Temperature and Solar Radiation Interactions on 3H-leucine Incorporation by Bacterioplankton in a Subtropical Estuary

Uncoupled phytoplankton-bacterioplankton relationship by multiple drivers interacting at different temporal scales in a high-mountain Mediterranean lake

Global-change stressors act under different timing, implying complexity and uncertainty in the study of interactive effects of multiple factors on planktonic communities. We manipulated three types of stressors acting in different time frames in an in situ experiment: ultraviolet radiation (UVR); phosphorus (P) concentration; temperature (T) in an oligotrophic Mediterranean high-mountain lake. The aim was to examine how the sensitivity of phytoplankton and bacterioplankton to UVR and their trophic relationship change under nutrient acclimation and abrupt temperature shifts. Phytoplankton and bacteria showed a common pattern of metabolic response to UVR × P addition interaction, with an increase in their production rates, although evidencing an inhibitory UVR effect on primary production (PP) but stimulatory on bacterial production (HBP). An abrupt T shift in plankton acclimated to UVR and P addition decreased the values of PP, evidencing an inhibitory UVR effect, whereas warming increased HBP and eliminated the UVR effect. The weakening of commensalistic and predatory relationship between phyto- and bacterioplankton under all experimental conditions denotes the negative effects of present and future global-change conditions on planktonic food webs towards impairing C flux within the microbial loop.

Away from darkness: a review on the effects of solar radiation on heterotrophic bacterioplankton activity

Heterotrophic bacterioplankton are main consumers of dissolved organic matter (OM) in aquatic ecosystems, including the sunlit upper layers of the ocean and freshwater bodies. Their well-known sensitivity to ultraviolet radiation (UVR), together with some recently discovered mechanisms bacteria have evolved to benefit from photosynthetically available radiation (PAR), suggest that natural sunlight plays a relevant, yet difficult to predict role in modulating bacterial biogeochemical functions in aquatic ecosystems. Three decades of experimental work assessing the effects of sunlight on natural bacterial heterotrophic activity reveal responses ranging from high stimulation to total inhibition. In this review, we compile the existing studies on the topic and discuss the potential causes underlying these contrasting results, with special emphasis on the largely overlooked influences of the community composition and the previous light exposure conditions, as well as the different temporal and spatial scales at which exposure to solar radiation fluctuates. These intricate sunlight-bacteria interactions have implications for our understanding of carbon fluxes in aquatic systems, yet further research is necessary before we can accurately evaluate or predict the consequences of increasing surface UVR levels associated with global change.

Growth conditions influence UVB sensitivity and oxidative damage in an estuarine bacterial isolate

The dose-dependent variation of oxidative cellular damage imposed by UVB exposure in a representative estuarine bacterial strain, Pseudomonas sp. NT5I1.2B, was studied at different growth phases (mid-exponential, late-exponential, and stationary), growth temperatures (15 °C and 25 °C) and growth media (nutrient-rich Tryptic Soy Broth [TSB] and nutrient-poor M9). Survival and markers of oxidative damage (lipid peroxidation, protein carbonylation, DNA strand breakage, and DNA-protein cross-links) were monitored during exposure to increasing UVB doses (0-60 kJ m(-2)). Oxidative damage did not follow a clear linear dose-dependent pattern, particularly at high UVB doses (>10 kJ m(-2)), suggesting a dynamic interaction between damage induction and repair during irradiation and/or saturation of oxidative damage. Survival of stationary phase cells generally exceeded that of exponential phase cells by up to 33.5 times; the latter displayed enhanced levels of DNA-protein cross-links (up to 15.6-fold) and protein carbonylation (up to 6.0-fold). Survival of mid-exponential phase cells was generally higher at 15 °C than at 25 °C (up to 6.6-fold), which was accompanied by lower levels of DNA strand breaks (up to 4000-fold), suggesting a temperature effect on reactive oxygen species (ROS) generation and/or ROS interaction with cellular targets. Survival under medium-high UVB doses (>10 kJ m(-2)) was generally higher (up to 5.4-fold) in cells grown in TSB than in M9. These results highlight the influence of growth conditions preceding irradiation on the extent of oxidative damage induced by UVB exposure in bacteria.

Influence of growth temperature and starvation state on survival and DNA damage induction in the marine bacterium Sphingopyxis alaskensis exposed to UV radiation

Despite the considerable volume of literature describing the individual effects of temperature or UV light on aquatic bacteria, little is known about their combined effects. The current study was conducted to learn about the effects of growth temperature and duration of starvation on the response of a marine bacterium, Sphingopyxis alaskensis to UV-B or simulated solar radiation. Cells grown at 12 degrees C or 24 degrees C, and harvested at early or late stationary phase, were exposed to UV-B or simulated solar radiation (>290 nm). The predominant forms of UV-induced DNA damage, namely cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts (6-4PP s), were quantified using a HPLC-mass spectrometry. While the commonly accepted view that DNA damage induced by UV-B radiation is temperature-independent, we observed in S. alaskensis that the yield of photoproducts for 12 degrees C was generally lower than for cells grown at 24 degrees C. The relative distribution of DNA photoproducts also varied with growth temperature, with an increased formation of TC 6-4PP for late compared to early stationary phase cells. In contrast, with the exception of cultures grown at 12 degrees C exposed to simulated solar radiation, the duration of stationary phase had no effect on total photoproduct formation. Collectively, these data indicate that growth temperature has more effect than duration of starvation on the formation of photoproducts in S. alaskensis.