UV radiation and its effects on P-uptake in arctic diatoms

Abstract

Due to severe alterations of the physical conditions in Arctic ice-covered ecosystems (decrease of sea ice, thinning of the ozone layer), the underwater light climate is changing both with respect to its intensity and spectral composition. It is commonly observed that phytoplankton respond differently to photosynthetically active radiation (PAR, 400-700nm) and ultraviolet radiation (UVR, 280–400nm) in terms of their elemental stoichiometry, where high levels of PAR tend to increase the ratios of carbon to phosphorus (C:P), while UVR has the opposite effect. Since this has importance not only for elemental cycling and P-sequestration, but also for the algal food quality for grazers, it is of considerable interest to reveal the role of different spectral regimes of elemental uptake and stoichiometry. There are ambiguous evidence as to whether UVR stimulates or reduces the uptake of inorganic P, and to test this for three common, arctic marine diatoms; Porosira glacialis, Thalassiosira sp. Synedropsis hyperborean, we performed 33P-uptake assays with exposure to either PAR alone or PAR+UVR. Neither of the species showed strong P-uptake responses to UVR exposure, yet the two former species had a negative trend, while Synedropsis, showed a slight increase in its P-uptake under moderate UVR exposure. The latter species also had a remarkably fast P-uptake kinetics. These mixed results indicate a species-specific complex role of P in the algal response to UVR induced light stress, where not only species affinities but also ambient conditions may yield different outcomes.