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

Abstract

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 Ϫ0.24 Ϯ 0.053 h Ϫ1 (mean Ϯ SE) in the experiments, whereas Synechococcus sp. showed the lowest decay rate of Ϫ0.021 Ϯ 0.008 h Ϫ1 (mean Ϯ 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 halflife 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% (LRD 50 ) 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 taxonspecific, 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.

Size-fractionated phytoplankton biomass and production in the tropical Atlantic

“…Accordingly, it has previously been argued that the small size of picophytoplankton should be a disadvantage under high solar radiation exposure as pico-sized organisms may find it difficult to accommodate within their cells sufficient photoprotective substances to avoid photosynthetically active radiation and ultraviolet radiation damage (García-Pichel, 1994;Finkel et al, 2010). 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.…”

Late Ordovician geographic patterns of extinction compared with simulations of astrophysical ionizing radiation damage

“…However, for simple organisms, especially phytoplankton, durations of hours or a few days are sufficient for serious effects to be observed (Boucher and Prezelin 1996;Neale et al 1998;Llabres and Agusti 2006;Kouwenberg et al 1999). Damage values in our modeling vary over timescales on order of weeks to a month.…”

Differences in cell viabilities of phytoplankton between spring and late summer in the northwest Pacific Ocean

“…It should be noted that field studies of the cell death of marine phytoplankton remain limited to several regions: the North Sea (van Boekel et al, 1992;Brussaard et al, 1995;Riegman and Winter 2003), the Adriatic Sea (Baldi et al, 1997), the Mediterranean Sea (Agustí et al, 1998;Agustí and Duarte, 2000;Agustí and Sánchez, 2002;Alonso-Laita et al, 2005), and the Atlantic Ocean (Veldhuis et al, 2001;Agustí, 2004;Alonso-Laita and Agustí, 2006;Llabrés and Agustí, 2006). No information was available on the cell death of phytoplankton in the Pacific Ocean.…”