Professor John Raven FRS FRSE
Photosynthetic organisms on Earth convert about 10 Pmol of carbon dioxide to organic carbon in net primary productivity each year. Almost half of this carbon is assimilated by phytoplankton living in the top 100 m or so of the ocean. Since more then two-thirds of the Earth’s surface is occupied by water, the ocean has a lower productivity per unit area than the land surface, despite the presence of significant areas of desert of very low productivity. Much of my work relates to the factors restricting primary productivity in the ocean. The constraints can be conceptually separated into ‘top down’ and ‘bottom up’ factors. ‘Top down’ factors, such as grazing and parasitism, remove primary producer biomass. ‘Bottom up’ factors limit the growth rate of primary producers, such as limitation of the supply of essential resources such as light and carbon dioxide for photosynthesis and of the nitrogen, phosphorus, iron and zinc needed to produce cells from photosynthetic products. Almost all of the work I have been involved in has involved ‘bottom up’ factors, and especially the results of interactions among resource supply limitations. As far as possible the laboratory experiments and mechanistic modelling relate to real world situations and ecologically important organisms.
One thread has involved studies of how constraints on the supply of light for photosynthesis interact with the capacity to assimilate other resources. Work on the important marine nitrogen-fixing cyanobacterium Trichodesmium in the Central Atlantic showed that growth, and hence the capacity to supply available nitrogen to eukaryotes that cannot use nitrogen gas, is limited by availability of light and of phosphorus.The supply of iron was not limiting in this area, despite the known high requirement for iron to produce nitrogen-fixing enzymes and the known restriction by iron supply on nitrogen fixation in other parts of the ocean. Subsequent work addressed the phylogenetic variations in elemental content among the great diversity of marine phytoplankton organisms when grown in nutrient-sufficient conditions, and the effects on these contents of growth with varying light supplies. In both cases there were significant differences between the measured cell quotas and those computed from mechanistic models using the elemental requirements for essential enzymes and quantities of each enzyme needed for balanced cell growth. While some of these discrepancies await a mechanistic explanation at the cell level, the observed variations in elemental stoichiometry have important repercussions for biogeochemical cycles. Attempts to find mechanism for the discrepancies continue.
A second thread has emphasised the significance of variations in the mechanisms by which marine phytoplankton acquire the carbon dioxide used in photosynthesis. Despite the apparent excess of inorganic carbon relative to available nitrogen, phosphorus and iron in surface seawater, the speciation of inorganic carbon in seawater, and the kinetics of the core carboxylase ribulose bisphosphate carboxylase-oxygenase, explain the almost universal occurrence of inorganic carbon concentrating mechanisms (CCMs) in marine phytoplankton. Recent work has investigated the phylogenetic distribution of the various CCMs, how their expression is influenced by the availability of other resources, and how some phytoplankton organisms can manage without CCMs. This work is continuing, with emphasis on responses to the changes in surface ocean chemistry attendant on increasing anthropogenic inputs of carbon dioxide to the atmosphere and hence to the surface ocean.