In this article we will discuss about the nutritional forms of microorganisms.
This group includes photosynthetic microalgae, cyanobacteria, and photosynthetic bacteria (purple sulphur bacteria and green sulphur bacteria).
In photosynthetic microalgae and cyanobacteria the external energy-source is light. One or more varieties of chlorophyll are present to trap the solar energy. Such microorganisms are, therefore, largely green. The hydrogen-source of all photoautotrophic microalgae and cyanobacteria is environmental water (H2O) which is split into oxygen and hydrogen with the help of light energy.
The oxygen is released as by-product and the free hydrogen liberated, as a result of photolysis of water reduces carbon dioxide (the inorganic raw material) to carbohydrate (organic metabolite food) which is used as food.
This pattern of food production may be symbolized as follows:
The hydrogen-source of photoautotrophic bacteria is not water, and oxygen is never a by-product of photosynthesis. These bacteria are adapted to live in sulphur springs and other sulphurous regions where hydrogen sulphide (H2S) is normally available. This compound generally serves as the hydrogen-source.
Two families of bacteria belong to photoautotrophic group, the purple sulphur bacteria (e.g., Chromatium, Thiospirillum) and the green sulphur bacteria (e.g., Chlorobium, Chloropseudomonas, Chlorobacterium). These bacteria possess a variety of special chlorophyll known as bacteriochlorophyll. It is green but its colour is masked by the additional yellow carotenoids which are also present.
For the photoautotrophic bacteria, therefore, the special pattern of photosynthesis becomes:
Elemental sulphur is the by-product. It is stored inside the cells in the purple sulphur bacteria, and is excreted from the cells in the green sulphur bacteria.
2. Chemolithoautotrophs (Chemoautotrophs):
This is a group of non-photosynthetic autotrophic microorganisms consisting entirely of bacteria. They cannot use light and their external energy sources in food manufacture are a variety of inorganic metabolites absorbed from the environment. In the most cases, these metabolites are combined with molecular oxygen in the cells, resulting in release of energy (exothermic reaction) and a variety of inorganic byproducts.
Water and carbon dioxide are the inorganic raw materials in subsequent food manufacture. The concept of chromo-autotrophy (chemolithotrophy) was formulated by Winogradsky. By studying Beggiatoa, he demonstrated for the first time that a living organism could oxidize H2S to elemental sulphur and then to SO2-4. This process of manufacturing food is called chemosyntliesis.
The general pattern is as follows:
Among the best known chemoautotrophic microorganisms are the sulphur-oxidizing bacteria, the iron- oxidizing bacteria, the nitrifying bacteria, and the hydrogen-oxidizing bacteria.
Sulphur-oxidising bacteria are those chemoautotrophs that oxidize sulphur compounds as electron donors and energy-releasing compounds. The sulphur compounds used by them are H2S, elemental sulphur (S), S2O2-3 and SO–3.
The best studied sulphur-oxidising chemoautotroph is the genus Thiobacillus that contains several gram- negative and rod-shaped bacteria such as T. thioparus , T. denitrificans, T. neopolitanus, T. thioxidans and T. intermedius. Other sulphur-oxidising chemoautotrophs are Acromatium, Beggiatoa, Thiothrix, Thioploca, Thiomicrospira, Thiosphaera, Thermothrix and Thiovulum.
The general pattern of chemoautotrophy used by these bacteria is the following:
Several species of Thiobacillus are acidophilic as they generate large amounts of sulphuric acid during the reactions. One such species, T. ferrooxidans, can also grow chemoautotrophically by the oxidation of ferrous iron and is a major biological agent for the oxidation of this metal. Achromatium commonly occurs in freshwater sediments containing sulphide.
Cells of Achromatium store elemental sulphur, like Chromatium (a photoautotrophic bacterium), internally inside the granules that later disappear as sulphur is oxidised to sulphate. Most species of Beggiatoa can obtain energy from the oxidation of inorganic sulphur compounds but lack enzymes of Calvin cycle and thus require organic compounds as carbon source.
