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Bacterial Metabolism & Photosynthesis

One of the most staggering aspects of bacteriology is the diversity of metabolism these organisms display. There is no known naturally-occurring carbon-containing molecule which cannot be metabolized by at least one species. Originally, two categories of metabolism were recognized:

However, this simple classification cannot accurately describe the metabolic diversity of bacteria. In order to accomplish this, bacterial metabolic types are usually defined on basis of both the energy and carbon source used:

Autotrophs:

Energy Source:

Carbon Source:

Name:

Example:

Light

Inorganic

Photoautotroph

Most photosynthetic bacteria, e.g. Chromatium (anaerobic), Cyanobacteria (aerobic)

Inorganic

Inorganic

Chemolithotrophic autotroph (chemoautotroph)

Nitrobacter

Organic

Inorganic

Chemoorganotrophic autotroph

Pseudomonas oxalaticus

Heterotrophs:

Energy Source:

Carbon Source:

Name:

Example:

Light

Organic

Photoheterotroph

Purple and green photosynthetic bacteria, e.g. Rhodospirillum

Inorganic

Organic

Chemolithotrophic heterotroph ("mixotroph")

Desulphovibrio

Organic

Organic

(Chemo)heterotroph

E. coli

The order Rhodospirillales contains three families of phototrophic bacteria:

Rhodospirillaceae: purple non-sulphur bacteria, e.g. Rhodospirillum. Cells of Rhodospirillum are helical, ~1 µm in diameter and variable in length. These cells contain bacteriochlorophyll a or b located on specialized membranes continuous with the cytoplasmic membrane. They are not able to use elemental sulphur as electron donor and typically use an organic electron donor, such as succinate or malate, but can also use hydrogen gas. The following digital video shows motile Rhodospirillum rubrum cells. Notice the purple colour of the culture at the start of the video:

Chromatiaceae: purple sulphur bacteria, e.g. Chromatium. These are short, Gram-negative rods, ~1 µm in diameter and 3-4 µm long. They are able to use sulphur and sulphide as the sole photosynthetic electron donor and sulphur can be oxidized to sulphate. These bacteria use an inorganic sulphur compound, such as hydrogen sulfide as an electron donor. Purple sulphur bacteria must fix CO2 to live, whereas non-sulphur purple bacteria can grow aerobically in the dark by respiration on an organic carbon source. The following digital video shows sulphur granules in Chromatium cells. The bright crystals inside the cells are the sulphur granules deposited as a by-product of photosynthesis and revealed here by phase-contrast microscopy. (Note that this is a mixed culture from an environmental isolate, so other types of bacteria are also present):

Chlorobiaceae: green sulphur bacteria. These cells contain bacteriochlorophyll c or d located in chlorobium vesicles attached to the cytoplasmic membrane.

 

Microbes are the predominant photosynthetic organisms in most aquatic environments. In aerobic conditions, (e.g. shallow water), algae, diatoms and Cyanobacteria predominate. In anaerobic conditions (polluted or eutrophic waters), other photosynthetic bacteria are dominant.

Cyanobacteria such as Spirulina (Arthrospira) are also photoautotrophs, able to use CO2 as their sole carbon source and light as their energy source. However, unlike other photosynthetic bacteria, Cyanobacteria use the same photosynthetic pathway as eukaryotic cells such as algae and higher plants (the "C3" or "Calvin" cycle). (Other photosynthetic bacteria use different light-harvesting pigments (bacteriochlorophyll) and metabolic pathways). Even more interesting is that we now know from DNA sequencing that eukaryotic chloroplasts evolved from Cyanobacteria, presumably after millions of years of symbiotic association.

 


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