USE OF DISSOLVED COMBINED AND FREE AMINO-ACIDS BY PLANKTONIC BACTERIA IN LAKE CONSTANCE

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 DC-AA and DFAA concentrations in 1-mum 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 [H-3]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 important substrates for the growth of planktonic bacteria in the lake.