Work Package 13 (Hillebrand, Brumsack/Pahnke, Feudel/Freund)

"Effect of growth rate and elemental availability on the stoichiometry of phytoplankton: beyond N and P"

Broader background of the proposed research project

The elemental composition of phytoplankton species constrains elemental fluxes within pe-lagic food webs and across ecosystem boundaries (Copin Montegut and Copin Montegut 1983, Sterner et al. 1992). The stoichiometry of elements, e.g. the canonical Redfield ratio of C:N:P content, has been in the focus of marine ecology (Goldman et al. 1979, Klausmeier et al. 2004). Recently, we summarized the available empirical information on N:P ratios in phytoplankton (Hillebrand et al. 2013) and found that faster absolute growth rates increase the relative P-content (as predicted by the growth rate hypothesis sensu Sterner et al. 1992) and constrain the variability of N:P ratios (as predicted by models on optimal ratios sensu Klausmeier et al. 2004). However, the elemental content did not converge on the Redfield ratio. Moreover, even at highest growth rates the N:P ratios showed an imprint of previous limitation and a difference between algal groups. So far, this information has mainly been assessed for N and P content and rarely for trace elements (Ho et al. 2003). Within EcoMol, we will combine quantitative meta-analysis with new laboratory assays to expand the knowledge on elemental composition of algae with respect to growth rate, phylogeny and abiotic conditions.

Outline for the proposed PhD research project

In this project, we will combine a quantitative meta-analysis on elemental content of marine phytoplankton with growth experiments conducted in flow-through cultures. For the meta-analysis, we will obtain data from published papers measuring the elemental content of algae (either single species or communities) including macronutrients (C, N, P, Si) and trace elements (e.g. Mo, Mn, Fe, Co). The stoichiometry of these elements will be assessed with regard to potential regulatory variables including temperature, cell size, growth rate, and phylogeny of the taxa involved. This meta-analysis tests the hypothesis that – in addition to N and P – also the content of other elements in phytoplankton is regulated by growth conditions. In a second stage, we will amend this literature by using chemical analysis (WP 14, nutrients and trace metals) of phytoplankton species isolated from the North Sea and growing in exponentially fed chemostats under different dilution (= growth) rates. Afterwards, these cultures will be grown in pairs and full mixtures to test the hypothesis that the presence of competitors alters the elemental incorporation per species.