ABSTRACT
Before the Earth's complete oxygenation (0.58 to 0.55 billion years [Ga] ago), the photic zone of the Proterozoic oceans was probably redox stratified, with a slightly aerobic, nutrient-limited upper layer above a light-limited layer that tended toward euxinia. In such oceans, cyanobacteria capable of both oxygenic and sulfide-driven anoxygenic photosynthesis played a fundamental role in the global carbon, oxygen, and sulfur cycle. We have isolated a cyanobacterium,
Pseudanabaena
strain FS39, in which this versatility is still conserved, and we show that the transition between the two photosynthetic modes follows a surprisingly simple kinetic regulation controlled by this organism's affinity for H
2
S. Specifically, oxygenic photosynthesis is performed in addition to anoxygenic photosynthesis only when H
2
S becomes limiting and its concentration decreases below a threshold that increases predictably with the available ambient light. The carbon-based growth rates during oxygenic and anoxygenic photosynthesis were similar. However,
Pseudanabaena
FS39 additionally assimilated NO
3
โ
during anoxygenic photosynthesis. Thus, the transition between anoxygenic and oxygenic photosynthesis was accompanied by a shift of the C/N ratio of the total bulk biomass. These mechanisms offer new insights into the way in which, despite nutrient limitation in the oxic photic zone in the mid-Proterozoic oceans, versatile cyanobacteria might have promoted oxygenic photosynthesis and total primary productivity, a key step that enabled the complete oxygenation of our planet and the subsequent diversification of life.
</jats:p
Is data on this page outdated, violates copyrights or anything else? Report the problem now and we will take corresponding actions after reviewing your request.