Nitrogen limitation in Dunaliella tertiolecta (Chlorophyceae) leads to increased susceptibility to damage by ultraviolet-B radiation but also increased repair capacity

Chl fluorescence was used to measure the photosynthetic capacity of the green alga Dunaliella tertiolecta in order to investigate interactions between susceptibility to acute UV-B radiation (UVBR, 280-320 nm) exposure and decreased nitrogen availability. Under UVBR exposure the decline in the fluorescence parameters F-v/F-m (the maximum effective quantum yield Phi(PSIIe-max)) and F-v'/F-m' (the operational quantum yield of PSII, (Phi(PSIIe)) were enhanced with higher UVBR fluxes, with the data well described by the Kok model, inferring that a dynamic balance existed between damage and repair with the repair proportional to the pool size of inactivated targets. When UVBR exposure was coupled with nitrogen limitation, the inhibition of photosynthesis was intensified. Under the more severely N-limited conditions, the damage rate increased. Unexpectedly, repair rates were also stimulated under N-limited conditions, although this was insufficient to counteract the increase in damage, so the overall effect of N limitation was an enhancement of UVBR-induced inhibition of photosynthesis.