles (in ciliates), and the simple diffusion of dissolved organic material through the cell membrane, as well as the sucking out of the cytoplasm of certain host cells (as in many parasitic protists). In the case of many symbiotic protists, methods for survival, such as the invasion of the host and transfer to fresh hosts, have developed through long associations and often the coevolution of both partners.

Reproduction and life histories

Algae regenerate by sexual reproduction, involving male and female gametes (sex cells), by asexual reproduction, or by both ways.

Asexual reproduction is the production of progeny without the union of cells or nuclear material. Many small algae reproduce asexually by ordinary cell division or by fragmentation, whereas larger algae reproduce by spores. Some red algae produce monospores (walled, nonflagellate, spherical cells) that are carried by water currents and upon germination produce a new organism. Some green algae produce nonmotile spores called aplanospores, while others produce zoospores, which lack true cell walls and bear one or more flagella. These flagella allow zoospores to swim to a favourable environment, whereas monospores and aplanospores have to rely on passive transport by water currents.

Sexual reproduction is characterized by the process of meiosis, in which progeny cells receive half of their genetic information from each parent cell. Sexual reproduction is usually regulated by environmental events. In many species, when temperature, salinity, inorganic nutrients (e.g., phosphorus, nitrogen, and magnesium), or day length become unfavourable, sexual reproduction is induced. A sexually reproducing organism typically has two phases in its life cycle. In the first stage, each cell has a single set of chromosomes and is called haploid, whereas in the second stage each cell has two sets of chromosomes and is called diploid. When one haploid gamete fuses with another haploid gamete during fertilization, the resulting combination, with two sets of chromosomes, is called a zygote. Either immediately or at some later time, a diploid cell directly or indirectly undergoes a special reductive cell-division process (meiosis). Diploid cells in this stage are called sporophytes because they produce spores. During meiosis the chromosome number of a diploid sporophyte is halved, and the resulting daughter cells are haploid. At some time, immediately or later, haploid cells act directly as gametes. In algae, as in plants, haploid cells in this stage are called gametophytes because they produce gametes.

The life cycles of sexually reproducing algae vary in some, the dominant stage is the sporophyte, in others it is the gametophyte. For example, Sargassum (class Phaeophyceae) has a diploid (sporophyte) body, and the haploid phase is represented by gametes. Ectocarpus (class Phaeophyceae) has alternating diploid and haploid vegetative stages, whereas Spirogyra (class Charophyceae) has a haploid vegetative stage, and the zygote is the only diploid cell.

In freshwater species especially, the fertilized egg, or zygote, often passes into a dormant state called a zygospore. Zygospores generally have a large store of food reserves and a thick, resistant cell wall. Following an appropriate environmental stimulus, such as a change in light, temperature, or nutrients, the zygospores are induced to germinate and start another period of growth.

Most algae can live for days, weeks, or months. Small algae are sometimes found in abundance during a short period of the year and remain dormant during the rest of the year. In some species, the dormant form is a resistant cyst, whereas other species remain in the vegetative state but at very low population numbers. Some large, attached species are true perennials. They may lose the main body at the end of the growing season, but the attachment part, the holdfast, produces new growth only at the beginning of the next growing season.

The red algae, as exemplified by Polysiphonia, have some of the most complex life cycles known for living organisms. Following meiosis, four haploid tetraspores are produced, which germinate to produce either a male or a female gametophyte. When mature, the male gametophyte produces special spermatangial branches that bear structures, called spermatangia, which contain spermatia, the male gametes. The female gametophyte produces special carpogonial branches that bear carpogonia, the female gametes. Fertilization occurs when a male spermatium, carried by water currents, collides with the extended portion of a female carpogonium and the two gametes fuse. The fertilized carpogonium (the zygote) and the female gametophyte tissue around it develop into a basketlike or pustulelike structure called a carposporophyte. The carposporophyte eventually produces and releases diploid carpospores that develop into tetrasporophytes. Certain cells of the tetrasporophyte undergo meiosis to produce tetraspores, and the cycle is repeated. In the life cycle of Polysiphonia, and many other red algae, there are separate male and female gametophytes, carposporophytes that develop on the female gametophytes, and separate tetrasporophytes.

