EFFECTS OF ENHANCED UVB RADIATION ON A MARINE BENTHIC DIATOM MAT

In order to study the ecophysiological response of a natural diatom-do minated microbial mat to an enhanced level of ultraviolet-B radiation (UVBR), intact sediment cores from a shallow microtidal bay on the Swe dish west coast were incubated in an outdoor flow-through system and e xposed to either no UVBR or to an artificially enhanced level of UVBR. The microbial mat was cohesive and dominated by the large (approximat e to 400 mu m) motile diatom Gyrosigma balticum. Functional response t o UVBR was assessed by measuring carbon fixation (C-14) and allocation of photosynthetic products. Sediment oxygen microprofiles were measur ed as indicators of the balance between the photosynthetic and respira tory activity of the community. Structural variables included biomass and composition of microalgae, pigment composition (HPLC), content of UV-absorbing compounds, and composition of fatty acids. An enhanced le vel of UVBR resulted in significant functional changes in the microalg al community. Significantly decreased carbon fixation and lower net ox ygen production (as calculated from oxygen microprofiles) suggest that primary productivity in the type of microbial mat studied is potentia lly sensitive to an enhanced level of UVBR. The pattern of carbon allo cation hinted a change as to the photosynthetically active part of the community when exposed to UVBR, indicating a differential sensitivity among algal cells. Despite significant functional effects, pigment or algal composition were not significantly affected by enhanced UVBR. T he concentration of UV-absorbing compounds was low and did not increas e at UVBR exposure. Due to the short duration of the experiment (4 d), a change in algal biomass or composition was not to be expected. More over, the thickness of the sampled sediment layer, may have contribute d to the lack of observed structural effects of enhanced UVBR. Visual observations suggested that UVBR affected the motility of G. balticum. This study, as well as other experiments with a similar diatom mat, s uggest that vertical migration is a key mechanism to be further studie d in relation to UVBR exposure of diatom-dominated microbial mats.