Solar Radiation‐induced Mortality of Marine Pico‐phytoplankton in the Oligotrophic Ocean^†

Abstract

We examined the response of pico‐phytoplankton communities sampled at the equatorial, tropical and temperate Central Atlantic Ocean to subsurface underwater solar radiation in order to test the generality of the reported cell mortality for these populations when exposed to high ultra violet radiation (UVR) and photosynthetically active radiation. The natural communities of pico‐phytoplankton populations tested experienced high cell mortality when exposed to high solar radiation, despite inhabiting tropical waters. Synechococcus and eukaryotes were more resistant to solar radiation than Prochlorococcus. The decay rates of all pico‐phytoplankton groups examined tended to be much higher when exposed to total solar radiation than when UVB‐R was filtered out. We also show that even short exposures of 30 min to high solar radiation were able to induce cell mortality in Prochlorococcus. The variability in the decay rates of living Prochlorococcus cells were strongly related to the condition of the original population. However, Synechococcus decay rates were higher in populations from the tropical area, with eukaryotes sensitivity increasing with increasing the trophic degree. The data reported in this study and in the literature revealed contrasting capacities of Prochlorococcus, Synechococcus and eukaryotes to survive under high solar radiation. Although the mechanisms involved are as yet unclear, their elucidation may help explain niche partitioning among these organisms in the ocean.

Effects of ultraviolet radiation on growth, cell death and the standing stock of Antarctic phytoplankton

“…In most of the UVR experiments, diatoms and flagellates experienced cell death, with percentages of dead cells decreasing when UVR was filtered out. In fact, cell death caused by UVR has also been described in phytoplankton from other marine environments, including tropical and temperate areas (Llabrés & Agustí 2006, Agustí & Llabrés 2007, but the role of UVR in causing cell death in natural phytoplankton from Antarctic waters has not been examined before. Comparable parameters, such as cell survival, were analysed before in culture studies with diatoms isolated from Antarctica (Karentz et al 1991, Davidson et al 1994) and in natural communities, but these were restricted to the analysis of the decay in cell abundance (Davidson & Belbin 2002).…”

The percentage of living bacterial cells related to organic carbon release from senescent oceanic phytoplankton

“…Although phototrophic picoplankton communities are typically dominant in oligotrophic waters (Agawin et al, 2000), they showed high variability in cell viability in the most oligotrophic waters sampled here. Surface populations are exposed to high PAR and UV radiation, resulting in high %DC of Prochlorococcus spp., which is strongly sensitive to high solar radiation (Llabrés and Agustí, 2006;Agustí and Llabrés, 2007;Llabrés et al, 2010), whereas Synechococcus is typically stressed by low light at deep layers, but shows higher cell survival in surface waters (Agustí, 2004;Llabrés and Agustí, 2006). In addition, the high cell mortality of Prochlorococcus sp.…”

“…The results presented here confirm these findings, with phytoplankton cell viability decreasing from upwelling-influenced waters to oligotrophic waters, particularly for diatoms, which showed a two-fold reduction in the percentage of living cells from the upwelling to the oligotrophic waters. However, the patterns displayed by the different populations composing the phytoplankton community were complex, as phytoplankton show intricate and differentiated niches of cell viability depending on cell size, irradiance, nutrient concentration, and temperature (Berges and Flakowski, 1998;Agawin et al, 2000;Agustí, 2004;Alonso-Laiìta and Agustí, 2006;Agustí and Llabrés, 2007;. The percentage of dead cells tended to increase with decreasing cell size, with more than 40 % of dead cells generally found in the phototrophic picoplankton community.…”

Size-fractionated phytoplankton biomass and production in the tropical Atlantic

“…Llabrés and Agustí (2006) have demonstrated that natural levels of visible and ultraviolet radiation induce considerable picophytoplankton cell mortality in the subtropical Atlantic Ocean. In addition, Agustí and Llabrés (2007) reported that picophytoplankton experience high cell death rate due to exposure to natural underwater levels of solar radiation in the UML.…”

“…The potential causes of this pattern were explored by Fernández et al (2003), who concluded that a higher light utilization efficiency of the larger phytoplankton fraction in the oligotrophic waters could enhance its contribution to primary production in comparison with that of picophytoplankton. In addition, it has been demonstrated that solar radiation induces considerable cell damage and mortality in the subtropical Atlantic (Llabrés and Agustí, 2006;Agustí and Llabrés, 2007), and Teira et al (2005) hypothesized that a considerable fraction of the picophytoplankton biomass should correspond to damaged and non-viable cells, thus resulting in discrepancies between their contribution to biomass and carbon fixation. Pérez et al (2006) characterized the vertical variability of phytoplankton biomass, size structure, production and growth in the Atlantic subtropical gyres.…”