Methods for the species-specific analysis of photosynthesis and top-down vs. bottom-up control in marine phytoplankton

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Phytoplankton builds the basis of all aquatic food webs. Species in an algal community differ in abundance and photosynthetic performance and production. These differences can have an essential effect on energy transfer in the food web. Abundant species with low production rates are possibly not grazed by zooplankton and thus do not contribute to energy transfer in the food web. In contrast, high production rates of less abundant species could indicate that they are grazed preferentially. Therefore, they might be of higher importance for energy transfer in the food web than abundant species.To test these hypotheses, methods for species-specific analysis of phytoplankton are to be tested in the Royal Netherlands Institute for Sea Research (NIOZ) in Yerseke, The Netherlands. Flowcytometry allows a fast characterization of the phytoplankton community (cell numbers and fluorescence as a measure of photosynthetic capacity), but only a coarse differentiation of algal groups. Single-cell analysis with a combination of microscope and pulse-amplitude modulation (PAM) fluorometry is much more detailed. Species can be determined accurately and their photosynthetic performance can be measured at the same time. This method is time-consuming and allows only for the measurement of a limited number of cells. A combination of flowcytometry and microscopy-PAM seems promising to test the hypotheses. The three months stay at the NIOZ in Yerseke will be used for optimizing the described methods for the single-cell analysis of phytoplankton. Afterwards, the phytoplankton community in the North Sea is to be characterized. Following the stay, the acquired knowledge will applied in a planned research project that will deal with energy transfer in the food web of the North and Baltic Sea.