Synechococcus and Prochlorococcus cell death induced by UV radiation and the penetration of lethal UVR in the Mediterranean Sea

11 páginas, 6 figuras, 5 tablas. -- Artículo Open Access.Irradiation experiments performed on natural communities of picocyanobacteria from the southwest Mediterranean Sea indicated that natural levels of solar ultraviolet radiation (UVR: 280 to 400 nm) induced important cell death in Prochlorococcus sp., although Synechococcus sp. appeared to be highly resistant. In the treatments where UVB radiation (UVBR, 280 to 315 nm) was excluded, Prochlorococcus also experienced high cell death with short half-life times of 3.01 ± 0.1 SE (h), showing the contribution of UVA radiation (UVAR, 315 to 400 nm) and photosynthetically active radiation (PAR, 400 to 700 nm) to Prochlorococcus cell death. Underwater radiometric measurements conducted during the cruise indicated that penetration of UVR was significant in the Mediterranean waters studied, with minimum diffuse attenuation coefficients of 0.165 and 0.071 m–1 for 313 and 380 nm, respectively. The lethal UV doses required to decrease the picocyanobacteria populations by half, UVLRD50, calculated experimentally, were related to underwater UVR penetration in the Mediterranean Sea measured during the cruise. By calculating (from incident irradiances and UVR penetration) the daily UV doses at different depths in the water column, we found that, for sunny days, experimental Prochlorococcus LRD50 (187 kJ m–2) could reach from 10 to 26 m depth. For Synechococcus, however, the depth receiving daily UVLRD50 (1375 kJ m–2) was always shallower, above 5 m depth. The differential sensitivity of the 2 genera and the UV transparency of the Mediterranean Sea suggest that solar radiation could be an important factor influencing the dynamics and distribution of cyanobacterial populations in the surface waters of this oligotrophic sea.Este estudio fue financiado por la concesión REN-2002-10606E/MAR para Susana Agustí, financiada por el Ministerio de España de Educación y Cienca, y por la Netherlands Science Foundation,Earth and Life Sciences (NWO-ALW, proyecto n. #812.03.001 para Gerhard J. Herndl), por el proyecto EUROTROPH (Nutrients Cycling and the Trophic Status of Coastal Ecosystems) financiado por el Quinto Programa Marco de la Unión Europea y por el proyecto SESAME financiado por la Unión Europea también. Moira Llabrés fue financiada por el proyecto REN2001-5060E/MAR.Peer reviewe