The impact of UV-B radiation on alkaline phosphatase activity in phosphorus-depleted marine ecosystems

The impact of UV-B radiation on alkaline phosphatase activity (APA) was examined in water samples from a phosphorus-depleted Norwegian fjord, Hylsfjord, in July 1995. APA was regularly measured for 4–7.5 h in water samples from the fjord (Exp. 1) and from a nutrient-manipulated enclosure (Exp. 2) incubated under three different light regimes; ambient radiation (AMB), ambient radiation with enhanced artificial UV-B (+UV), 1.10 W m −2 UV-B, and ambient radiation screened for UV-B (−UV). Water samples exposed to UV-B (+UV; AMB) had significantly lower APA than water samples screened for UV-B (−UV), both in raw water samples as well as in water samples prefiltered through 1 and 0.2 μm filters. The decline could be explained by photodegradation of dissolved and cell-bound APA, with dissolved APA being more affected than APA associated with organisms. The results suggest that UV-B radiation inhibits APA in phosphorus-depleted aquatic environments and has the potential to strengthen the phosphorus limitation of microbial ecosystems.