Seasonal biochemical composition of the pelagic tunicate Salpa thompsoni

Salps are pelagic tunicates, which have been observed in marine habitats worldwide. They are omnivorous filter feeders filtering water for nutrition while swimming by contracting their circular muscle bands and pumping water trough their oral siphon. By filtering water for nutrition salps produce large, fast-sinking pellets, which are dense and rich in carbon, nitrogen, phosphorous and trace elements. Next to their significant role in the marine biological pump by exporting particulate carbon from surface waters to the seafloor, it has been shown that especially the salp species Salpa thompsoni is considered to be an important link between surface production and benthic communities. Continuous global warming and the accompanied decrease of sea ice cover in the Antarctic Ocean have been correlated to decreasing krill stocks and increasing salp populations. Furthermore, a southward shift in the distribution of S. thompsoni has been observed. However, it is still unknown how salps perform during seasonal and temperature related changes. Therefore, the first aim of my thesis is to compare possible seasonal differences in biochemical composition of S. thompsoni over the change from summer to winter. Salpa thompsoni specimen were collected in austral summer 2012/2013, in winter 2016 and in autumn 2018 in the Antarctic waters. On board of the research vessels, the salp samples were sorted and the body length was measured. Afterwards the salps were snap frozen immediately until further processing in the home institute. By analysing the energy stores (total body protein, carbohydrate and glycogen) and specific activities of the key enzymes 3-hydroxyacyl-CoAdehydrogenase (HOAD) and malate dehydrogenase (MDH) seasonal differences in body composition can be detected. Especially analysis of glycogen content in S. thompsoni specimen show a significant increase from summer to winter month. The second part of my thesis addresses the differences in biochemical activity between both life history stages. The life cycle of Salpa thompsoni, and for all tunicates characteristic, involves two life history stages with asexual reproducing oozoids (solitaries) and the sexual reproducing blastozooids (aggregates). The results show seasonal differences within each life history stage as well as significant differences between aggregates and solitaries. Noteworthy is the MDH activity in winter, where aggregates have higher activity compared to solitaries. Additionally, solitary specimen show significant decreasing values from autumn to winter, suggesting a metabolic slow-down in winter