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Bacterioplankton and Picoplankton

Use of modern epifluorescence microscopy techniques has substantially changed our perception of the role of bacteria in aquatic ecosystems over the recent past. Bacterioplankton productivity is far greater than previously thought, having high division and turnover rates (Fuhrman and Noble, 1995). The productivity is comparable to or exceeds phytoplankton primary productivity (Herndl, 1997) Bacterioplankton is no longer regarded solely as a final decomposer of organic material (Fig. 4.3). According to the "microbial loop hypothesis," bacterioplankton is seen in the center of a food web, having a similar function to phytoplankton and protists (Pomeroy and Wiebe, 1988). It has been shown that bacterioplankton plays a central role in the carbon flux in aquatic ecosystems by taking up dissolved organic carbon (DOC) and remineralizing the carbon.

Fig. 4.3 Microbial loop (continuous arrows) in an aquatic habitat.

The effect of solar UV on bacterioplankton depends on the spectral attenuation coefficients in the water column and the time pattern of exposure and protection for the organisms as they are passively moved in the mixing layer. Bacterioplankton seems to lack UV screening pigments such as mycosporines or scytonemins, possibly because of their small size (Karentz et al., 1994; Garcia-Pichel, 1994). As a consequence, bacterioplankton are more prone to UV-B stress than larger eukaryotic organisms and exposure produces about double the amount of cyclobutane dimers as shown in a case study in the Gulf of Mexico (Jeffrey et al., 1996 a,b). This damage is at least partially offset by photoreactivation (Nicholson, 1995). The equilibrium between UV damage and photorepair is governed by the passive movement of the cells within the mixing layer where they are alternately exposed to high levels of damaging solar UV near the surface and beneficial UV-A/blue at greater depths. Other macromolecular components of the bacterial cells, as well as ectoenzymes responsible for the cleavage of external organic matter, are affected by solar UV-B (Müller-Niklas et al., 1995). The bacterioplankton serves as food for heterotrophic flagellate picoplankton (<1 µm). The bacterial plankton population is limited by UV damage, viruses and heterotrophic flagellates (Aas et al., 1996; Sommaruga et al., 1995). This effect is partially offset by an effective repair mechanism using the photolyase enzyme. UV/blue radiation (360-430 nm) is most effective in the induction of the activity. It should be mentioned that also the viruses and nanoflagellates show a high sensitivity to solar UV radiation (Sommaruga et al., 1996).


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