Description: 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. I, 47, 1621) 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.

Description: Blooms of large diatoms dominate the CO2 drawdown and silicon cycle ofthe Southern Ocean in both the past and present. The growth of theseAntarctic diatoms is limited by availability of iron (and light). Here wereport the first assessment of growth rates in relation to ironavailability of two truly oceanic Antarctic diatom species, the large,chain-forming diatom Chaetoceros dichaeta and the small, uni-cellulardiatom C. brevis. In filtered natural, untreated Southern Ocean water, amaximum specific growth rate of 0.62 ± 0.09 d-1 and a Km for growth of 1.12x 10-9 M dissolved iron was calculated for C. dichaeta. This response couldonly be seen during a long-day light period. C. brevis maintained growthrates of 0.39 ± 0.09 d-1 with and without iron addition, even under-shortday light conditions, and could only be forced into iron limitation byadding the siderophore desferri-ferrioxamine B (DFB), an iron immobilisingagent. Using this approach, the low Km value for growth of 0.59 x 10-12 Mdissolved Fe was calculated for this species. The size class dependentgrowth response to iron (and light) confirms the key role of theseparameters in structuring Southern Ocean ecosystems, and thus the CO2dynamics and the silicon cycle.