Effects of flocculated humic matter on free and attached pelagic microorganisms

Abstract

The effect of dissolved and particulate humic matter on pelagic microorganisms was studied in laboratory experiments. A humic concentrate was added to cultures of natural bacterial populations in either dissolved or partly flocculated form. Due to the occurrence of epibacteria attached to particles, a markedly higher bacterial yield was obtained when a fraction of the added carbon was flocculated. Attached bacteria apparently utilized floes as well as dissolved organic carbon surrounding the particles. Both free and attached bacteria were grazed by phagotrophic flagellates. In addition to grazing attached bacteria, attached flagellates could ingest free bacteria. In 11 oligotrophic lakes, ranging from those with clear water to those heavily colored with humic substances, a linearly increasing amount of both particulate material and particle-associated epibacteria was found in the pelagial with increasing humic content. We suggest that suspended detritus particles (humic flocculates) have a positive influence on the secondary productivity of humic lakes due to an enrichment of the heterotrophic epibacterial-bacterivorous flagellate segment of the microbial food web.In our understanding of pelagic ecosystems, recent information indicates that the environment of pelagic microorganisms is spatially heterogeneous. Gradients of oxygen and pH occur on the microscale in association with suspended detrital particles (Alldredge and Cohen 1987), and there are numerous examples of higher abundance and specific activity of bacteria and protists in association with particles than in surrounding water (Caron et al. 1982(Caron et al. , 1986 Hodson et al. 198 1;Paerl and Merkel 1982). Detrital aggregates may be responsible for the existence of several protozoan species in the pelagic zone (Caron et al. 1986), making the distribution of species variable on a I Present address:

Cycling of biologically available nitrogen in riverine humic substances between marine bacteria, a heterotrophic nanoflagellate and a photosynthetic dinoflagellate

“…Growth of C. roenbergensis probably lagged some days behind the growth of bacteria, as observed elsewhere in natural communities (Azam et al 1983, Andersen & Ssrensen 1986 and in laboratory studies (Fenchel 1982, Tranvik & Sieburth 1989. Thus, C .…”

Differences in consumption of allochthonous DOC under limnic and estuarine conditions in a watershed

“…higher ionic strength), which may influence the bioavailability of dissolved terrigenous compounds. It is known that organic material aggregates in saline environments (Sholkowitz 1976, Forsgren et al 1996, and that organic aggregates can be more susceptible to bacterial degradation than DOC (Tranvik & Sieburth 1989).…”

Lack of coupling between heterotrophic nanoflagellates and bacteria: A general phenomenon across aquatic systems?

“…However, there is considerable empirical and experimental evidence of resource control on bacteria. Bacterial numbers are consistently correlated with chlorophyll (Bird and Kalff 1984;Cole et al 1988), phosphorus (Curtie 1990) or dissolved organic matter in brownwater lakes (Tranvik and Sieburth 1989), suggesting that predatory control of bacterial numbers may be of minor importance in many systems and is perhaps more important in eutrophic than oligotrophic systems (Bird and Kalff 1984;Cole et al 1988). Wright (1988) presented a mathematical model for control of planktonic bacterial density and productivity which considers both predation and resource control acting simultaneously.…”