Pelagic Interactions & Food Webs

From tiny cells to large drifting animals, plankton organisms shape oceanic ecosystems and biogeochemical fluxes. Their interactions and behaviors determine how much carbon dioxide is fixed - and how much is transferred into the food web or exported into the deep sea. We use a suite of observational methods in the field, including bottle and net sampling (Fig. 1) as well as in situ optical and acoustic systems. With these, we determine how planktonic organisms are distributed in the ocean in relation to environmental drivers, e.g. low-oxygen eddies. Using data on abundance and vertical migration as well as species-specific metabolic rates under in situ conditions, we estimate their impact on elemental cycling in the water column, both in the sunlit surface and in the twilight and dark zones. Some pelagic organisms migrate vertically over hundreds of meters every morning and evening and contribute substantially to carbon transport. In some regions, however, they need to endure extremely low oxygen concentrations, which significantly reduces their activity (Fig. 2). This research area is led by Dr. Helena Hauss (who is also pictured in the Dark Ocean Research Topic photo).

Other studies in this area are on the diversity and interactions of surface ocean microbes – including algae and their predators as well as other entities, such predators and their symbionts and viruses. This area  is led by Alexandra Worden with participation from several team members, including Camille Poirier (now at Oxford), Susanne Wilken (now a Professor in Amsterdam), Kristin Bergauer (now a group lead at GEOMAR) and on the analysis side David Needham (now a Junior Professor at University of Kiel). We use multiple approaches, including Stable Isotope Probing (SIP) and at sea single-eukaryote cell sorting to tease apart interactions between protists and other biological entities in the sea. Sorting is followed by DNA sequencing and, from there, characterization of genomes, protein functions and transcriptional activities in the field (Fig. 3). In collaboration with the Alm Lab at MIT we are developing additional approaches as well. Some of our work in this area can be seen at Needham et al. PNAS 2019, Needham et al Proceedings of the Royal Academy B. Understanding interactions is important for then being able to predict the fate of fixed carbon – does it end up being exported into the deep sea, or is it respired while still in the surface ocean?