Bacteria play a central role in the cycling of nutrients and energy flow to higher trophic levels, yet the effects of ultraviolet-B (UV-B) radiation upon bacterioplankton have been largely overlooked. Using a highly specific radloimmunoassay, measurements of solar-induced DNA photodamage (cyclobutane pyriinldine dimers) were taken in planktonic samples collected from the northern Gulf of Ilcxico. Diel patterns of dimer accumulation and repair were observed in both the bacterloplankton ~1 7 1 ' fraction (<0.8 pm) and in the larger eukaryotic plankton size fraction (>0.8 pm < 120 pm), although damacje induction was approximately twice as much in the bacterioplankton fraction. Depth profiles of Dii \ damage in the bacterioplankton size fraction during cdlnl and moderate seas demonstrated the infl~iencc ot mixlng on t h e distribution of W radiation effects. During calm seas, damage was greatest in surface waters, decreased with depth, and could be detected to 10 m. In moderate seas, however, no net accumulation of damage was observed, even at the surface. The results demonstrate that bacteria are more susceptible to UV-B damage and may wrve as a more sensitive indicator of UV stress than other microorganisms. Wave action and mixing strongly influence the effects of UV-B in surface waters, demonstl-ating that UV-B effects may not always be predictable from measures of UV radiatio~ attenuation.

We investigated the in situ destruction rates of marine viral particles as well as the decay rates of infectivity for viral isolates along an -400&n transect from oligotrophic offshore waters to productive coastal waters in the Gulf of Mexico. Light-mediated decay rates of viral infectivity averaged over the solar day ranged from 0.7 to 0.85 h ' in surface waters at all stations and decreased with depth in proportion to the attenuation of UVB (305 nm). The destruction rates of viral particles also decreased with depth, although the rates of particle destruction were only 22-61% of infectivity when integrated over the mixed layer. The rates of viral particle destruction indicated that at three of four stations 6-12% of the daily bacterial production would have to be lysed in order to maintain ambient viral concentrations. At the fourth station, where there was a dense bloom of Synechococcus spp. and the mixed layer was shallower, 34-52% of the daily bacterial production would have to be lysed. A comparison of the difference between destruction rates of viral particles and infectivity integrated over the depth of the mixed layer implies that host-mediated repair must have restored infectivity to 39-78% of the sunlight-damaged viruses daily. The calculated frequency of contacts between viral particles and bacterial cells that resulted in infection (contact success) ranged from -18 to 34% in offshore waters, where the frequency of contacts between viruses and bacteria was much lower, to -1.0% at the most inshore station, where contact rates are much higher. This suggests that in offshore waters bacterial communities are less diverse, and that there is less selection to be resistant to viral infection. This paper provides a framework for balancing viral production, destruction, and light-dependent repair in aquatic viral communities.Viruses are ubiquitous and abundant in marine environments, where they infect bacteria, phytoplankton, and heterotrophic flagellates (see reviews by Fuhrman and Suttle 1993; Borsheim 1993;Bratbak et al. 1994). Estimates of viral-induced mortality of marine bacteria range from a small percentage of production to rates that imply that viruses are a major mechanism of bacterial mortality in aquatic systems

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In temperate lakes, asynchronous cycles in surface water temperatures and incident ultraviolet (UV) radiation expose aquatic organisms to damaging UV radiation at different temperatures. The enzyme systems that repair UV-induced DNA damage are temperature dependent, and thus potentially less effective at repairing DNA damage at lower temperatures. This hypothesis was tested by examining the levels of UV-induced DNA damage in the freshwater crustacean Daphnia pulicaria in the presence and absence of longer-wavelength photoreactivating radiation (PRR) that induces photoenzymatic repair (PER) of DNA damage. By exposing both live and dead (freeze-killed) Daphnia as well as raw DNA to UV-B in the presence and absence of PRR, we were able to estimate the relative importance and temperature dependence of PER (light repair), nucleotide excision repair (NER, dark repair), and photoprotection (PP). Total DNA damage increased with increasing temperature. However, the even greater increase in DNA repair rates at higher temperatures led net DNA damage (total DNA damage minus repair) to be greater at lower temperatures. Photoprotection accounted for a much greater proportion of the reduction in DNA damage than did repair. Experiments that looked at survival rates following UV exposure demonstrated that PER increased survival rates. The important implication is that aquatic organisms that depend heavily on DNA repair processes may be less able to survive high UV exposure in low temperature environments. Photoprotection may be more effective under the low temperature, high UV conditions such as are found in early spring or at high elevations.

Heterotrophic microbial communities cycle nearly half of net primary productivity in the ocean, and play a particularly important role in transformations of dissolved organic carbon (DOC). The specific means by which these communities mediate the transformations of organic carbon are largely unknown, since the vast majority of marine bacteria have not been isolated in culture, and most measurements of DOC degradation rates have focused on uptake and metabolism of either bulk DOC or of simple model compounds (e.g. specific amino acids or sugars). Genomic investigations provide information about the potential capabilities of organisms and communities but not the extent to which such potential is expressed. We tested directly the capabilities of heterotrophic microbial communities in surface ocean waters at 32 stations spanning latitudes from 76°S to 79°N to hydrolyze a range of high molecular weight organic substrates and thereby initiate organic matter degradation. These data demonstrate the existence of a latitudinal gradient in the range of complex substrates available to heterotrophic microbial communities, paralleling the global gradient in bacterial species richness. As changing climate increasingly affects the marine environment, changes in the spectrum of substrates accessible by microbial communities may lead to shifts in the location and rate at which marine DOC is respired. Since the inventory of DOC in the ocean is comparable in magnitude to the atmospheric CO2 reservoir, such a change could profoundly affect the global carbon cycle.

