Microbial food web experiments

Oliver Müller
University of Bergen

Lasse M. Olsen
University of Bergen

Christine Gawinski
PhD fellow
UiT The Arctic University of Norway

All microorganisms – from viruses via ciliates to copepods – are linked via nutritional needs and predation in the microbial food web. For example, when algae are infected by viruses and die, bacteria can utilize the organic matter and increase in numbers and become pray for larger microorganisms which at some point in the food web affects top predators like fish.

In the conceptual Minimum Marine Microbial Food Model, bacteria, autotrophic flagellates and diatoms compete for mineral nutrients (nitrogen and phosphorus). Bacteria and diatoms are additionally controlled by the supply of carbon and silicate respectively, while the autotrophic flagellates and the diatoms also depend on light. The predators, heterotrophic flagellates, ciliates and mesozooplankton exert a top-down control on smaller organisms and constitute a grazing food chain. Manipulating top predator level and nutrient conditions can give insights on how the system is controlled, how the groups interact (top-down, bottom-up, cascading effects) and how the nutrients flow through the system.

During the Nansen Legacy cruises we get access to ice covered areas of the Barents Sea, where we can take samples to identify the microorganisms and design experiments to understand their processes. Water samples for the experiments are prepared by gentle reverse filtration of surface water to retain organisms of different size fractions (<0.8µm; <3µm; <90µm) and are incubated for several days at close to in situ light and temperature. Additional manipulations include the addition of different organic matter sources (permafrost soil and algae exudates) and different zooplankton predators (Oithona and Calanus) to investigate bottom up and top-down controls.