Volume 13, Part 3, August 1999

Mycology AnswersIn terms of cell structure and function, how do the fungi differ from

other microbes?The term microbe is used to describe anorganism composed of a single cell or group ofcells that are able to live independently. Thesemicroscopic organisms include the bacteria,algae, protozoa and fungi. The range of formsand life styles that occur in the microbial worldis enormously diverse. The ordering of livingspecies on the basis of evolutionary relationshipsis known as phylogeny and the development ofmolecular methods has made it possible tocompare large molecules in living organisms. Theuse of such techniques (molecular phylogeny)has resulted in the defmition of three kingdomsnow known as domains, each distinctly differentfrom the others. All organisms belong in one ofthese domains, namely Bacteria (formerlyEubacteria), Archaea (formerly Archaebacteria)and Eukarya (including animals, plants, fungi,and protists).

Cells are the basic units of living organisms.They are bounded by a cell membrane that actsas a semi-permeable barrier from thesurroundings and many cells are also protectedby an outer cell wall. A major distinguishingfeature of cells is the arrangement of the geneticmaterial (genomic DNA) in the cytoplasm. Ineukaryotic cells (members of the Eukarya) linearchromosomes are contained within a membranebound nucleus. In prokaryotes (Bacteria andArchaea) that lack true nuclei , the geneticmaterial is free in the cytoplasm and organisedas a single circular chromosome. The prokaryoticmicrobes are also smaller than eukaryoticmicrobes (algae , protozoa and fungi).Additionally, cells of the Bacteria and Archaealack the discrete structures (organelles)associated with specific biochemical activitiesthat occur in the cytoplasm of m embers ofthe Eukarya.

The Bacteria and Archaea are microscopicand all are prokaryotes, but members of thesetwo domains are quite distinct from one another.The Archaea are mostly anaerobic organisms,able to grow where oxygen is unavailable orpresent in very low amounts, and are found inunusual habitats and extreme environments.Some have a high requirement for salt, some

require high levels of sulphur and some grow atextremely high temperatures (100 - 110°C). Some(methanogens) produce methane underanaerobic conditions. Archaea are found in hotsprings, geothermal vents and volcanic habitats,regions which would be regarded as hostile forthe organisms we encounter most often.Members of the Bacteria are also found in anextremely wide range of habitats. Themycoplasmas lack cell walls and theactinomycetes have a filamentous , ramifyinggrowth form. The remaining ba cteria can bedivided into two groups depending on thestructure of the cell wall , general morphologyand nutritional behaviour. Bacterial cells have asimple structure. Morphologically the cells areusually spherical (cocci) , rod shaped (bacillus),curved (vibrio) or spiral (helix). The cell wallscharacteristically contain peptidoglycan withvarying amounts of lipoplysaccharide. Bacteriacan use a wide range of organic chemicals asenergy sources. The cyanobacteria andphotosynthetic bacteria use light en ergy formetabolism. Some can make use of inorganicenergy sources and have a vital role to play inthe cycling of elements in the environment on aglobal scale . Many are also capable of growth inanaerobic conditions.

In the Eukarya the microorganisms (algae,protozoa and fungi) are generally largerthan the prokaryotes . Most of the algaecontain chlorophyll and use light energy forphotosynthesis. Many are small and unicellularalthough some marine algae can grow to reachmuch larger sizes. Many algae or their gametes(germ cells) move (are motile) by means offlagella and some aggregate and co-operat etogether in colonial forms. Algae grow well inaquatic environments , in soil and on dampsurfaces. They are divided into groups by thepigments they contain and by tile composition ofthe cell walls which contain cellulose, xylans andmannans. Protozoa are unicellular and motile butlack a protective cell wall. They are also found inaquatic habitats and very damp situations wherethey obtain nutrients by the ingestion of othermicrobes or particulate organic materials. Some

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Volume 13, Part 3, August 1999

are of great importance because they causeserious diseases in man and animals (e.g.malaria, amoebic dysentery, giardiasis).

Hyphae, the units of fungal growth, are fine(1-30 [Lm, but usually 5-10 [Lm diameter) and areessentially tubular with a characteristic internalstructure that enables invasive growth andsubstrate exploitation supporting the fungalmode of life. Most hyphae are surrounded by acell wall containing chitin and glucan (somecontain cellulose and glucan). This has animportant role in maintaining the shape of thehypha and is a protective layer against osmoticchanges and potential bursting. Hyphae containtrue nuclei, although they are typically verysmall and not easily seen using a lightmicroscope. They also have mitochondria (sitesof respiratory activity and energy metabolism),endoplasmic reticulum (extensive systems ofmembrane, sites of protein synthesis), vacuoles(membrane-bound storage compartments) andvesicles (very tiny membrane-bound bodies thatprobably carry specific enzymes and/or cellcomponents). The arrangement of theseorganelles is very specific and related to thegrowth form. Fungal hyphae grow by extension ofthe cell wall at the apex only. Growing tips areparticularly rich in vesicles (clusters of vesiclesare seen at growing apices), almost to theexclusion of other organelles. These vesiclesprobably provide wall components for theextending tip. Immediately behind these, activemitochondria are present, usually arrangedwith their longitudinal axes lying inthe same direction as the hypha. Nuclei willbe found much further back, away fromthe growing region, and vacuoles are formedfurther away.

Since physiological and biochemical activitiestend to be compartmentalised and associatedwith organelles there is a zonation of activityalong fungal hyphae. Some enzymes are alsoassociated with the fungal cell wall. Tip regionsthat have newly extended into fresh substratewill be involved in nutrient digestion and uptakewhereas more distal zones will be associatedwith nutrient storage and the processes ofdifferentiation and/or sporulation. There is somemixing of cytoplasmic contents by streamingaction from one part of a hypha to another. Thisstreaming probably counters problems thatmight otherwise occur with relatively long

distance diffusion. Some hyphae are able togrow, sometimes for extended periods, oversubstrates with little nutrient and water content.This allows the eventual exploitation of newregions of substrate but would not be possiblewithout the initial maintenance of hyphalconnections. Many growing fungal colonies willeventually produce spores (by sexual or asexualreproduction). These may be agents of rapiddispersal or may be resting structures, withthickened pigmented walls, designed towithstand adverse conditions (overwintering).Fungal spores are often highly resistant,representing an interruption in thedevelopmental sequence of a fungal colony, andare a very efficient means of survival. Spores alsocontain all the components necessary for theresumption of growth although the organellespresent may be immature or temporarily non-functional. When growth resumes the fungalcharacter will quickly be re-established.

Fungi lack chlorophyll and derive energy frompre-formed organic compounds. They absorbsoluble nutrients through the cell wall andmembrane. However, they are able to exploit avery wide range of habitats and can use a widevariety of substrates as nutrient materials. Thefungi demonstrate an invasive growth form,penetrating into the substrate. Owing to their lifestyles they act as agents of decay, nutrientrecycling, and disease production, and also havegreat industrial importance.

Susan IsaacSchool of Biological Sciences,

Life Sciences Building, University of Liverpool,Crown Street, Liverpool, L69 7ZB

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