The life cycles of diatoms, which are diploid, are also unique. Diatom walls, or frustules, are composed of two overlapping parts (the valves). During cell division, two new valves form in the middle of the cell and partition the protoplasm into two parts. Consequently, the new valves are generally somewhat smaller than the originals, so after many successive generations, most of the cells in the growing population are smaller than their parents. When such diatoms reach a critically small size, sexual reproduction may be stimulated. The small diploid cells undergo meiosis, and among pennate (thin, elliptical) diatoms the resulting haploid gametes fuse into a zygote, which grows quite large and forms a special kind of cell called an auxospore. The auxospore divides, forming two large, vegetative cells, and in this manner the larger size is renewed. In centric diatoms there is marked differentiation between nonmotile female gametes, which act as egg cells, and motile (typically uniflagellate) male gametes.

Environmental Biotechnology and Safety Hydrogen Production by Photofermentations

Photosynthetic bacteria have long been studied for their capacity to produce significant amounts of H 2. Photosynthetic bacteria evolve molecular H2 catalyzed by nitrogenase under nitrogen-deficient conditions using light energy and reduced compounds. The overall reaction of hydrogen production is given as

The versatile metabolic capabilities of these organisms and the lack of PSII, which automatically eliminates the difficulties associated with O2 inhibition of H2 production, provide an advantage over biophotolysis. Another advantage with the use of photosynthetic bacteria is that these organisms are able to utilize a wide range of cheap organic compounds for H2 production.

Photoheterotrophic bacteria capable of converting organic acids (acetic, lactic, and butyric acid) to H2 and CO2 under anaerobic conditions in the presence of light include Rhodobacter sphaeroides, Rhodobacter capsulatus, Rhodovulum sulfidophilum W-1S, and Rhodopseudomonas palustris. The highest conversion efficiency reported in the literature was obtained when lactic acid was used as the sole carbon source. R. rubrum and R. palustris P4, which express a CO-dependent dehydrogenase (CODH) enzyme, have also been reported to be able to produce H2 from CO or other organic acids [11] . The optimum growth temperature and pH for the photosynthetic bacteria was reported to be in the range of 30–35 °C and pH 7.0, respectively. Hydrogen production by these bacteria requires anaerobic conditions under illumination [12] . Even though these organisms prefer organic acids as carbon source, other industrial effluents are amenable to H2 production [13] .

The main factors that affect H2 production rates by photoheterotrophic bacteria are light intensity, carbon source, and the type of microbial culture. Nitrogenase, however, is the key enzyme catalyzing H2 production by these bacteria. The presence of oxygen, ammonia, and high N/C ratios are known to inhibit the activity of nitrogenase. For example, H2 production by R. sphaeroides is completely inhibited at ammonia concentrations above 2 mM [13] .

The presence of high nitrogen induces metabolic shifts to the utilization of organic substrates for cell synthesis rather than H2 production, resulting in excess biomass growth and reduction in light penetration. Therefore, ammonium-limited and oxygen-free conditions are required. Proteins such as albumin, glutamate, and yeast extract can be used as nitrogen sources. Uptake hydrogenase enzymes in photofermentative bacteria oxidize H2 and are antagonistic to nitrogenase activity therefore, uptake hydrogenase activity should be eliminated for enhanced H2 production. Two to three times more H2 production was achieved by using hydrogenase-deficient mutant cultures of photofermentative bacteria [14] .

Light intensity is another important parameter affecting the performance of photofermentations. Increasing light intensity has a stimulatory effect on H2 yields and production rates, but has an adverse effect on the light conversion efficiencies. It was reported that the reduced antenna mutant of R. sphaeroides MTP4 produces H2 more efficiently under high light intensity as compared to the wild-type strain. Reduced antenna mutants have been studied for many biotechnological applications, since the benefits derived from a reduced absorption of light may affect a number of physiological pathways, in different microorganisms. Torzillo et al. [15] reported on green algae that indeed a great benefit can be derived from such mutants.

Light intensity also affects the consumption rates of organic acids. For example, butyrate consumption requires higher light intensities ( 4000 lux) as compared to acetate and propionate. Exposure time to light also affects H2 production. Alternating 14 h light/10 h dark cycles yielded slightly higher H2 production rates and cell concentrations compared to continuous illumination. More frequent exposure to dark/light cycle has a better effect on H2 production [16] .

