Picophytoplankton cell death induced by UV radiation: Evidence for oceanic Atlantic communities

We analyzed the effect of ambient levels of visible and ultraviolet radiation (UVR) on picophytoplankton cell death by exposing natural communities of picophytoplankton (Prochlorococcus, Synechococcus, and picoeukaryotic cells) from the Atlantic Ocean to different levels of natural solar radiation, from that received just below the surface to 23% lower levels and dark conditions. Underwater oceanic levels of UVR and visible light can induce significant cell death in picophytoplankton communities. The decay rates of living cells induced by solar radiation was highest for Prochlorococcus sp., which showed an average decay rate of -0.24 ± 0.053 h^-1 (mean ± SE) in the experiments, whereas Synechococcus sp. showed the lowest decay rate of -0.021 ± 0.008 h^-1 (mean ± SE) in treatments ranging from the full incident irradiance to 23% of the irradiance incident below the ocean surface. Decay rates decreased significantly upon removal of UVR, demonstrating a major effect of UVR on cell death, although ambient levels of visible light alone still induced cell death in Prochlorococcus and picoeukaryotic populations, but not in Synechococcus sp. The high cell death of Prochlorococcus induced by total solar radiation resulted in short half-life values for this genus, ranging between 1.5 and 13.4 h across treatments. The half-life times for Synechococcus sp. and eukaryotic picoplankton cells exposed to UVR were longer, varying from 8.8 to 14.7 h and from 2.1 to 31.7 h, respectively. The UVR doses required to reduce the picophytoplankton populations by 50% (LRD₅₀) differed among the three groups, with considerably lower doses required for Prochlorococcus sp. Prochlorococcus sp. is highly sensitive to solar radiation, contrasting with the higher tolerance of Synechococcus sp. High, but taxon-specific, phytoplankton mortality induced by ambient UVR levels may act as a primary driver of the community structure of autotrophs and affect the dynamics of the microbial food web in clear, oligotrophic, oceanic waters.