This document discusses different types of microbial phototrophs. It describes oxygenic phototrophs like cyanobacteria which produce oxygen during photosynthesis using water as the electron donor. It also describes anoxygenic phototrophs which do not produce oxygen, including purple sulfur and nonsulfur phototrophs which use inorganic sulfur compounds or hydrogen as electron donors, green sulfur phototrophs which also use inorganic sulfur compounds, and green nonsulfur phototrophs and heliobacteria. Each group is characterized by their electron donors, habitats, and examples of genera.

1. Microbial metabolism
Nidhi Jodhwani

2. Unit 1- Bacterial photosynthesis
• What is photosynthesis?
• Photosynthesis is the conversion of light energy into chemical energy used for
growth. Organisms that obtain most or all of their energy in this way are
called phototrophic or photosynthetic

3. • 1.1 types of photosynthetic micro organisms:
• The phototrophic prokaryotes are a diverse assemblage of organisms that
share the common feature of being able to use light as a source of energy for
growth.
• Their classifi cation is based upon physiological differences, including whether
they produce oxygen, and what they use as a source of electrons for
biosynthesis

5. Oxygenic phototrophs
• use H2 O as the electron donor for the photosynthetic reduction of
NADP
• Most of the species belong to the cyanobacteria
• widely distributed in nature, occurring in fresh and marine waters,
and in terrestrial habitats
• They have only one type of chlorophyll (chlorophyll a), and light-
harvesting pigments called phycobilins

7. • another kind of oxygenic microbial phototroph consists of the
prochlorophytes
• There are three genera among the prochlorophytes
Prochloron
• Obligate symbionts of ascidians
Prochlorothrix
• Filamentous, free living
• Fresh water lakes
Prochlorococcus
• Free living
• Marine env.

8. Anoxygenic phototrophs
• The other phototrophic prokaryotes do not produce oxygen (i.e., H2
O is not a source of electrons). Instead, they use organic compounds,
inorganic sulfur compounds, or hydrogen gas as a source of electrons.

9. • Purple sulfur phototrophs
• use hydrogen sulfide as the electron donor
• CO2 as the carbon source
• habitats are freshwater lakes and ponds, or marine waters
• oxidize sulfide to elemental sulfur, which accumulates as granules
intracellularly
• E.g. Chromatium
• Ectothiorhodospira, which deposits sulfur extracellularly

10. • Purple nonsulfur phototrophs
• Originally it was thought that these bacteria were not able to utilize sulfi de as
a source of electrons, hence the name “nonsulfur.”
• However, it turns out that the concentrations of sulfi de used by the purple
sulfur bacteria are toxic to the nonsulfur purples and, provided the
concentrations are suffi ciently low, some purple nonsulfur bacteria (viz.,
Rhodopseudomonas and Rhodobacter) can use sulfi de and/or thiosulfate as
a source of electrons during photoautotrophic growth sources of energy and
carbon
• They are found in lakes and ponds with low sufi de content. Representative
genera are Rhodobacter and Rhodospirillum.

11. • The purple nonsulfur phototrophs can grow photoautotrophically or
photoheterotrophically in anaerobic environments
• Photo auto trophic growth uses H2 as the electron donor and CO2 as
the source of carbon
• Photoheterotrophic growth uses simple organic acids such as malate
or succinate as the electron donor and source of carbon, and light as
the source of energy.

12. • Green sulfur phototrophs
• strict anaerobic
• photoautotrophs
• use H2 S, S°, S 2− 2 3 O (thiosulfate), or H2 as the electron donor
• CO2 as the source of carbon
• Their light-harvesting pigments are located in special inclusion bodies called
chlorosomes
• E.g. Chlorobium

13. • Green nonsulfur phototrophs (green gliding bacteria)
• The best-studied green nonsulfur photosynthetic bacterium is Chloroflexus
• thermophilic, fi lamentous, gliding green phototroph
• Chlorofl exus can be isolated from alkaline hot springs, whose pH can be as
high as 10.
• Most isolates have a temperature optimum for growth from 52 to 60 °C.
Chloroflexus can be grown as a photoheterotroph, photoautotrophically with
either H2 or H2 S as the electron donor, or chemoheterotrophically in the
dark in the presence of air

14. • Heliobacteria
• The heliobacteria are recently discovered, strictly anaerobic
photoheterotrophs that differ from the other phototrophs in having
bacteriochlorophyll g as their reaction center chlorophyll, in containing few
carotenoids
• The two known genera are Heliobacterium and Heliobacillus
• most species of heliobacteria make endospores that resemble those
produced by Bacillus or Clostridium