Industrial effluents are amenable for H2 production by photosynthetic organisms as long as the color of wastewaters does not inhibit light penetration. Industrial effluents should also have a reasonable or tolerable amount of ammonia, which inhibits the nitrogenase enzyme and reduces the H2 productivity. Therefore, removal of ammonia and toxic compounds (heavy metals, phenols, etc.) and dilution of high organic matter content (COD) in industrial effluents may require pretreatment before use for biohydrogen gas production.

An extensive summary of H2 production studies from some food industry wastewaters is given by Kapdan and Kargi [16] . Glucose, sucrose, starch, wheat starch, lactose, food waste, potato processing waste, apple, domestic sludge, molasses, rice winery, biosolids, filtrate, sweet potato starch residues, and organic fraction of municipal solid wastes have been used as substrates for H2 production. In addition, acetate, butyrate, lactate, malate, and succinate have also been studied. Biohydrogen production from pretreated sugar refinery wastewater (SRWW) in a column photobioreactor using R. sphaeroides OU 001 achieved a H2 production rate of 3.8 ml l −1 h −1 at 32 °C in batch operation with 20% diluted SRWW [17] . Addition of malic acid (20 g l −1 ) into SRWW enhanced the production rate and resulted in a H2 production rate of 5 ml l −1 h −1 . Significant dilution (3–4%) of highly concentrated olive mill effluent (OME) was needed to eliminate the inhibitory effects of high organic content and dark color of the OME before a reasonable amount of H2 production was observed [18] . Two percent dilution was reported to be the best for H2 production potential of 13.9 l H2 l −1 wastewater at 32 °C with R. sphaeroides OU 001, and ∼35% COD reduction was obtained [19] .

Tofu wastewater, which is a carbohydrate and protein-rich effluent, was also used for H2 production. Hydrogen yield from tofu wastewater (1.9 l H2 l −1 wastewater at 30 °C) was reported to be comparable to H2 yield from glucose (3.6 l H2 l −1 wastewater) using R. sphaeroides RV immobilized in agar gel [20] . No ammonia inhibition (2 mM) was observed, and 41% of total organic carbon (TOC) was removed. Similarly, the dilution of the wastewater at a ratio of 50% resulted in an increase in H2 yield of up to 4.32 l H2 l −1 wastewater and 66% TOC removal [20] .

Other agro-based waste materials such as potato starch, sugarcane juice, and whey were also investigated in terms of H2 production using Rhodopseudomonas sp. Sugarcane juice yielded the maximum level of H2 production (45 ml (mg DW h) −1 ) as compared to potato waste (30 ml (mg DW h) −1 ) and whey (25 ml (mg DW h) −1 ). There was no H2 production by the photosynthetic bacterium, Rhodobium marinum, using raw starch as the substrate [21] .

Development of efficient photobioreactors is another critical factor for photobiological H2 production. The major types of photobioreactors reported for H2 production are tubular, flat panel, and bubble column reactors. Attempts to intensify the process in laboratory conditions by increasing the concentration of cells in the photobioreactor were unsuccessful due to the exponential decay of light intensity with increasing density of the cell culture. This fact necessitates rigid requirements for bioreactors with all photosynthesizing microorganisms. The most common experimental photobioreactors have suspension layer thicknesses of 1–5 cm. Therefore, the rates of H2 evolution are, according to different authors, 0.08–0.26 l H2 l −1 suspension h −1 . The concentration of cells inside the reactor can also be increased by immobilization on light transmitting matrices with high surface area/volume ratios. In this case, up to 12 g of cells (dry weight) can be immobilized in 1 l of matrix. Therefore, the rates of H2 evolution per unit volume increase considerably. Thus, R. sphaeroides immobilized on a porous glass steadily evolved H2 at a rate of 1.1 l l −1 matrix h −1 for > 1000 h. The maximum volumetric H2 rate attained was 3.8 l l −1 matrix h 1 , with an 80% conversion of the organic acid substrate [22] .

