Effect of light, CO2 and nutrient limitation on photosynthesis in marine diazotrophic cyanobacteria.

The ocean plays a central role in the global carbon cycle. Uptake of carbon dioxide by the oceans has reduced the increase in atmospheric carbon dioxide that has arisen from fossil fuel burning and deforestation. It has long been know that the ocean biota play a major role in sequestering carbon dioxide on very long time scales (>1000 years). Recent evidence also suggests that the ocean biota play an important role on shorter time scales (10-100 years) as well. The balance between phytoplankton photosynthesis and community respiration determines the ability of the oceans to take up carbon dioxide. Nitrogen is generally considered to be the nutrient that limits phytoplankton photosynthesis. But what limits the amount of N in the ocean? Unlike most phytoplankton, which are N-limited, nitrogen fixing cyanobacteria have an unlimited supply of N. This is the N2 gas that is dissolved in seawater. Nitrogen-fixing cyanobacteria play a significant role in ocean nutrient and biogeochemical cycles as they are a major source of N, providing N for up to 50% of primary productivity in the most nutrient impoverished regions of the ocean. Nitrogen fixation is a key process that modulates the ability of the oceans to sequester carbon dioxide on time scales of 10 to 10,000 years. Limitation of nitrogen fixation results in lowered N availability for other primary producers reducing the potential of oligotrophic oceans to sequester carbon. This brings us to the issue of 'What limits the amount of nitrogen fixation in the ocean?' Amongst the environmental factors that may limit nitrogen fixation are temperature, light, carbon dioxide concentration and P- or Fe-limitation. It is argued that whereas N is the proximate limiting nutrient for phytoplankton photosynthesis in the sea, the ultimate limiting nutrient is either P (or Fe) because this nutrient limits the amount of nitrogen fixation. This proposal will examine the effects of light, carbon dioxide, P-limitation and Fe-limitation on photosynthetic properties and nitrogen fixation of nitrogen-fixing cyanobacteria. Research will be conducted under defined culture conditions in two species. One of these species, Trichodesmium, is documented to be of global significance. In addition, nitrogen fixation by unicellular cyanobacteria has recently been recognized to be significant. Therefore, the second species is one of these unicellular nitrogen fixers, Crocosphaera. The outcomes of this study will provide new insights into the mechanisms by which phosphorous and iron limit photosynthesis and nitrogen fixation in cyanobacteria. It will also provide new insights into the interaction with environmental factors such as light and carbon dioxide. This research will ultimately assist with several aspects of oceanographic studies on nutrient cycling and modeling the future importance of the oceans as C sinks.