Such a nutritional lifestyle is called mixotrophy. Beggiatoa, the filamentous gliding sulphur-oxidising bacteria, occurs in nature mainly in H2S -rich sulphur springs, decaying seeweed beds, mud layers of lakes, and waters polluted with sewage.
In these habitats the filaments of Beggiatoa are usually filled with sulphur granules. An interesting habitat of Beggiatoa is the rhizosphere of plants (rice, cattails, and other swamp plants) living in flooded, and hence anoxic, soils.
Such plains pump oxygen down into their roots so a sharply defined oxic/anoxic boundry develops between the root and the soil. Beggiatoa (and probably other sulphur-oxidising bacteria) develop at this boundry, and plays a beneficial role for the plant by oxidising (as thus detoxifying) the H2S.
Iron-oxidising bacteria cause aerobic oxidation of iron from ferrous (Fe2+) to ferric (Fe3+) state that yields energy. Ferric (Fe2+) compounds are soluble, whereas the ferric (Fe3+) compounds, e.g., ferric hydroxide [Fe(OH)3] are insoluble. The insoluble ferric hydroxide precipitates in water and imparts reddish colour to it.
However, the general pattern of nutritional lifestyle of iron-oxidising bacteria can be represented as:
Thiobacillus ferroxidans, Leptospirillum, Gallionella, Ferrobacillus, Leptothrix, and Cladothrix are the representatives of iron-oxidising bacteria. The best studies ones are T. ferrooxidans and Leptospirillum ferrooxidans, which are very common in acid polluted environments such as coal-mining dumps. Other iron- oxidising bacteria are commonly found in water-logged soils, bogs, and anoxic lake sediments.
One of the most common forms of iron in nature is pyrite (FeS2), which is formed from the reaction of sulphur with ferrous sulphide (FeS) to form a highly insoluble crystalline structure, and is very common in bituminous coals and in many ore bodies.
The bacterial oxidation of pyrite is very significant in the development of acidic conditions in mining operations. Additionally, oxidation of pyrite by iron-oxidising bacteria is of considerable significance in the process called microbial leaching of ores.
Nitrifying bacteria are those chemoautotrophic bacteria that grow at the expense of reduced inorganic nitrogen compounds. No nitrifying bacterium is known that carries out the complete oxidation of ammonia (NH3) to nitrate (NO–3); thus, nitrification in nature results from the sequential action of two separate groups of nitrifying bacteria, the ammonia-oxidising bacteria (the nitrosifyers) and the nitrite-oxidising bacteria or true nitrifying (nitrate -producing) bacteria.
Nitrifying bacteria are widespread in soil and water and occur in highest numbers in those environments where considerable amounts of ammonia is present (e.g., sites where extensive protein decomposition takes place and in sewage treatment facilities). Nitrifying bacteria flourish especially in lakes and streams that receive inputs, of sewage or other waste waters because these are frequently rich in ammonia.
Most of the nitrifying bacteria are obligate chemoautotrophs (chemolithotrophs). Species of Nitrobacter are an exception as they may grow chemoheterotrophically (chemoorganotrophically) on acetic acid or pyruvic acid as sole carbon and energy source.
Ammonia-oxidising bacteria (the nitrosifyers):
The ammonia-oxidising bacteria or nitrosifying bacteria typically have genus names beginning in “Nitroso”. Nitrosomonas, Nitrosococcus, Nitrosospira, Nitrosolobus, and Nitrosovibrio are the major genera of nitrosofyers. These bacteria possess a key enzyme (ammonia monooxygenase) that oxidises ammonia (NH3) to hydroxylamine (NH2OH), which then is oxidised to nitrite (NO–2) by nitrosifying bacteria.