Mutant photosynthetic bacteria have been used by many researchers to enhance the light conversion efficiency, and hence H2 production rate. Although an improvement was observed by mutant type, the light conversion efficiency was ∼6% [23] , which is still less than the theoretical efficiency. A maximum light conversion efficiency of 9.23% was achieved by using a light-induced and diffused photobioreactor at 300 W m −2 light intensity. The light penetration length (i.e., width of the bioreactor) is important for the hydrogen productivity. In relation to solar energy-driven H2 production, the light conversion efficiency was reported to be less during midday because of high light intensity (1.0 kW m −2 ). In addition, a delay of 2–4 h was observed in maximum hydrogen production rate (3.4 l H2 (m 2 h) −1 ) after the highest light intensity at noon with an average light conversion efficiency of 1.4% [23] . A 3.5% light conversion efficiency with an over 0.8 kW m −2 light intensity at midday was obtained using a photo-bioreactor system with light shade bands, whereas photoinhibition was observed at 0.4 kW m −2 in photobioreactors without shade bands [24] .

The other important parameter that has an effect on H2 production in photobioreactors is mixing. Gas injection using argon gas is commonly used for mixing, although not cost-effective. On the other hand, continuous argon sparging inhibits the growth of Rhodopseudomonas in a pneumatically agitated photobioreactor because of CO2 loss, whereas recirculation provided better growth of the culture. A novel flat-panel airlift photobioreactor with baffles provided a significant increase in the biomass productivity, and therefore, it could also be used for H2 production [25] .

Another commonly used reactor system is multitubular photobioreactor, which is generally used for the cultivation of Spirulina. Tubular reactors are made up of parallel transparent tubes filled with water. The system is inclined with a 10–30% slope to allow gas bubbles to rise. The hydrogen production rate from lactate using a modified tubular reactor reached 2 l m −2 h −1 with light conversion efficiency of 2% in outdoor experiments [26] .


CAROL A. SHEARER , . JOYCE E. LONGCORE , in Biodiversity of Fungi , 2004


The Chytridiomycetes, which are true Fungi ( Förster et al. 1990 Bowman et al. 1992 ), and the Hyphochytriomycetes, which are allied with the straminipiles ( Barr 1992 Van der Auwera et al. 1995 ), are treated together because species from both groups are outwardly similar, occupy the same habitats, and are studied with the same techniques. The hyphochytrids consist of a single order with three families and four genera ( Karling 1977 Fuller 1990 ). Members characteristically produce zoospores with a single, anterior, tinsellated flagellum. The five orders that comprise the class Chytridiomycetes are defined on the basis of differences in ultrastructural characters of zoospores (D. J. S. Barr 1990 , 2001 ). Members of all orders except some species in the specialized Neocallimastigales have zoospores with a single, posteriorly directed flagellum. The Chytridiales, Monoblepharidales, and Blastocladiales are found in water and in soils. The Spizellomycetales primarily inhabit soils but may be found at the margins of lentic and lotic aquatic habitats. Representatives of the Neocallimastigales are obligate anaerobes and, thus far, have been isolated only from the digestive systems and feces of herbivorous animals, with the exception of a single isolation from a farm pond ( Wubah and Kim 1995 ).

The Blastocladiales contains five families and 13 genera. Some taxa, including the well-known, experimental organisms, Allomyces and Blastocladiella , are saprotrophic, but many others are specialized parasites of invertebrates (e.g., Coelomomyces species in mosquito larvae). Physoderma, a genus whose members parasitize aquatic and semiaquatic plants, also belongs to this order. Monoblepharidales contains four families and six genera, all saprotrophic ( Forget et al. 2002 ). Members of the Chytridiales first were described in the 1850s currently four families and about 80 genera containing more than 500 species are recognized. In 1980, Spizellomycetales, which contains four families and 10 genera, was separated from Chytridiales on the basis of ultrastructural characters of zoospores ( Barr 1980 ).

