Effects of UV radiation on the taxonomic composition of natural bacterioplankton communities from Bahía Engaño (Patagonia, Argentina).

In order to gain insights into the effects of solar ultraviolet radiation (UVR, 280-400 nm) on the composition of marine bacterioplankton communities from South Atlantic waters - Bahía Engaño (Patagonia, Argentina), we performed microcosms experiments during the Austral summer of 2010. Water samples were exposed to three solar radiation treatments in 25 L microcosms during 8 days: PAR+UV-A+UV-B (280-700 nm; PAB treatment), PAR+UV-A (320-700 nm; PA treatment), and PAR only (400-700 nm; P treatment). The taxonomic composition of the bacterial communities, at the beginning and at the end of the experiment, were studied by the analyses of 16S rDNA gene libraries. Multivariate and phylogenetic analyses demonstrated substantial differences in the community composition so that the samples exposed to PAR and PAR+UV-A presented more similar taxa assemblages among them than compared to the PAR+UV-A+UV-B exposed one. Our results indicate that overall, exposure to different radiation treatments can shape the taxonomic composition of marine bacterial populations, grown in microcosms, from this Patagonian area.

Ultraviolet radiation reduces the photoprotective capacity of the marine diatom Phaeodactylum tricornutum (Bacillariophyceae, Heterokontophyta).

This study demonstrates that UV radiation (UVR) reduces the photoprotective capacity of the diatom Phaeodactylum tricornutum by affecting xanthophyll cycle (XC) activity. The short-term reduction of photosystem II (PSII) maximum efficiency of charge separation (F(v)/F(m)) in cells exposed to UVR could be explained mainly by a reduced photoprotective capacity under this condition. Phaeodactylum tricornutum cells acclimated to two different photosynthetically active radiation (PAR) intensities, high light (HL, 200 micromol quanta m(-2) s(-1)) and low light (LL, 50 micromol quanta m(-2) s(-1)), were exposed to saturating irradiance (1100 micromol quanta m(-2) s(-1)) in the presence (PAR + UVR) and absence of UVR (PAR). HL cells exhibited a greater reduction in F(v)/F(m) in PAR + UVR when compared with the PAR treatment that was related to a reduction in the de-epoxidation of XC pigments. In contrast, in LL cells, UVR did not considerably affect XC de-epoxidation even though the reduction in F(v)/F(m) was greater than in HL cells. The negative effect of UVR on photoprotection was more pronounced in HL cells because they synthesized more XC pigments than LL cells. This was confirmed when XC activity was blocked with dithiothreitol and when PSII repair was inhibited with chloramphenicol (CAP). The differential reduction of F(v)/F(m) between PAR + UVR and PAR treatments disappeared when XC was blocked in HL cells. A higher reduction and an incomplete recovery of F(v)/F(m) were observed in cells incubated with CAP in the presence of UVR. Such responses confirm that UVR had a negative effect on photoprotective mechanisms causing an enhancement of damage by PAR, especially in HL-acclimated cells in which heat dissipation is important for PSII regulation.