Nitrite-oxidising bacteria (the true nitrifying bacteria):
The nitrite-oxidising bacteria (the true nitrifying bacteria or nitrite-producing, bacteria) have genus names begining in “Nitro”. Nitrobacter, Nitrospira, Nitrococcus, and Nitrospira are the main representative genera of nitrate-producing bacteria. These bacteria oxidize nitrite (NO–2) produced by nitrosifyers to nitrate (NO–3).
The general pattern of nutritional lifestyle of nitrite -oxidising bacteria is as follows:
Hydrogen-oxidising bacteria represent the group of bacteria that oxidise H2 (the sole electron donor) and reduce O2 (the electron acceptor) via “knallgas” reaction, the reduction of O2 with H2. This reaction yields energy (-273 kJ/reaction), which is used in CO2 fixation. These bacteria are both gram-positive and gram- negative.
The best studied genera of this group of bacteria are Ralstonia, Pseudomonas, Paracoccus, and Alkaligenes; others are Acidovorax, Aquaspirillum, Hydrogenophaga, Hydrogenobacter, Bacillus, Aquifex, and Mycobacterium. All hydrogen-oxidising bacteria possess one or more hydrogcnase enzymes that function to bind hydrogen (H2) and use it either to generate ATP or for reducing power for chemoautotrophic growth.
Most hydrogen-oxidising bacteria flourish best under micro-aerobic conditions when growing chemoautotrophically (chemolithotrophically) on hydrogen because hydrogenases are oxygen-sensitive enzymes. Typically, oxygen levels of about 5-10% support best growth of these bacteria.
The general pattern of the nutritional life-style of chemoautotrophic (chemolithotrophic) hydrogen-oxidizing bacteria is as follows:
Almost all hydrogen-oxidising bacteria are facultative chemoautotrophs, i.e, they can also grow chemoheterotrophically (chemoorganotrophically) with organic compounds as energy source. This means that the hydrogen-oxidising bacteria have ability to switch between chemoautotrophic and chemoheterotrophic (chemoorganotrophic) modes of metabolism and generally do so in nature whenever required.
This is a major distinction between hydrogen-oxidising bacteria and many sulphur-oxidising bacteria or nitrifying bacteria; most of the latter two groups are obligate chemoautotrophs (i.e., their growth fails to occur in the absence of the inorganic energy source).
3. Photoorganoheterotrophs (Photoheterotrophs):
Some bacteria use light as energy source with organic compounds as the carbon source to grow. These bacteria are called photoorganoheterotrophs (photoheterotrophs) and belong to the group of purple nonsulphur bacteria. The latter have been called “nonsulphur” because it was originally thought that they were unable to use sulphide as an electron donor for the reduction of CO2 to cell material.
Recent studies clarified that most species of these bacteria can use sulphide although the level of sulphide utilised by them is quite low than that by purple sulphur bacteria. Most purple nonsulphur bacteria have additional ability to grow aerobically in darkness utilising organic compounds as electron donor.
It is their great ability to practice photoheterotrophy that likely accounts for their competitive success in nature. Purple non-sulphur bacteria are typically nutritionally diverse with respect to photoheterotrophy as they can use fatty, organic, or amino acids; sugars; alcohols; and even aromatic compounds like benzoate as carbon sources.
Purple non-sulphur bacteria possessing additional ability to photoorganoheterotrophy (e.g., Rhodopseudomonas, Rhodospirillum. Rhodobacter, Rhodovulum, Rhodopila, Rhodobaca, Rhodocyclus, Rhodoferax, Phaeospirillum, Roseospira, Roseospirillum, Rubrivivax, Rhodoplanes, Rhodobium, Rhodomicrobium) are considered to be intermediate between photolithoautotrophs and chemoorganoheterotrophs.
The general pattern of nutritional lifestyle of these bacteria can be represented as:
Majority of heterotrophic microorganisms belong to this nutritional category. Since they cannot synthesize their own food (organic substances) they obtain it directly from external environment using chemical energy- source. A clear-cut distinction between the carbon-source and the energy-source, characteristics of the three preceding nutritional forms, loses its clarity in the context of chemoorganoheterotrophs.