Most of the diversity of the Chytridiomycetes lies within the Chytridiales, but unfortunately, studies of that group are severely limited by the lack of adequate species descriptions, mentors who can help with identifications, and recent identification guides. The most recent monograph that includes Chytridiomycetes and Hyphochytriomycetes is Sparrow's Aquatic Phycomycetes (1960), which he later followed with a key to genera ( Sparrow 1973 ). More than 23 genera and 300 species have been described since Sparrow's monograph and are listed along with other changes in taxonomy in a bibliography by Longcore (1996 ). Taxa described since 1960, however, have not been incorporated into taxonomic keys. Zarys hydromikologii ( Batko 1975 ) is dedicated exclusively to aquatic fungi, but its usefulness is limited in many countries because the keys are written in Polish and are not comprehensive in their coverage of species. Chytridiomycetarum Iconographica ( Karling 1977 ) contains commentaries on genera of chytrids and hyphochytrids and many drawings. Because it is tempting to use Karling's drawings to “picture-key” aquatic fungi, we emphasize that his book portrays only a fraction of the described species. Investigators who wish to identify chytrids and hyphochytrids, particularly those described since 1960, need to refer to original descriptions.

Reviews of the ecology of freshwater fungi ( Sparrow 1968 Dick 1976 Powell 1993 ) provide an overview of what is known about the diversity of zoosporic fungi. Knowledge of the diversity of zoosporic fungi in several geographic areas has accrued as a consequence of the career research of mycologists who specialize in those groups. Notable in this regard with respect to aquatic habitats are the papers of H. M. Canter and colleagues, which document chytrids associated with algae in the Lake District of England (see references in Karling 1977 ). Knowledge of the aquatic mycota of the Lake District was broadened further by L. G. Willoughby's studies of the saprotrophic chytrids of the margins and muds of several lakes in that area (see references in Karling 1977 ).

Similarly, a general knowledge of the diversity of chytrids and other aquatic fungi found in northern Michigan, in the United States, exists as a result of the research of F. K. Sparrow and colleagues. Over a span of about 20 years, those researchers published papers on zoosporic fungi from aquatic sites and soils throughout the northern counties of the lower peninsula of Michigan. Their studies emphasized the fungi, not the habitat.

The Chytridiomycetes and Hyphochytriomycetes are denoted as “aquatic” fungi because they disperse through water with motile spores. After rain or snow melt, most soils are transformed into an “aquatic” habitat in which chytrids and hyphochytrids are widespread. Consequently, many aquatic fungi have been studied from soil samples, which contain resting spores of zoosporic fungi, because they are relatively easy to collect and transport. For example, Karling based his reports of zoosporic fungi from India, Africa, New Zealand, Oceania, and various South American countries mostly on his studies of soil samples, although some of the soil samples came from dried aquatic habitats. Willoughby (1962a , 1962b ) reported differences between the species found in lakes and those usually found in soils, but many species have been reported from both aquatic habitats and soils.

Chytrids associated with discrete, countable substrata such as algae have been quantified for reviews see Masters (1976 ) and Powell (1993 ). In northern North America, conifers produce an annual shower of pollen, which falls in such abundance that yellow pollen lines surround lake shores. Ulken and Sparrow (1968 ) used a modification of the most probable number (MPN) method used by bacteriologists and found that the number of chytrid zoospores that attack pollen grains per liter of lake water peaked during the 2 weeks following the height of the pollen shower.

Endemism has not been known among chytrids and hyphochytrids however, recent molecular evidence (Morehouse et al. 2000) suggests that Batrachochytrium dendrobatidis, a chytridialean pathogen of amphibians, may have recently spread to several continents. Sparrow's experience with Michigan fungi allowed him to observe that several of the fungi he found in a bog in the Hawaiian Islands belong to the same species that occur in bogs in northern Michigan ( Sparrow 1965 ). Chytrid species with morphologies so distinctive as to preclude misidentification have been reported from different continents. The prevailing hypothesis is that chytrid species are distributed worldwide, with occurrence determined by local conditions rather than geography ( Sparrow 1968 ).


For over 140 years, lichens have been regarded as a symbiosis between a single fungus, usually an ascomycete, and a photosynthesizing partner. Other fungi have long been known to occur as occasional parasites or endophytes, but the one lichen–one fungus paradigm has seldom been questioned. Here we show that many common lichens are composed of the known ascomycete, the photosynthesizing partner, and, unexpectedly, specific basidiomycete yeasts. These yeasts are embedded in the cortex, and their abundance correlates with previously unexplained variations in phenotype. Basidiomycete lineages maintain close associations with specific lichen species over large geographical distances and have been found on six continents. The structurally important lichen cortex, long treated as a zone of differentiated ascomycete cells, appears to consistently contain two unrelated fungi.

