Sensitivity of phytoplankton to future changes in ocean carbonate chemistry: current knowledge, contradictions and research directions

Despite their microscopic size, marine phytoplankton are responsible for about half of the global primary production and represent the basis of the marine food web. This diverse group of organisms drives important biogeochemical cycles, exporting massive amounts of carbon to deep waters and sediments, and strongly influencing ocean - atmosphere gas exchanges. Anthropogenic climate change will result in significant alterations in the marine environment over the next 100 yr and beyond. The increase in atmospheric CO, has already caused significantly higher aquatic CO2 concentrations and lower pH values ('ocean acidification') than in pre-industrial times. Rising temperatures will also impact surface ocean stratification, which in turn will affect the surface-water light regime and nutrient input from deeper layers. Phytoplankton will be affected by these environmental changes in many ways. In this article we assess the possible responses of different phytoplankton groups with regard to the expected physico-chemical changes. In addition to summarizing laboratory and field studies, we outline the current understanding of the underlying mechanisms that cause processes such as photosynthesis, calcification, and nitrogen fixation to be sensitive to ocean acidification. We describe different approaches to manipulate carbonate chemistry (e.g. acid/base or CO2 addition), discuss their potential to simulate future ocean acidification, and allude to common problems in experiments caused, for instance, by high biomass or the use of buffers. In addition to guidelines for CO2 perturbation experiments, we argue that it is essential to look at multiple environmental factors in combination with CO2, to aim for process-understanding rather than correlation, and to assess a wider diversity of phytoplankton species both in laboratory and field studies.