Short-term effects of UVB radiation on chlorophyll fluorescence, biomass, pigments, and carbohydrate fractions in a benthic diatom mat

The effects on UVB radiation on a subtidal, cohesive-sediment biofilm domin ated by the diatom Gyrosigma balticum (Ehrenberg) Rabenhorst were investiga ted, Chlorophyll fluorescence parameters (F-v/F-m, phi(PSII)), pigment conc entrations, cell densities, and carbohydrate fractions were measured in fou r treatments (no UVBR, ambient UVBR, +7%, and +15% enhancement with UVBR). Enhanced UVBR was provided by a computer-controlled system directly linked to natural diel UVBR levels, increases in phi(PSII) values in the UVBR-enha nced treatments and a decrease in the steady-state fluorescence yield (F-s) from the surface of the biofilms during the middle and latter part of dail y exposure periods suggested that G, balticum responded to enhanced UVBR by migrating down into the sediment. Diatoms in the +15% UVBR treatment also had significantly higher concentrations of beta-carotene after 5 days of tr eatment, Although G. balticum responded to enhanced UVBR by migration and i ncreased beta-carotene concentrations, significant reduction in maximum qua ntum yield of PSII (F-v/F-m) and in minimal fluorescence (F-o) and decrease s in cell densities occurred after 5 days. Concentrations of different carb ohydrate fractions (colloidal carbohydrate, glucan, exopolymers [EPS]) asso ciated with diatom biomass and motility also decreased in the UVBR-enhanced treatments. Short-term responses (migration) to avoid UVBR appear insuffic ient to prevent longer-term decreases in photosynthetic potential and biofi lm carbohydrate concentration and biomass.