Video. Studies of gene activity have now revealed that many lichens are not a twosome but instead a threesome, with two fungi in the mix (Courtesy Science Magazine).

Of baobabs, bats and elephants - a relationship of epic proportions

This is not a garden example of a symbiotic relationship The mighty baobab begins with a fruit bat. The large white nocturnal flowers of the baobab attract large fruit eating bats that act as the baobab's pollinator. In exchange the baobab provides food and shelter for the bats.

The fruits encasing the fertilised seeds are then eaten by elephants and carried miles away. The gastric juices of the elephant weaken the shell of the seeds necessary for successful germination. The seed is then dumped, encased in its unique blend of potting soil, ripe for growing into the largest tree in the savannah. Both the elephant and baobab are keystone species and play important ecological roles in nature. Although it should be said when the pachyderms meet pachycauls the baobab comes off second best as elephant strip the tree to get water in times of drought.

Nutrition in Fungi (With Diagrams)| Botany

The fungi utilise both organic compounds and inorganic materials as the source of their nutrient supply. In other words, organic and inorganic compounds constitute their food. No fungus is able to make any increase in its dry weight in the absence of organic food materials, why?

Lacking chlorophyll the fungi are unable to photosynthesize or use carbon dioxide to build up organic food materials. They are, thus heterotrophic for carbon (organic) food compounds which they in their natural habitats obtain by living as saprophytes or parasites from dead or living plants, animals or micro-organisms or their wastes.

Essential Elements:

The constituent elements of the organic and inorganic substances which fungi make use of are C, O, H, N, P, K, Mg, S, P, Mn, Cu, Mo, Fe and Zn. Calcium is required by some fungi but not all. These elements which fungi require as food are termed the essential elements. Some of these elements, the fungi need in extremely small trace amounts and the others in comparatively larger amounts.

The former are called the trace or micro elements and the latter macro elements. The fungal growth is adversely affected or the fungus fails to grow if any one of the essential elements is lacking in the culture medium. Examples of the macro elements are C, N, O, H, S, P, K and Mg. The macro elements are body builders and provide energy for metabolic processes.

Sources of Macro Elements:

The organic substances usually utilized by fungi are very varied in nature. Yeasts, for example, can use acetates as sources of carbon but for most fungi the chief sources of carbon are the carbohydrates. The carbohydrates are needed for building up the body and also as a source of energy. In a typical fungus, 50% of the dry weight is carbon of the carbohydrate source of carbon, most fungi use simple sugars.

Glucose, for instance, is suitable for almost all fungi. Next in preference are the fructose. Less commonly used are the hexose sugars and some pentoses. Xylose among the pentoses has been reported to be superior to glucose for some fungi. Mannitol is equivalent to glucose for many fungi. Maltose which occurs in nature as a byproduct of starch hydrolysis is utilized by many fungi. Sucrose is also a good source of carbon for some.

From among the polysaccharides, starch and cellulose are utilised by a fewer fungi which can synthesize the appropriate hydrolytic enzymes. Some fungi are able to make good growth on fats as the only source of carbon.

Organic acids are generally poorer sources of carbon for most fungi. Basidiomycetes include most of the lignin-utilizing fungi. Proteins, lipids some organic acids and higher alcohols are utilised by some fungi as a sole energy source’ Growth, however, is always better on a substance containing a suitable carbohydrate.

Besides carbon, fungi require nitrogen. To obtain nitrogen, they utilise both organic and inorganic materials as the source. The chief organic sources of nitrogen are protein, peptide or an amino acid certain groups of fungi show specializations in respect of certain nitrogen sources For example, the Saprolegniaceae and Blastocladiales include a number of species which grow only with organic nitrogen such as amino acid.

In nature, fungi decompose proteins and other materials to obtain their supply of nitrogen. In pure cultures amino acids, peptides, or peptones gelatin, casein and egg albumin can serve as sources of organic nitrogen for building up protoplasm. Urea is also considered as a utilisable nitrogen source for some fungi.

Many fungi, however, obtain nitrogen from inorganic sources. A number of fungi are known which use both nitrate and ammonium salts. Robbin (1939) and Lindberg (1944) reported that Absidia sp., Mucor hiemalis, Lenzites trabea and Marasmius sp. use ammonia but are incapable of utilizing nitrate salts. Fewer fungi are able to utilize nitrate salts.

