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

Abstract

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.

Limnological conditions in Subglacial Lake Vostok, Antarctica

“…In theory, higher freezing rates should increase the partition coefficient (Killawee et al 1998); therefore, our extrapolations to actual lake concentrations of NPOC should be viewed as an underestimate. Acid-extractable amino acids have been shown to represent a significant fraction of the bioavailable NPOC in lake and ocean ecosystems (Weiss and Simon 1999;Rosenstock and Simon 2001). The acid-extractable amino acid concentration from core depths between 3,539 and 3,622 m mirrored the NPOC concentration (Fig.…”

How Daphnia copes with excess carbon in its food

“…It means that daphniids, depending on food quality, could be able to regulate their disposal of C-rich compounds through DOC excretion. Biochemical description of these excreted organic compounds as a function of food C:P ratio would be of great interest in the understanding of zooplankton grazing impact on bacterial productivity (Rosenstock and Simon 2001;Richardot et al 2001). …”

Relaciones entre las abundancias del bacterioplancton y del fitoplancton en tres ecosistemas lénticos de los Andes Colombianos

“…Dicha relación depende de factores que afectan a las dos comunidades, como temperatura y disponibilidad de fósforo, e influye sobre su productividad, abundancia y composición. Se ha demostrado que la abundancia y producción bacterianas están relacionadas positivamente con la producción primaria del fitoplancton (Cole and Caraco 1993) y que el bacterioplancton utiliza gran parte de las proteínas y aminoácidos provenientes del fitoplancton (Rosenstock and Simon 2001). Sin embargo, estos resultados no son concluyentes; otros autores han encontrado que la respuesta de cada comunidad es independiente, ya que el bacterioplancton está limitado por el fósforo y el fitoplancton por el nitrógeno (vrede et al 1999).…”