[7] Between 2.5 and 3.0 billion years ago, cyanobacteria started using the energy from light to split water, releasing oxygen into the anaerobic, reducing environment.
As Earth's climate became more oxidated, the process of fixing nitrogen became unfavorable, and natural selection eliminated some of the necessary genes for the nitrogenase protein complex to increase evolutionary fitness.
[5][9] The rate of oxygen created by photosystem II is much higher when Cyanothece does not fix nitrogen (when the medium is nitrogen-replete).
[5][14] Cyanothece balances the production of oxygen through photosynthesis and oxygen-sensitive nitrogen fixation and fermentation all in one cell.
[13] In a very energy-intensive process, nitrogenase is first synthesized[13][14] and then takes N2 from the air, combining it with protons and electrons to produce ammonia and hydrogen gas.
[5] Different Cyanothece species metabolize nitrogen-containing compounds through a variety of pathways; all have an arginine decarboxylase, but vary after that point.
[5] To provide the anoxic environment needed by nitrogenase, Cyanothece boosts its respiration as night begins by using its glycogen stores[12] while turning off photosynthesis.
[5] Some Cyanothece species also are capable of tryptophan degradation, methionine salvage, conversion of stored lipids into carbohydrates, alkane and higher alcohol synthesis, and phosphonate metabolism.
[5] They can switch between a photoautotrophic and photoheterotrophic metabolism depending on the environmental conditions that maximize their growth, employing the pathways that use the least amount of energy.
Between 4367 and 6642 coding sequences are an amalgamation of genes encoding capabilities for fermentation and aerobic nitrogen fixation (like filamentous cyanobacteria).
[5][15] The linear DNA encodes enzymes for glucose and pyruvate metabolism[15] (recall that glycerol is the only organic carbon source on which Cyanothece has grown successfully[10]), lactate fermentation,[8] transposons, and CRISPR proteins.
[2] Typically they are associated with water in benthic marine environments,[5] rice fields,[5] acidic marshes,[4] peaty bogs,[2] intertidal zones,[4][7] moors[3] and clear lakes,[3] but sometimes are found in mountain soils.
[3] Cyanothece species have a thin mucilaginous layer around a thick outer wall that contains spherical, glassy vesicles of unknown function.
[7] The granules are rapidly consumed to boost respiration, so remove the oxygen from the cell at the onset of nitrogen fixation.
When cultures are entrained in light-dark cycles, the nitrogenase and uptake hydrogenase are both active during the "night", with many copies per cell.
ATCC 51142 from cultures that were grown in continuous light at 30 μmol photons/(m2s), anaerobic conditions, 50 mM glycerol, and without any nitrate (so the nitrogenase was active).
[18] A parallel study has demonstrated concomitant and uninterrupted production of both H2 and O2 in continuously illuminated photobioreactor cultures, upon nitrogen-deprivation of ammonium-limited chemostat growth.