| Quantification of rates, patterns and mechanisms that control uptake of CO2 by phytoplankton and the fate of the resultant organic carbon is an important central theme in oceanographic research. Phytoplankton production sets upper limits to both the overall activity of the pelagic food web and the quantity of organic carbon exported downwards. The nature and activity of the phytoplankton community are strongly influenced by physical and chemical factors that determine their light and nutrient availability. Phytoplankton losses by viral infection-induced death, grazing and sinking, however, restrain primary production and are thus equally important for ocean ecosystem productivity. These controlling processes influence the cycling of energy and biogeochemically relevant elements each very differently, directly affecting the production/respiration ratio of the ocean and the efficiency of the biological pump. As appropriately formulated by Kirchman, "how phytoplankton die largely determines how other marine organisms live". Phytoplankton biomass that sinks from the euphotic zone has a strong impact on carbon sequestration in the oceans, whereas grazed algae are channelled to higher trophic levels. Viral lysis directly affects the standing stock of dissolved organic carbon which forces the food web towards a more regenerative pathway. The increasing awareness that global change may affect stratification patterns in the ocean6 and consequently phytoplankton productivity and community structure, emphasizes the significance of studying the growth and fate of phytoplankton. Comprehensive and integrated studies are, however, largely lacking. As part of an integral and synergetic research program, the present subproject aims to mechanistically understand the ecological relevance of phytoplankton loss factors (viral lysis, grazing, sinking) in relation to changes in vertical stratification. The main aim of the present subproject translates into the following specific objectives: 1. To determine the phytoplankton, grazer and algal virus community abundance and composition in the upper mixed layer (UML), deep chlorophyll maximum (DCM) and onboard translocation experiments. 2. To link phytoplankton viral lysis and grazing in the UML, DCM and translocation experiments to phytoplankton community abundance, size class and composition. 3. To assess the ecological importance of loss factors (i.e. viral lysis and grazing) structuring the phytoplankton community in the surface ocean, DCM and translocation experiments in relation to physical/chemical water column characteristics. 4. To study the sensitivity for viral infection, grazing and sinking of representative phytoplankton species in laboratory cultures under different light, nutrient and temperature conditions. 5. To relate our findings to changes in seasonal stratification and vertical transport caused by climate change. Modifications in vertical mixing and stratification due to global warming will not only alter primary production and phytoplankton species composition, but also its fate. Shifts in algal abundance and species composition due to changes in stratification patterns will directly affect viral lysis and grazing rates. Viral infection is dependent on encounter rate between host and virus, and viruses have also a stringent host-specificity (often to strain level). Furthermore, grazers can be selective in their choice of prey (nutritious quality prey). During previous ALW-funded cruises (IRONAGES and MOMAP) supervised by the PI, it became clear that phytoplankton mortality due to viral infection and microzooplankton grazing can indeed differ for the relevant groups, size classes and population abundance. For the smallest picoeukaryotes in the subtropical oligotrophic North Atlantic Ocean viral lysis was responsible for 50-100% of the total cell losses. In contrast, cyanobacteria losses seemed to be dominated by grazing. It has to be noted that only the lytic viral life cycle was studied. Other studies suggested that the significance of lytic vs lysogenic infection decreases with increasing nutrient limitation. Additionally, our data suggested that mortality rates at the DCM were higher than in the mixed surface waters. This needs to be verified because data were very limited, but if this is true it would have direct implications for the carbon flux in the ocean. Changes in the extent of viral lysis and microzooplankton grazing will affect the relative importance of regenerated production (of special importance for oligotrophic environments). Because stratification can suppress or enhance primary productivity (depending on nutrient availability) and thus affect all of the above mentioned aspects, it is clear that there is need for more studies to clarify the relationships between viral infection and grazing at contrasting conditions as found along the North- South transect in the Northeast Atlantic. Furthermore, changes in vertical stratification may directly and indirectly (e.g. shift towards smaller-sized phytoplankton in response to more severe nutrient limitation upon reduced vertical mixing) impinge on the sedimentation rate of algae from the euphotic zone, of special importance for the mid- and higher-latitudes of the North Atlantic. |
