Picophytoplankton; a comparative study of their biochemical composition and photosynthetic properties

Picophytoplankton are a small or major component of the phytoplankton community and present in all oceanic systems, from pole to pole. They dominate in the low chlorophyll biomass areas, such as the (sub)tropical regions, but also contribute considerably (up to 20%) in the high chlorophyll biomass areas. The ecosystems of occurrence contrast significantly in physical and chemical settings. This includes a strongly mixed upper water column replete in nutrients as well as a strongly thermally stratified euphotic zone depleted in nutrients at the surface with a steep inverse light and nutrient gradient. These changes impose a strong impact on the composition of the picophytoplankton community but also on the biochemical and physiological properties of the species present. In particular, the pigmentation and cellular carbon, nitrogen and phosphorus quota and requirement will differ from a stratified compared to a well-mixed water column. As a result no characteristic values for the parameters required for this specific algal group in a global phytoplankton carbon model (the SWAMCO model, Lancelot et al. (2000), Deep-Sea Res. 1, 47, 162 1) can be given. In the present paper an inventory is made of the biochemical, physiological and photosynthetic parameters of two species of cyanobacteria (Prochlorococcus and Synechococcus) and the pico-size class fraction of the eukaryote phytoplankton component. Other groups of phytoplankton, such as diatoms, Trichodesmium, Phaeocystis and coccolithophorids, will be discussed in separate papers in this issue. This inventory is a mixture of laboratory experiments using well-defined algal populations as well as data derived from field surveys including a mixture of species. Where possible, the relevance of the parameters will be discussed in relation to the nature of the physico-chemical conditions of the area of occurrence. (C) 2004 Elsevier B.V. All rights reserved.