The Ecology of Anoxygenic Photosynthetic Bacteria

1. Requirements for anoxygenic photosynthesis
   1. Electron donors
      1. Unlike water, H2S is not universally available
      2. H2S and O2 will react (slowly), so the two do not coexist for very long
      3. Other electron donors
   2. Light
      1. Penetration of light depends upon absorbtion of water & pigments in the water
      2. Absorbtion spectra are different depending upon the organism's pigmentation
      3. The organism on the top gets the first chance at the light; deeper organisms have to use what they can get
      4. Thus the organism in the top layer can control what organsim can surviv in lower layers
   3. Other important factors
      1. Temperature
      2. Salt concentration
   4. Extreme environments can reduce competition
   5. Toxicity of H2S and O2 -- dancing with the devil
      1. Relatively low sulfide tolerance in proteobacteria, particularly non-sulfur proteobacteria
      2. Relatively high oxygen tolerance in proteobacteria
      3. High tolerance for H2S, low tolerance for oxygen in Green Sulfur Bacteria
   6. Diurnal variation
      1. Photosynthesis occurs in the light
      2. O2 production, H2S depletion
2. Stratified Environments
   1. Stratification in the Chesapeake Bay
3. Freshwater lakes
   1. Holomectic lakes (seasonally stratified)
      1. Euphotic zone (depends upon definition of photosynthetically active radiation)
      2. Chemocline
      3. Thermocline
      4. For anoxygenic photosynthesis to occur, chemocline must lie above lower bound of euphotic zone
   2. Factors affecting mixing
      1. Waves and wind action
      2. Salinity
   3. H2S (or H2) production by fermentation of organic matter under anaerobic conditions
   4. Meromectic lakes (permanently stratified)
4. Shallow bodies of water
   1. Lakes < 15m in depth, anaerobic zone is confined to muck on bottom
      1. Bacterial bloom on surface of sediments
   2. Spectacular seasonal blooms can occur in ditches and small ponds with substantial nutrient input
5. Mud flats and other sediments
   1. Similar to stratified lake, but on a solid sediment
   2. Bacterial mats
      1. Layering of taxa occurs in any stratified environment, but is very dramatic in bacterial mats
      2. Layers are compressed into a very small area
      3. Gliding motility is most useful here
   3. Salt marshes
   4. Salt flats
      1. Low solubility of oxygen in hypersaline waters
6. Hot springs and mineral springs
   1. Geochemical production of H2S
   2. Cyanobacteria are generally limited to relatively moderate temperatures
   3. Two ways of life for Chloroflexus, depending upon pH and H2S concentration
7. Motility
   1. Ability to control position in the environment
   2. Flagella
   3. Gas Vesicles
   4. Gliding
8. Symbiosis
   1. Photosynthetic proteobacteria are sometimes found as endosymbionts of marine sponges, which presumably have anaerobic pockets in their tissues.

Required Reading: M&C, pp 71-92 (you should already have read this), and Chapter 8

Supplementary Reading:

Schlegel, H.G., and B. Bowien. 1989. Autotrophic Bacteria. Science Tech Publishers, Madison WI. (Chapter 6)