In the latter, both carbon and energy can usually be derived from the metabolism of a single organic compound. The chemoorganoheterotrophs include protozoa, fungi (including slime molds), and the great majority of bacteria.
The chemoorganoheterotrophic form of nutrition can further be divided into two categories on the basis of the physical state in which organic nutrients enter the cell. These categories are: holotrophit nutrition and absorptive nutrition.
This form of nutrition is accomplished by free-living protozoa and slime molds which directly engulf food, digest it inside their body, and egest unusuable remains. Such microorganisms are called “holotrophs” and they use two processes, the phagocytosis and the pinocytosis, to engulf and digest the food.
Phagocytosis represents the uptake of solid objects through small invaginations in the cell membrane that form intracellular vesicles. Majority of protozoa and slime molds obtain food by engulfing microorganisms like small protozoans, diatoms, rotifers, viruses, and micro fungi through this method.
The protozoan cell that engulfs microorganisms via phagocytosis is called “phagocyte”. The phagocyte extends small pseudopods around the microbe after the latter adheres on its surface. These pseudopods fuse and form a vacuole by means of invagination of the phagocyte-plasmamembrane engulfing or surrounding the object (process called “endocytosis”).
The vacuole is now called a phagosome. Lysosome granules move towards the phagosome, fuse with it to form a phagolysosome, and discharge, their contents (hydrolytic enzymes such as lysozyme, phospholipase, ribonuclease, deoxyribonuclease and several proteases) into the phagolysosome.
The hydrolytic enzymes initiate the killing and digestion of the entrapped microorganism and finally within a few hours, the victim is completely degraded and the contents are absorbed. If the engulfed object is not digestible, it is passed outside by the converse mechanism called exocytosis. The process of phagocytosis is shown schematically in Fig. 18.2.
Pinocytosis represents the uptake of fluids and soluble nutrients through small invagination in the cell membrane that subsequently form intracellular vesicles (Fig. 18.3). This process is also called “cell drinking” and is usually used by amoebae, and some flagellate and ciliate protozoa. The entry of the fluid or soluble nutrient occurs in a similar fashion to that of phagocytosis.
Saprotrophic and symbiotic microorganisms (most of the bacteria, colourless microalgae, micro-fungi) except slime molds and protozoa obtain their food from external environment in ‘dissolved form’ by the process of absorption. This form of nutrition is called absorptive nutrition.
Saprophytic micro-organisms decompose the complex organic matter of dead organisms, dung, sewage, excretory products etc. by secreting extracellular enzymes and absorb the resulting usuable organic metabolites as food. Symbiotic microorganisms, though live in association with other living organism, also employ similar mechanism for their food procurement.
Micro fungi are unique as one-to-all of them have adopted absorptive mode of nutrition, whether they live as saprotrophs or as symbionts, or they develop haustoria or not, they procure their food from external environment by absorption accompanied with external digestion.
The simple soluble molecules such as monosaccharides, amino acids, fatty acids etc. directly diffuse through fungal cell wall and move across the plasmamembrane by active or passive transport.
But, if the prefabricated food is in the form of insoluble complex organic polymers such as cellulose, hemicellulose, pectins etc., the fungal hyphae secrete a variety of extracellular digestive enzymes like cellulases, proteases, pectic enzymes, lipases, amylases etc. in their external environment. Each cell of the fungal hypha may feed independently and is able to synthesize variety of digestive enzymes.
The micro fungus as a whole can make use of different kinds of food at the same time. Digestive enzymes, however, break-up (digest) the insoluble complex organic polymers stepwise (Fig. 18.4) by a process known as hydrolysis (or digestion) finally into simpler soluble organic molecules.
After the external digestion is complete, the simple soluble organic molecules diffuse through the cell wall and move across the plasma membrane by active or passive transport. The absorbed organic metabolites (food) are used in metabolic processes and their excess is stored usually in the form of glycogen. The waste product, if any, leaves the fungal cell by diffusion.