Ultraviolet-B radiation resistance of benthic diatoms isolated from tidal flats in the Dutch Wadden Sea

Seven species representative of the benthic diatom community of the tidal flats in the Dutch Wadden Sea hardly differed in their sensitivity to ultraviolet-B radiation (UVBR). Some isolates had been cultured in the laboratory for up to 20 yr. Cell numbers of all species increased at a rate similar to unexposed cultures up to a DNA-weighted daily UVBR dose of 3.5 kJ m(-2) d(-1) (biologically effective dose, normalized at 300 nm); only at higher UVB irradiance levels did the growth rate become reduced. No clear relationship between mean cell size and UVBR sensitivity was observed. The benthic diatoms that were tested are apparently adapted to the natural, high UVB irradiance incident at tidal flats during spring and summer. Thus, even a sharp UVBR increase resulting from severe stratospheric ozone reduction would hardly affect tidal flat diatom communities by influencing cell division rate. In contrast, growth of representatives of the phytoplankton community was already seriously affected by doses that were 10 times lower. This is in agreement with their natural, low UVBR exposure

Wavelength-dependent induction of thymine dimers and growth rate reduction in the marine diatom Cyclotella sp. exposed to ultraviolet radiation

Cultures of the marine diatom Cyclotella sp. were subjected to various polychromatic exposures of UVB radiation (280-320 nm), UVA radiation (320-400 nm) and photosynthetically active radiation, PAR (400-700 nm). Changes in growth rate and residual thymine dimer content (a measure for DNA damage) were measured during prolonged exposure (6 to 7 d) to these conditions. Also, changes in mean cell size were studied as an indication of UV radiation induced cell cycle arrest in Cyclotella sp. Growth rate reduction was strongly related with residual thymine dimer content in treatments including wavelengths below 302 nm. Additionally, significant increases in mean cell size were found in these cultures. This suggests that UVB-induced residual DNA damage is followed by cell cycle arrest and growth rate reduction in Cyclotella sp. We discuss how these results can be interpreted in relation to changes in the solar spectrum as a result of stratospheric ozone reduction

In situ impact of solar ultraviolet radiation on photosynthesis and DNA in temperate marine phytoplankton

In situ experiments were conducted at various depths in the water column to determine the impact of solar UV radiation (280 to 400 nm) upon photosynthesis and DNA of natural phytoplankton assemblages from mid-latitudes of Patagonia (Bahia Bustamante, Chubut, Argentina; 45 degreesS, 66.5 degreesW). The effects of UV radiation were significant at the surface; however, the impact decreased rapidly with depth: at 3 m there was no measurable DNA damage accumulation, whereas at 6 m photosynthetic inhibition was almost zero. UV-A radiation (315 to 400 nm) was mostly responsible for photosynthetic inhibition, while UV-B radiation (280 to 315 nm) had a lesser effect on this process. However, UV-B radiation was very effective in damaging the DNA through the formation of cyclobutane pyrimidine dimers (CPDs) in surface waters. The high initial CPD level found in the natural phytoplankton assemblage decreased when samples were incubated at 3 or 6 m, indicating that at these depths repair, dilution or disappearance of damage occurred. Phytoplankton assemblages were dominated by cells less than 2 mum in effective diameter; this cell size category seems to be more resistant to photosynthetic inhibition, but vulnerable to CPD accumulation, as compared with larger eukaryotic phytoplankters (i.e., Phaeodactylum sp.)

Springtime phytoplankton dynamics in Arctic Krossfjorden and Kongsfjorden (Spitsbergen) as a function of glacier proximity

The hydrographic properties of the Kongsfjorden-Krossfjorden system (79 degrees N, Spitsbergen) are affected by Atlantic water incursions as well as glacier meltwater runoff. This results in strong physical gradients (temperature, salinity and irradiance) within the fjords. Here, we tested the hypothesis that glaciers affect phytoplankton dynamics as early as the productive spring bloom period. During two campaigns in 2007 (late spring) and 2008 (early spring) we studied hydrographic characteristics and phytoplankton variability along two transects in both fjords, using high-performance liquid chromatography (HPLC)-CHEMTAX pigment fingerprinting, molecular fingerprinting (denaturing gradient gel electrophoresis, or DGGE) and sequencing of 18S rRNA genes. The sheltered inner fjord locations remained colder during spring as opposed to the outer locations. Vertical light attenuation coefficients increased from early spring onwards, at all locations, but in particular at the inner locations. In late spring meltwater input caused stratification of surface waters in both fjords. The inner fjord locations were characterized by overall lower phytoplankton biomass. Furthermore HPLC-CHEMTAX data revealed that diatoms and Phaeocystis sp. were replaced by small nano-and picophytoplankton during late spring, coinciding with low nutrient availability. The innermost stations showed higher relative abundances of nano-and picophytoplankton throughout, notably of cyanophytes and cryptophytes. Molecular fingerprinting revealed a high similarity between inner fjord samples from early spring and late spring samples from all locations, while outer samples from early spring clustered separately. We conclude that glacier influence, mediated by early meltwater input, modifies phytoplankton biomass and composition already during the spring bloom period, in favor of low biomass and small cell size communities. This may affect higher trophic levels especially when regional warming further increases the period and volume of meltwater

UV damage to plant life in a photobiologically dynamic environment:The case of marine phytoplankton

The effect of UV-B radiation on growth of marine phytoplankton was investigated in relation to DNA damage induced by a range of biologically effective doses (BEDs). Emiliania huxleyi (Prymnesiophyceae) was chosen as a model organism of the ocean's phytoplankton because of its importance in global biogeochemical cycling Of carbon and sulphur, elements that influence the world's climate as components of the trace gases carbon dioxide (CO2) and dimethylsulfide (DMS). A marine diatom, Cyclotella, was studied for its capacity to repair the DNA damage, quantified as thymine dimers by the application of a monoclonal antibody against these photoproducts. DNA repair was shown to be complete after just a few hours of exposure to visible light; the repair rate increased with PAR intensity. E. huxleyi appeared to be most sensitive to UV-B radiation: growth was already affected above a dose of 100 J m(-2) d(-1) (biologically effective radiation, weighted with Setlow's DNA action spectrum), probably through effects on the cell cycle related to damage to nuclear DNA: mean specific growth rates were inversely correlated with thymine dimer contents in cells. Near the ocean's surface UV-B radiation conditions that induce the changes observed by us in cultures can be expected during the growing season of phytoplankton, not only in the tropics but also at higher latitudes. Nevertheles, blooms of species such as E. huxleyi are often excessive in the field. It is suggested that exposure duration of cells near the surface of the ocean can be shorter than our artificial mixing reaches depths greater than the layer where most UV-B is attenuated, negative effects on cells through W-A-induced inhibition of photosynthesis may prevail over DNA damage, the action spectrum of which has been shown to be limited to the UV-B part of the spectrum. Moreover, the radiation wavelengths that induce DNA damage repair (UV-A and visible) are attenuated vertically much less than UV-B. The photobiological situation in the upper ocean is much more complicated than on land, and effects of UV radiation on plankton biota can only be modelled realistically here when both the spectrally differential attenuation in the UV and visual part of the spectrum and the rate of vertical mixing are taken into account. Action spectra of both damage and repair of DNA and of photosynthesis inhibition of representative microalgal species are the second conditio sine qua non if we want to predict the effect of stratospheric ozone depletion on marine phytoplankton performance