Thiamin (vitamin B1) in the aquatic food web

Sammanfattning: Thiamin (vitamin B1) is required for several life-sustaining processes in most organisms and cells, e.g. in the conversion of food to energy. It also serves as an antioxidant and is important for proper nerve signaling. Thiamin is produced predominantly by bacteria and phytoplankton in the aquatic food web. Not all bacteria and phytoplankton, nor any organisms in higher trophic levels can produce thiamin; instead, they rely on a continuous external supply and uptake of this essential compound. Thiamin deficiencies occur episodically in a wide range of taxa, especially in higher trophic levels. In the Baltic Sea, thiamin deficiencies has been most pronounced in salmon (Salmo salar) and recently deficiencies are also reported for other fish species and birds.This thesis focuses on thiamin dynamics in lower trophic levels, covering primary producers as well as primary consumers, as this topic has not received much attention previously. Thiamin content of common phytoplankton and zooplankton species and the transfer between them was investigated in experiments and field studies. The relationship between thiamin deficiency and underlying environmental factors was also investigated using monitoring data.Thiamin content differed among phytoplankton classes, species and even strains. Filamentous Cyanophyceae had considerably higher thiamin content than other classes. However, thiamin transfer to copepods was lower, probably associated with difficulties ingesting the filaments. Moreover, thiamin content in seston varied seasonally, being highest during summer when both Prymnesiophyceae and filamentous Cyanophyceae were more abundant. Thiamin content in the two size fractions correlated strongly and was always higher in the smaller size fraction, illustrating the importance of picoplankton and bacteria in the food web. Also, seston thiamin content was higher in the Baltic Proper than in the Skagerrak. Copepods differed in thiamin content among genera, as well as between locations. Acartia sp. had the highest thiamin content and copepods from the Skagerrak had higher levels than congeners from the Baltic Sea. Thiamin deficiency syndromes in salmon was associated with changes in all trophic levels, as well as changes in several abiotic variables.In all, this thesis provides new insights on thiamin content and seasonal dynamics in various phytoplankton and zooplankton species, the transfer of this vitamin between trophic levels as well as the overall importance of thiamin in the aquatic food web.