The ecophysiological basis for co-variability in light-limited and saturated rates of phytoplankton photosynthesis.

The phytoplankton are a diverse group of single celled organisms which live in the sunlit upper layer of the oceans. These organisms are responsible for the vast majority of the photosynthetic conversion of light energy to chemical energy in oceans. Phytoplankton thus form the basis of the marine food chain and are ultimately responsible for around half the biologically mediated global production of oxygen and removal of carbon dioxide from the atmosphere each year. Due to their key role in controlling the cycles of these major elements, it is important that we understand what controls phytoplankton photosynthesis and ultimately how susceptible these organisms might be to ongoing climate change. A necessary first step in understanding the global impact of phytoplankton is an accurate measure of the amount of carbon they take up. Phytoplankton carbon fixation can be reasonably well approximated as a function of the amount of light available for photosynthesis and the amount of the pigment, chlorophyll, which absorbs this light. Both of these variables can be accurately measured using a number of methods ranging from small scale ship based measurements to large scale year round repeated measurements using earth observing satellites. However, a simplistic treatment of phytoplankton carbon fixation simply as pigment multiplied by light ignores important variability in phytoplankton physiology. Indeed, the efficiency with which phytoplankton convert the light they absorb into carbon varies hugely in the oceans. Much of this variability is currently poorly explained, introducing large uncertainties into our best estimates of the amount of carbon fixed each year. The current study aims to address some of this uncertainty by studying a specific aspect of unexplained variability in phytoplankton photosynthesis in a range of environments. During research cruises we will make simultaneous measurements of the overall rate of phytoplankton light absorption (which is equivalent to the conversion of photons of light to electrons in their photosynthetic apparatus) and the rate of carbon fixation. At the same time we will measure a number of factors which we suspect are responsible for causing the unexplained variability in the ratio of light absorption and carbon fixation. By performing this work we will not only increase our understanding of the physiology of these important organisms, but will also make significant progress towards increasing the accuracy of our current best estimates of phytoplankton carbon fixation.