Phytoplankton in the Elbe estuary: New insights into community composition and mixotrophy, from metabarcoding, flow cytometry, and laboratory experiments

Abstract

Estuaries, the lower river areas that merge into the oceans, are centers of human lifes, home for a diverse wildlife and important in global carbon cycles. Phytoplankton are the basis of the food chains in estuaries but may suffer from harsh conditions here. For instance, turbidity can darken the water body, but light is needed for photosynthesis. Species that live under such conditions may play an important role because they provide food for zooplankton and take up CO2. However, there is not much knowledge about estuarine life. This is also the case in the Elbe estuary, one of Europe’s largest estuaries, where the lower ca. 70 km are largely unexplored. Even in explored areas of this habitat, certain groups of phytoplankton may have been overlooked so far due to the choice of methods.

In our study, we applied molecular tools (metabarcoding) and automated cell counting techniques (flow cytometry) to assess phytoplankton composition in the Elbe estuary. Our focus was to identify important phytoplankton groups in the unmonitored mid to lower estuary and to compare our results from further upstream with available microscopy data. We also carried out laboratory experiments, to assess the ability of phytoplankton from the Elbe estuary to use organic substances (e.g. sugars), a strategy considered mixotrophy.

In compliance with former knowledge, phytoplankton communities were overall dominated by diatoms. However, we found that the smallest players among the phytoplankton - the picophytoplankton - and potentially mixotrophic flagellates were more dominant than so far reported. Due to their fragile nature and small size, those groups were likely overlooked in former microscopy-based studies. In our laboratory study, we furthermore found that all included phytoplankton species were able to use organic substances. Our results altogether provide support for the importance of mixotrophy in the Elbe estuary.