Socioeconomic
Data and Applications Center
Environmental Effects of Ozone Depletion 1998 Assessment |
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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