Sources and sinks of dissolved free amino acids and protein in a large anddeep mesotrophic lake

We studied the microbial cycling of dissolved free amino acids (DFAAs) and protein in mesotrophic Lake Constance, Germany, by examining their release by phytoplankton and various heterotrophic organisms and incorporation by h eterotrophic bacterioplankton. Release processes of both substrate classes, measured by an isotope dilution approach, comprised, as an annual mean, 15 % of primary production and as much as 64% during the clearwater phase. DFA As accounted for similar to 70% of total release during the spring bloom, i n the early phase predominantly as photosynthetic extracellular products of rapidly growing algae and toward the end as a result of copepod grazing. T hereafter, during the clear-water phase, when daphnids were most abundant, release was dominated by protein. At this time, and again in late summer, l ysis of grazing-damaged and senescent algae, including as well the hydrolyt ic activity of attached bacteria was one of the most important sources of p rotein. Rotifers, protozoans, and release processes in the fraction <1 <mu> m were minor sources of DFAA + protein. Concentrations of dissolved combine d amino acids (DCAAs) and protein ranged between 750-1,900 and 1-280 nM, re spectively, and peaked during phytoplankton blooms in spring and summer. As an annual mean, concentrations of labile protein constituted 8% of DCAAs, and the ratio of DFAAs to DCAAs was 0.16. About 50% of the DCAAs occurred i n the molecular weight fraction between DFAA and 3 kDa and 30% in that >30 kDa. Concentrations of DCAAs >3 kDa were closely correlated to chlorophyll a, suggesting their phytoplankton origin and thus a ready availability. Pro tein was the preferred bacterial substrate. As an annual mean, its incorpor ation supported 45% of bacterial biomass production, compared with 13% by D FAAs. During winter and spring, when DFAA concentrations were highest, DFAA incorporation constituted up to 40% of bacterial production. Annually, the sum of DFAA + protein supported 58% and 80% of the bacterial C and N deman d, respectively, indicating that they were the most important bacterial C a nd N sources.