Photosynthesis anoxic

The balance between relative rates of aerobic respiration and water movement were considered in Section 4.3.4. We saw that a subsurfece concentration minimum, the oxygen minimum zone (OMZ), is a common characteristic of vertical profiles of dissolved oxygen and is produced by in situ respiration. Waters with O2 concentrations less than 2.0 ppm are termed hypoxic The term anoxic is applied to conditions when O2 is absent. (Some oceanographers use the term suboxic to refer to conditions where O2 concentrations fall below 0.2 ppm but are still detectable.) As illustrated by Figure 4.21b, this water column is hypoxic in the OMZ. The dissolved oxygen concentrations are presented as % saturations in Figure 4.21c. With the exception of the mixed layer, the water column is undersaturated with respect to dissolved oxygen with the most intense undersaturations present in mid-depths. Surface supersaturations are the result of O2 input from photosynthesis and bubble injection. 

Nonconservative elements elements that do not remain in constant proportion due to biological (e.g., uptake via photosynthesis) or chemical (e.g., hydrothermal vent inputs) processes. In estuaries, as well as other oceanic environments (e.g., anoxic basins, hydrothermal vents, and evaporated basins), the major components of seawater can be altered quite dramatically due to numerous processes (e.g., precipitation, evaporation, freezing, dissolution, and oxidation). 

Many phototrophs do not produce O2 as a waste product. Such anoxygenic phototrophs are comprised of purple sulfur, purple nonsulfur, green sulfur, and green nonsulfur bacteria. Although purple sulfur bacteria are typically found in anoxic zones of lakes and sediments, many are capable of photosynthesis under oxic conditions (Van Gemer-den, 1993). Most fix N2 and store S° intra- or extracellularly, and some are capable of chemo-lithoautotrophic growth. Extreme halophilic, sulfi-dic, and mildly thermophilic environments harbor... 

In those organisms that perform photosynthesis, further phosphorylation of D-ribulose 5-phos-phate into D-ribulose 1,5-diphosphate by phosphoribulokinase represents an important prerequisite in CO2 fixation. As has already been mentioned, the key and extensively studied enzyme involved in this reaction is D-ribulose 1,5-bisphosphate carboxylase/oxygenase. The subfamily divergence in this multigene family has been studied in certain plants of Triticeae and other families [13]. The occurrence of this enzyme in anoxic Archaea is very interesting, since it had to evolve in the absence of molecular oxygen [14]. 

Anoxic photosynthesis utilizes light energy, but does not produce oxygen. Green and purple bacteria of the sulfur and nonsulfur kind are anoxic photosynthesizers. The sulfur bacteria oxidize sulfide to elemental sulfur and can even oxidize it further to sulfate. 

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