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Abstract
Due to the stratospheric ozone depletion, several organisms will become exposed to increased biologically active UVB (280–320 nm) radiation, not only at polar but also at temperate and tropical latitudes. Bloom forming cyanobacteria are exposed to UVB radiation on a mass scale, particularly during the surface bloom and scum formation that can persist for long periods of time. All buoyant species of cyanobacteria are at least periodically exposed to higher irradiation during their vertical migration to the surface that usually occurs several times a day. The aim of this study is to assess the influence on cyanobacteria of UVB radiation at realistic environmental intensities. The effects of two UVB intensities of 0.5 and 0.99 W/m2 in up to 0.5 cm water depth were studied in vitro on Microcystis aeruginosa strains, two microcystin producing and one non-producing. After UVB exposure their ability to proliferate was estimated by cell counting, while cell fitness and integrity were evaluated using light microscopy, autofluorescence and immunofluorescence. Gene damage was assessed by TUNEL assay and SYBR Green staining of the nucleoide area. We conclude that UVB exposure causes damage to the genetic material, cytoskeletal elements, higher sedimentation rates and consequent cell death. In contrast to microcystin producers (PCC7806 and FACHB905), the microcystin non-producing strain PCC7005 is more susceptible to the deleterious effects of radiation, with weak recovery ability. The ecological relevance of the results is discussed using data from eleven years’ continuous UVB radiation measurements within the area of Ljubljana city (Slovenia, Central Europe). Our results suggest that increased solar radiation in temperate latitudes can have its strongest effect during cyanobacterial bloom formation in spring and early summer. UVB radiation in this period may significantly influence strain composition of cyanobacterial blooms in favor of microcystin producers.
Highlights
Cyanobacteria evolved in an extreme environment under anoxic conditions, high temperature, large variations of available nutrients and strong solar radiation including UV
Influence of UVB Radiation on Cell Proliferation After photosynthetically active radiation (PAR) exposure all three strains in controls were able to proliferate normally and approximately doubled their concentration in 48 h, while the Dark-acclimated group was unable to proliferate in the absence of light (Figure 2)
After UVB exposure (0.51 W/m2 for six hours) lower cell count was observed in all three PAR acclimated strains where both MC producers (PCC7806 and FACHB905) recovered from the stress induced and resumed proliferation
Summary
Cyanobacteria evolved in an extreme environment under anoxic conditions, high temperature, large variations of available nutrients and strong solar radiation including UV These are major adverse factors that influenced their early life on Earth [1]. Current best estimates suggest that slow recovery of the ozone layer may be expected only during the half of the 21st century [2] and that it could take decades before the ozone layer recovers to pre21980 values [3] Freshwater ecosystems, especially those in mid to low latitudes, may be affected to a much higher degree by the persisting fluxes of UV radiation [4]. In relatively shallow freshwater bodies, climate change and stratospheric ozone depletion may act synergistically to increase the exposure of organisms to UVB radiation
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