Nutritional types

Nutritional types of bacteria

All the bacteria (in fact all living organisms) essentially need energy, electron donors and carbon for their normal cellular activity, growth and reproduction. Energy in the form of ATP, electrons by oxidizing chemical compounds (electron donors) and carbon for their cellular activity are synthesized from different sources.

Energy: The bacteria, which derive energy from sunlight are called ‘phototrophs’ and those from chemicals are called ‘Chemotrophs’.

Electron: If an organism uses organic chemical sources (like glucose), they were referred as ‘Organotrophs’, those uses inorganic chemicals (like nitrate, ammonia, sulphate), are called as ‘lithotrophs’.

Carbon: If an organism uses CO2 as their sole carbon source, they are called ‘autotrophs’ and those use organic carbon sources are called as ‘heterotrophs’.

Based on the energy, electron and carbon requirements, there will be eight combinations of nutritional types can be derived.

Photo lithotrophic autotroph (These are generally called as photoautotrophs): Those organisms derive energy from sunlight; inorganic chemicals for electron and fixes CO2 as their carbon source are called photolithotrophic organisms. All plants, algae are phototrophs. In bacterial world, cyanobacteria are photolithotrophs (as that of plant). The process of photolithothrophic autotrophy is photosynthesis. Apart from these, purple Sulphur and Green Sulphur bacteria also perform the photolithotrophic life. They use H2S instead of H20 as source of electron.

Photo organotrophic heterotroph:Those organisms use sunlight for the energy and organic chemicals for electron and carbon are referred as photoorganotrophic heterotrophs. Ex. Rhodopseudomonas, Rhodobacter. These bacteria uses light-harvesting systems (like plants), but cannot use CO2, rather uses simple organic compounds as their carbon and electron sources.

Photo lithotrophic heterotroph: No organism reported so far.

Photo organotrophic autotroph: No organism reported so far.

Chemo lithotrophic autotroph: Those organisms use simple inorganic chemicals for their energy and electron requirement and fixed CO2 (like plants) are called Chemo lithotrophic heterotrophs. Example: Nitrosomonas, Nitrobacter. These organisms oxidize ammonia and nitrite respectively to nitrite and nitrate and derive their energy and electron. They fix the CO2 through Kalvin cycle. These kinds of organisms are numerously found in soil ecosystem, which are responsible for nutrient cycling.

Chemo lithotrophic heterotroph: Only one organism is example for this type of nutrition (Oceanithermus sp). Which is a typical organotrophic life, but can also utilize hydrogen as its electron source.

Chemo organotrophic autotroph: The organism which prefers the organic compounds for their energy and electron, but also fixes CO2 are called chemoorgano autotrophs. Example methanogens. They use organic compounds (malate, succinate, etc) as their energy and electron and reduces CO2 to CH4.

Chemo organotrophic heterotroph: Those organism solely dependent of organic compounds for their energy, electron and carbon are chemoorganotrophic heterotrophs. Human, animals, fungi are belong to this group. Most of the bacteria such as E. coli, Pseudomonas, Bacillus, Rhizobium, Azospirillum, food-borne pathogens, etc are also there in this group.

Animation of photosynthesis by plants and cyanobacteria

Animation of bacterial photosynthesis

All sorts of combinations may exist in nature. For example, most cyanobacteria are photoautotrophic, since they use light as an energy source, water as electron donor, and CO₂ as a carbon source. Fungi are chemoorganotrophic since they use organic carbon as both an electron donor and carbon source. Eukaryotes are generally easy to categorise. All animals are heterotrophic, as are fungi. Plants are generally photoautotrophic. Some eukaryotic microorganisms, however, are not limited to just one nutritional mode. For example, some algae live photoautotrophically in the light, but shift to chemoorganotrophy in the dark. Even higher plants retained their ability to respire heterotrophically on the starch at night which had been synthesised phototrophically during the day.

Prokaryotes show a great diversity of nutritional categories. For example, purple sulfur bacteria and cyanobacteria are generally photoautotrophic whereas purple non-sulfur bacteria are photoorganotrophic. Some bacteria are limited to only one nutritional group, whereas others are facultative and switch from one mode to the other, depending on the nutrient sources available.

Some, organisms can switch between different metabolic modes, for example between photoautotrophy, photoheterotrophy, and chemoheterotrophy. Those were referred as mixotrophs. Such mixotrophic organisms may dominate their habitat, due to their capability to use more resources than either photoautotrophic or organoheterotrophic organisms.

An oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments. Oligotrophs are characterized by slow growth, low rates of metabolism, and generally low population density. Ex. Cyanobacteria. Copiotrophs are fastidious growing organism with high population and diversified metabolic behavours. Ex. E. coli.

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