Responses of estuarine intertidal microphytobenthic algal assemblages to enhanced ultraviolet B radiation

Abstract

AbstractAs a result of ozone depletion, ground doses of ultraviolet B (UVB) radiation in the mid latitudes of the Northern Hemisphere have increased since the 1980s, and current predictions indicate no possible alleviation until at least post 2020. Mudflats and sandflats are important coastal‐zone habitats, and support extensive biofilms of benthic microalgae (microphytobenthos). In intertidal situations, these assemblages are exposed to high levels of UVB radiation during periods of tidal exposure. Exposure of intertidal biofilms dominated by epipelic (mud‐inhabiting) diatoms to 0, 0.18 or 0.35 W m−2 UVB radiation for between 4 and 10 days resulted in no significant decreases in the maximum PSII quantum efficiency (Fv/Fm) throughout diel exposure periods. Although the quantum efficiency of electron transport (Fq′/Fm′) showed significant reductions early in some experiments, the major response was an increase in Fq′/Fm′ in UVB exposed biofilms. This increase in Fq′/Fm′ was suggestive of a protective vertical migration down into the sediment. Single‐cell and whole biofilm fluorescence imaging demonstrated, for the first time, that motile diatoms are able to detect UVB radiation independently of UVA or photosynthetically active radiation (PAR) and migrate rapidly down (within 15 min) into the sediments to avoid it. This behavioural acclimation mechanism appears to prevent significant accumulation of UVB induced damage to the algae. UVB exposure had no significant effect of biofilm photosynthesis (measured by 14C carbon fixation), but did alter organic carbon allocation patterns, with significantly less new carbon allocated to intracellular storage (glucan) and extracellular colloidal carbohydrate fractions. Significant reductions in the sediment standing stocks of chlorophyll a (Chl a), colloidal carbohydrates, extracellular polymeric substances (EPS) were seen after 7 days of UVB exposure. This study showed that marine intertidal benthic diatoms use a behavioural strategy to avoid exposure to UVB and that this response is effective as a short‐term protection mechanism against UVB damage. However, altered carbon allocation patterns feed forward over time into changes in biofilm biomass and sediment carbohydrate dynamics. This suggests that continual long‐term exposure to UVB may impact on sediment carbon cycling and trophic interactions and on the stabilization of sediments by microalgal biofilms through their production of extracellular carbohydrates.