Picophytoplankton is one of the most successful algal groups and contributes 25-55% of global phytoplankton biomass, and plays an important role as a primary producer in the microbial loop. It includes picoeukaryotes (≤ 2-3µm) and picoprokaryotes, represented by Prochlorococcus and Synechococcus.

To project the impact of climate change on the marine ecosystem, it is necessary to understand all major processes inside the ocean, including the physiological response of picophytoplankton to changes in light, temperature and nutrient regimes. Ocean biogeochemical models project an increase in the extent of the subtropical gyres where picophytoplankton are the dominant algal group, suggesting this group could gain even more importance.

In this study we present new laboratory data which will contribute to the marine World Ocean Atlas of marine Ecosystem data (MAREDAT) to help to better parameterise this small plankton functional type in marine biogeochemistry models.

We show that their physiological response to light, calculated as initial slope of the photosynthesis versus irradiance curve (α^Chl) is about 2 times higher for picoprokaryotes than for picoeukaryotes (8.6 and 4.2 g C mol^-1 photons m^-² g^-1 Chlorophyll a). This agrees with their common distribution at the deep chlorophyll maximum. However, α^Chl shows no increased affinity to light compared to other major phytoplankton groups.

Temperature experiments showed optimum growth temperatures of 22.7 ± 2.0 °C for picoprokaryotes and 23.6 ± 3.1 °C for picoeukaryotes with maximum growth rates of 0.47 ± 0.17 d^-1 and 1.05 ± 0.47 d^-1. These are significantly lower than those of diatoms, but not significantly different from Coccolithophores which allows a characterisation of picophytoplanton as K-strategists.

Preliminary nutrient experiments showed strong deviations from redfield ratios with significant differences between phosphorus and nitrogen limitation.

For investigating the impact of climate change on picophytoplankton in the future ocean, we modified physiologically relevant parameters derived from our experiments (α^Chl, chlorophyll to carbon ratio (θ), maximum growth rate at 0°C (µmax₀) and Q₁₀) in an ocean biogeochemical model. We compared preindustrial conditions to the high emission scenario RCP8.5.

We show that there are no significant changes in total biomass of picophytoplankton from 1800 to 2100. However their contribution to total primary production increases in tropical and subtropical regions by up to 16% leading to decreased export of carbon particles of up to 30% to the deep ocean in these areas.