We measured the use of dissolved combined (DCAA) and free amino acids (DFAA) by planktonic bacteria during the growing season in Lake Constance (Bodensee). Utilization was estimated from the decrease of DCAA and DFAA concentrations in l-pm filtrates and compared to the C demand for the net production of bacterial biomass measured from the increase of bacterial abundance. During the phytoplankton spring bloom and the clear-water phase, DFAA accounted for 30-48% and DCAA 5-62% of the bacterial C demand. From August until November use of DFAA comprised ~5% and DCAA 50-148% of the bacterial C demand. In aphotic depths, amino acids accounted for a higher fraction of the C demand relative to euphotic depths. From mid-July until early September use of DCAA exceeded the bacterial C and N demand, leading to release of ammonium. During the spring bloom and the clearwater phase, DFAA + DCAA accounted for < 100% of bacterial C demand, suggesting that other substrates were also used. Concentrations and utilization rates of DFAA covaried, whereas maxima of DCAA use followed maxima of DCAA concentration with a time-lag of several weeks. Measurements of the extracellular isotope dilution of [3H]amino acids indicated that after the diatom bloom, bacterial hydrolysis and use of DCAA were closely coupled and release of DFAA accounted for ~7% of the DCAA used. During this time, turnover of the DFAA pool could be explained on the basis of release and utilization. Our results show that DCAA and DFAA are imnortant substrates for the growth of planktonic bacteria in the lake.

We investigated the physiological response of planktonic bacteria to varying C and N sources in mesotrophic Lake Constance (Bodensee) by studying the amino acid composition and the isotope dilution (ID) of [3H] amino acids in the intracellular pool.

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 heterotrophic 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. DFAAs accounted for ϳ70% of total release during the spring bloom, in the early phase predominantly as photosynthetic extracellular products of rapidly growing algae and toward the end as a result of copepod grazing. Thereafter, during the clear-water phase, when daphnids were most abundant, release was dominated by protein. At this time, and again in late summer, lysis of grazing-damaged and senescent algae, including as well the hydrolytic activity of attached bacteria was one of the most important sources of protein. Rotifers, protozoans, and release processes in the fraction Ͻ1 m were minor sources of DFAA ϩ protein. Concentrations of dissolved combined amino acids (DCAAs) and protein ranged between 750-1,900 and 1-280 nM, respectively, 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 in the molecular weight fraction between DFAA and 3 kDa and 30% in that Ͼ30 kDa. Concentrations of DCAAs Ͼ3 kDa were closely correlated to chlorophyll a, suggesting their phytoplankton origin and thus a ready availability. Protein was the preferred bacterial substrate. As an annual mean, its incorporation supported 45% of bacterial biomass production, compared with 13% by DFAAs. During winter and spring, when DFAA concentrations were highest, DFAA incorporation constituted up to 40% of bacterial production. Annually, the sum of DFAA ϩ protein supported 58% and 80% of the bacterial C and N demand, respectively, indicating that they were the most important bacterial C and N sources.

We assessed growth dynamics of heterotrophic picoplankton and concentrations and turnover of dissolved protein, amino acids, and neutral monosaccharides in the Atlantic sector of the Southern Ocean in austral summer (December-January) and fall (March-May). Phytoplankton biomass (chlorophyll a) and biomass production of heterotrophic picoplankton in summer was twice as high as in fall. This difference was also reflected in protein turnover rate constants and in concentrations of dissolved combined neutral polysaccharides (DCCHO). Turnover rate constants of dissolved free amino acids (DFAA) were in the same range in both seasons, but turnover rate constants of glucose were higher in fall as compared to summer. In summer, dissolved protein was the major substrate for growth of heterotrophic picoplankton, followed by DFAA and dissolved free neutral monosaccharides (DFCHO). During summer, heterotrophic picoplankton production was closely correlated to concentrations and incorporation of dissolved protein. In fall, heterotrophic picoplankton production was only significantly correlated to glucose turnover rate constants. The latter were also inversely correlated to DCCHO concentrations in fall. The reduced supply of organic substrates by phytoplankton in fall not only resulted in an equal reduction of heterotrophic picoplankton production but also in a shift of the supply in dissolved protein, DFAA, and DFCHO to heterotrophic picoplankton. Dissolved protein was the major substrate for heterotrophic picoplankton growth in summer, but in fall, when supply of dissolved protein was reduced, DFAA and DFCHO were relatively more important substrates.

The decomposition of dissolved organic matter (DOM) in pelagic ecosystems is mediated primarily by heterotrophic bacteria, but transformation by short-wave solar radiation may play an important role in surface waters, in particular when humic substances constitute a substantial fraction of the DOM pool. Most of the studies examining bacterial decomposition and photochemical transformation of DOM stem from limnetic and coastal marine systems and much less information is available from oceanic environments. To examine the bacterial decomposition of humic and non-humic DOM in the Southern Ocean we carried out microcosm experiments in which we measured bacterial growth on isolated fractions of humic and non-humic DOM of the size classes <3 kDa and >3 kDa. Experiments carried out at the Polar Front showed a preferential bacterial growth on non-humic DOM and in particular on the size fraction <3 kDa. Bacterial growth, measured as bacterial biomass production, on non-humic DOM accounted for 74% to 88% of the total growth on all four DOM fractions. In experiments in the Antarctic circumpolar current and the coastal current under pack ice, bacterial growth was 6x lower than at the Polar Front, and humic and non-humic DOM was consumed to equal amounts. The size fraction <3 kDa was always preferred. Experiments examining the effect of solar radiation on the release of dissolved amino acids (DAA) and carbohydrates (DCHO) and their subsequent bacterial utilization showed a stimulating effect on glucose uptake and the release of DAA at the Polar Front but an inhibition in the eastern Weddell Sea. Ultraviolet-B was the most effective component of the solar radiation spectrum tested. Effects of UV-B on glucose uptake and release of DAA were positively correlated with concentrations of humic-bound DAA. The data imply that at low concentrations, e.g., <100 nM (amino acid equivalent), UV-irradiation reduces, whereas at concentrations >100 nM UV-irradiation stimulates glucose uptake and release of DAA as compared to dark conditions.