Inhibitory effect of solar radiation on thymidine and leucine incorporation by freshwater and marine bacterioplankton

Abstract

We studied the effect of solar radiation on the incorporation of [ 3 H]thymidine ([ 3 H]TdR) and [ 14 C]leucine ([ 14 C]Leu) by bacterioplankton in a high mountain lake and the northern Adriatic Sea. After short-term exposure (3 to 4 h) of natural bacterial assemblages to sunlight just beneath the surface, the rates of incorporation of [ 3 H]TdR and [ 14 C]Leu were reduced at both sites by up to ϳ70% compared to those for the dark control. Within the solar UV radiation (290 to 400 nm), the inhibition was caused exclusively by UV-A radiation (320 to 400 nm). However, photosynthetically active radiation (PAR) (400 to 700 nm) contributed almost equally to this effect. Experiments with samples from the high mountain lake showed that at a depth of 2.5 m, the inhibition was caused almost exclusively by UV-A radiation. At a depth of 8.5 m, where chlorophyll a concentrations were higher than those in the upper water column, the rates of incorporation of [ 3 H]TdR were higher in those samples exposed to full sunlight or to UV-A plus PAR than in the dark control. In laboratory experiments with artificial UV light, the incorporation of [ 3 H]TdR and [ 14 C]Leu by mixed bacterial lake cultures was also inhibited mainly by UV-A. In contrast, in the presence of the green alga Chlamydomonas geitleri at a chlorophyll a concentration of 2.5 g liter ؊1 , inhibition by UV radiation was significantly reduced. These results suggest that there may be complex interactions among UV radiation, heterotrophic bacteria, and phytoplankton and their release of extracellular organic carbon. Our findings indicate that the wavelengths which caused the strongest inhibition of TdR and Leu incorporation by bacterioplankton in the water column were in the UV-A range. However, it may be premature to extrapolate this effect to estimates of bacterial production before more precise information on how solar radiation affects the transport of TdR and Leu into the cell is obtained.

Effects of solar radiation on dissolved organic matter cycling in a subtropical seagrass meadow

“…The enhancement of bacterial production occurred in both UV and PAR treatments, and primary production was similar and high enough in both of the treatments to support the measured increase in bacterial production. The enhancement of bacterial production due to the presence of phytoplankton was reported in other studies of the effect of UV radiation on bacterial activity (Aas et al 1996;Sommaruga et al 1997). Although phytoplankton ex- Table 5.…”

“…other studies have reported the inhibition of bacterial activity due to exposure to natural levels of UVB and UVA (Bailey et al 1983;Sieracki and Sieburth 1986;Herndl et al 1993;Aas et al 1996;Lindell et al 1996;Kaiser and Herndl 1997;Reitner et al 1997;Sommaruga et al 1997;Pakulski et al 1998). Most of these studies exposed water samples for 2-12 h before measuring bacterial production in the dark.…”

Lead and copper speciation in remote mountain lakes

“…Average water column values can vary considerably, however; e.g., in September 2000 the pH of Ladove was about 7.3, whereas the following July it was only 6.1, caused by melt water draining into the lake, which tends to cause a temporary decline in pH, alkalinity, calcium, and major ion concentrations. The transparency of mountain lakes is known to be very high, leading to deep penetration of UV light, which is thought to be a cause for DOC breakdown to the bottom of the shallower lakes (Sommaruga and Psenner 1997). (1997)(1998) water samples were filtered in-line through 0.45-m membrane filters, whereas subsequent samples were filtered where feasible using a cartridge with a 0.2-m cutoff and an internal 0.45m prefilter.…”

Carbon monoxide and chromophoric dissolved organic matter cycles in the shelf waters of the northern California upwelling system

“…Our model assumes a constant k bio and P dark within the chemophotic zone in light of these results. Further complications may arise in determining k bio solely from dark incubations because of reported UV inhibition of microbial growth as indicated by general bacterioplankton metabolic studies of various marine assemblages [Herndl et al, 1993;Sommaruga et al, 1997]. The only estimates of this effect in the literature range from 10% to a factor of 10 in the oligotrophic Sargasso Sea [Tolli and Taylor, 2005;Xie et al, 2005].…”