Does ambient UV-B radiation influence marine diatom-dominated microbial mats? A case study

Intact natural microbial mats collected from a shallow microtidal bay were incubated in the presence or absence of ambient ultraviolet-B radiation (UVBR) in an outdoor flow-through system for 2 and 6 wk, respectively. The microbial mats were cohesive and dominated by a large, motile diatom species, Gyrosigma balticum (Ehrenberg) Rabenhorst. The variables studied included biomass, composition of microalgae and meiofauna, bacterial biomass, pigment composition and presence of UV-absorbing compounds. Carbon assimilation and sediment oxygen microprofiles were measured as indicators of photosynthetic and respiratory activity. No significant effects were found on any of the measured variables when natural UVBR was excluded, suggesting that ambient UVBR did not exert any strong selective pressure on the particular type of microbial mat studied. Biochemical screening by UV-absorbing compounds did not appear to be of major importance for avoiding UVBR damage. Visual observations, and to some extent oxygen profiles, indicated downward migration of the dominant diatom species as a response to high light levels. This may have been an important strategy also to avoid exposure to UVBR. Thus, vertical migration of benthic diatoms appears to be a factor that should be further investigated in relation to effects of both present and enhanced levels of UVBR.