Nitrites can be toxic. Organic sources of nitrogen can also serve as sources of carbon. There is not much evidence to support nitrogen fixation in saprophytic fungi. Metcalfe and Chayen (1954) reported that soil inhabiting Rhodotorula and yeast-like Pullularia pullans fix atmospheric nitrogen. It is certain however that nitrogen fixation is not a widespread ability in fungi.

Hydrogen and oxygen are supplied in the form of water which is the major constituent of fungus mycelium forming about 85-90% of the entire weight.

The chief among the inorganic nutrients which the fungi require in fairly large amounts for their mineral nutrition are sulphur, phosphorus, potassium and macronutrients the fungi obtain from simple inorganic salts or sources such as suIphates for sulphur, and phosphates for phosphorus.

These must be supplied in any culture medium. Calcium is not known to be needed by the fungi in general. Some, however, require it as a micronutrient. Some fungi are reported to require only minute traces of iron, zinc, copper, manganese and cobalt and molybdenum.

These trace elements or micronutrients are considered essential of growth. The form in which the major and the minor metallic element requirements are utilised is the anion. Fungi store excess food in the form of glycogen or lipids.

The fungi like all other organisms require minute amounts of specific, relatively complex organic compounds for growth. These are the vitamins or growth factors. Many fungi synthesize their own supply of appropriate growth factors from a simple nutrient solution of defined composition. Such fungi are thus autotrophic for vitamins and are called need no exogenous supply.

There are others which depend in whole or in part on an external source because they are unable to synthesize one or more of the essential growth substances. The fungi heterotophic for their needs of growth factors are termed auxo-trophic. There are marked difference between the vitamin demands of the different species of a genus or even the strains of a single species.

The important fungal vitamins, which may function in enzyme systems include thiamine (B1), biotin, pyredoxine (B6) and riboflavin (B2). A few fungi also need nicotinic acid and pantothenic acid. The vast majority, however, require thiamine (B1). The growth factors are catalytic in their actions.

To sum up, the basic nutritional needs of fungi are:

(i) A suitable organic compound as a source of carbon and energy.

(ii) A suitable source of nitrogen.

(iii) Inorganic ions of sulphur, phosphorus, potassium and magnesium in significant amounts.

(iv) Inorganic ions of iron, zinc, copper, manganese and molybdenum only in minute traces,

(v) Certain vitamins or organic growth factors in trace amounts.

Besides the nutritional requirements listed above the growth of fungi is habitat factors such as temperature, oxygen supply, moisture, pH value and by-products of metabolism.

Modes of Nutrition:

The fungi lack chlorophyll. They are, therefore, unable to synthesize carbohydrate food from inorganic materials and get it readymade from themselves. These heteromorphs according to their method of obtaining food are divided into two categories, namely, the saprophytes or saprobes and parasites.

The organic nutrients directly through the cell membrane from the substratum which abounds in dead organic matter of both animal and plant origin. The saprophytes cannot ingest solid food. Yeast and molds are the common examples of saprophytic fungi.

The parasite lives in or the Living body of a plant or animal and absorbs organic molecules as nutrients through the cell walls from the tissues of the host. Rusts and smuts are the common parasites.

Mechanism of Nutrition (Fig. 1.15):

The whole mycelium may have the power to absorb these nutrients or this task may be assigned to special portions of the mycelium. In saprophytic fungi the hyphae (Mucor mucedo) or rhizodial hyphae (Rhizopus stolonifer) come in intimate contact with nutrients in the substratum (A) and absorb soluble smaller molecules such sugars and amino acids.

Insoluble complex substances such as proteins, lipids and Poly are first broken into soluble monomeres (digested) by secreting extra-cellular enzymes and then absorbed.

The fungal hyphae secrete enzymes which convert insoluble complex food materials in the substratum to soluble ones. The latter are then absorbed by direct diffusion either through the hyphal walls of the hyphae that penetrate the substratum or by the rhizoidal hyphae.

