Bacterial Consumption of Humic and Non-Humic Low and High Molecular Weight DOM and the Effect of Solar Irradiation on the Turnover of Labile DOM in the Southern Ocean

Abstract

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.

Estimation of Uptake of Humic Substances from Different Sources by Escherichia coli Cells under Optimum and Salt Stress Conditions by Use of Tritium-Labeled Humic Materials

“…That finding coincided with the observations of Rosenstock et al (42). They studied decomposition by heterotrophic bacteria of different fractions of humic dissolved organic matter (DOM) in pelagic ecosystems and showed preferential bacterial growth on a 3-kDa-size fraction of humic DOM compared to one of Ͼ3 kDa.…”

Tracing inputs of terrestrial high molecular weight dissolved organic matter within the Baltic Sea ecosystem

“…The model theory was proven by several other studies (Sulzberger and Durisch-Kaiser, 2009, and references therein); however, there were other studies showing contradictory results (e.g. Rochelle-Newall et al, 2004;Rosenstock et al, 2005). In our data we could not find a correlation between salinity and the calculated DOM recovery, which could be used as an indicator of a change in the size of the DOM.…”

Dissolved organic matter assimilation by heterotrophic bacterial groups in the western Arctic Ocean

“…Microautoradiography may not detect slow protein degradation and assimilation of the by-products during the relatively short incubations we used. Nevertheless, in other environments, protein is often less important than amino acids in supporting heterotrophic production (Keil & Kirchman 1999, Rosenstock et al 2005, and the Arctic environment may not be different in this aspect. More cells affiliated with 1 bacterial group in particular, the Alphaproteobacteria, assimilated amino acids compared to other compounds in the Arctic (the present study) and in the northwest Atlantic .…”