The mycelium of the parasites is rarely ectophytic but frequently it grows inside the host. The hyphae either ramify in the intercellular space between the host cells (D) or penetrate into the host cells (G). The former are called intercellular hyphae and the latter intracellular hyphae.

The intercellular hyphae obtain nutrition through the cell walls or membranes of the host cells. This they do by secreting an enzyme upon the plasma membrane of the host cell.

It makes the membrane more permeable to the contained solutes. The latter diffuse out and are absorbed by the hyphal walls. The hyphal walls of the intracellular hyphae come in direct contact with the host protoplasm (G) and obtain food by direct diffusion.

The intercellular hyphae of some highly specialised (obligate) plant parasites give out slender lateral outgrowths. The hyphal outgrowth punctures the host cell wall making a minute pore through which it enters the host cell. Within the host cell, it enlarges to form a globose (D), lobed (B), or branched (F) absorptive organ.

This type of feeding organ of the parasitic fungi is called a haustorium. It is markedly specialised in structure to absorb nutrition from the host tissues. The haustonum is intracellular and thus robs the host of its food without killing it. Haustoria are characteristic of obligate parasites.

They vary in shape and size in different fungi. In Albugo the haustorium is a button-like (D) or spherical structure. Peronospora parasitica has sac-like haustoria (E) in the leaf cells of Capsella. Peronospora calotheca produces branched filamentous haustorium in the stem cells of Galium (F). Erysiphe graminis forms an elongated branched haustorium inside the host cell (B).

Each haustorium (Fig. 1.16) usually consists of two parts, a constricted region which is in the form of a narrow penetration tube and the expanded or branched region on the host cell. The penetration tube is usually clasped by a ‘collar’ of host wall material. The enlarged or expanded region of the haustorium causes Invagination of the cytoplasmic membrane of the host cell.

The latter remains closely appressed to the wall of the haustorium. There is a zone of apposition enclosing the haustorium between the fungal wall and the unbroken cytoplasmic membrane of the host. Its origin is in dispute.

The secretion from the haustorium upon the plasma membrane of the host makes it permeable to solutes contained in the sap cavity. They diffuse out and are then absorbed by the haustorium parastic fungi do not produce haustoria in artificial cultures. The haustona are also not produced by fungi which live as parasites on animals.

The fungi, as mentioned above, are unable to synthesize sugars from carbon dioxide and water. They, however, can synthesize from soluble sugars the more complex carbohydrates which I the chief components of their cell walls.

They are also able to synthesize proteins and eventually protoplasm if supplied with carbohydrates and simple nitrogen compounds such as ammonium salts. Besides ammonium salts, they can absorb and utilize many complex but soluble organic nitrogenous compounds.

Many fungi obtain nutrition by living in mutually beneficial associations with other p ants. The Association is not causal but permanent and is established during long process of evolution. The two best known examples of mutualisitc associations of fungi with other plants are Symbiosis and Mycorrhiza.

The common example of symbiosis is an association of a fungus and an alga in a lichen thallus. The two organisms in this association are so intertwined as to form a single composite thallus plant which different from either of the partners in form and habit.

The duty of alga in this partnership is to synthesize food with the help of green chloroplasts and share it with its fungal partner. The fungus absorbs minerals in solution and water from the substratum and press on to the alga. The fungal hyphae which form the bulk of the lichen thallus provide shelter to the alga, in addition.

(b) Mycorrhiza. (pl. Mycorrhizae or mycorrhizas):

It is defined as the symbiotic association between the hypha of certain fungi and roots of plants.

The fungal hyphae in this case form a complete envelope around the root tip and also penetrate and extend into the first few cortical layers to form an intercellular network of hyphae known as the Hartignet.

The hyphal strands that extend into the substrate from the envelope absorb water and nutrients from the soil and pass them on to the roots of the plant through the Hartig’s net. The presence of the fungus thus increases root absorption. In return the fungus receives food and shelter.

The fungal hyphae, in this case, penetrate root hairs, epidermis and reach the cortex where they grow intracellularly forming fungal knots in the cortical cells. A portion of the mycelium lives in the soil but it forms no dense hyphal growth (envelope on the surface of the root).

(iii) Ectoendomycorrhiza:

It is a combination of the two. The fungal hyphae form a sheath at the surface of the root. Within the root, they grow intercellularly and intracellulary.

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