A station located in the Gerlache Strait (Antarctic Peninsula) was sampled during early spring to determine the vertical distribution of marine planktonic archaea and to further describe the dynamic environment where they live. Chlorophyll concentration indicated that sampling occurred during the early stages of austral spring algal development. As expected, prokaryote abundance was higher at surface than at depth, and prokaryotic activity estimated by leucine incorporation was low at the surface and extremely low at 500 m. The relative abundance of planktonic archaeal, eucaryal, and bacterial ribosomal RNA was determined by quantitative rRNA hybridization, and the performance of two different universal probes used to normalize group-specific probe hybridization response was compared. Archaeal rRNA was detected in all samples analyzed, and was more abundant at depth (up to 25% of total rRNA) than at the surface. In both years, the archaeal signal decreased during the sampling period, particularly at surface. Most of the archaeal signal was attributable to group I archaea, affiliated with the kingdom Crenarchaeota. Planktonic euryarchaeotes (group II archaea) showed a larger contribution to the archaeal assemblage in surface waters than at depth. In total, our results verify that planktonic archaea are dynamic and abundant components in marine picoplankton assemblages of the Antarctic Peninsula.

The effects of ultraviolet ( U V ) r a d~a t~o n on thymldine and leucine incorporation were evarmned In surface waters from the Gulf of Mexico and Santa Rosa Sound a mesotrophlc estuary In no] thwest Florida USA Whole dnd 0 8 ].[m flltered surface waters were incubated w~t h 3 H -t h y m~~n e and 14C-leucine in IJV transparent containers under natural solar radlatlon Solar radlation was either not filtered (samples exposed to UV-B UV-A and photosynthetically active radlatlon PAR), filtered through Mylar 500D (samples exposed to UV-A and PAR), or filteled through Acryllte OP3 (samples exposed only to PAR) I11 Santa Rosa Sound thymldine lncorporatlon was lnhlblted an average of 44 % relatlve to dark controls when exposed to unflltered solar radlatlon PAR contnbuted 23% to the total thynudlne ~n h i b i t~o n lvhlle UV-A and UV-B contributed 37 % and 39% respectively to total inhlb~tion L e u c~n e incorporation In Santa Rosa Sound was lnhlblted 29% by full solar r a d~a t~o n The malonty of the total l e u c~n e inhlb~tlon was d u e to UV-B (83%) whlle PAR only treatments showed leucine lncorporatlon rates 1 0 "~ higher than dark controls For the Gulf of Mexico expenments full solar radlation ~nhlbited thym~dine inco~polatlon approximately twlce as much as leuclne lncorporatlon However there were no consistent patterns In differences d u e to d~fferent wavelengths Both thymidlne and l e u c~n e incorporation were lnhlbited to a greater extent ~n < 0 8 pm filtered water samples than In whole water samples suggesting that the presence of primary producers may mediate the d e t~l m e n t a l effects of solar radiation on bacterioplankton Surface rvater was also incubated In s~t u wlth thymidlne at flved depths in UV transparent and darkened containers at 3 locations in the Gulf of b l e x~c o Total ~nhlbltion was 60 to 7 0 % at the surface and was evldent to 15 m Comparison with radiometric data and DNA doslmeters lndlcated that UV-B exerted the greatest effect In the upper 5 m vvhlle below that the lnhlbltion was most l~k e l y due to longer wavelengths Our results suggest that both UV and visible solar radlation can negatively affect bacterial metabolism and fallure to take Into account the effects of light may result In the overest~rnat~on of bacterioplankton production in surface waters KEY WORDS: Ultraviolet radlation . Bacterla Thymidine .

Abstract— There has been much recent concern about the effects of increased UV radiation at certain locations on the earth's surface. There have been extensive studies of ultraviolet radiation effects on phytoplankton and primary production, yet the effects of UVB upon bacterioplankton have been largely overlooked. Bacteria play a central role in the cycling of nutrients and energy flow to higher trophic levels, serving as both mineralizers and secondary producers that are consumed by higher organisms. We have begun to investigate the induction of DNA photodamage by UVB in marine planktonic communities using a highly specific radioimmunoassay to measure cyclobutane pyrimidine dimers in samples collected from the northern Gulf of Mexico. DNA damage in the bacterioplankton size‐fraction (< 0.8 μ.m) was greater than in the larger eukaryotic size fraction (>0.8 μm <120 μm) in 9 of 10 samples. Diel patterns of dimer accumulation and repair were observed in surface waters over a 48 h period in the bacterioplankton size fraction and in the larger eukaryotic plankton size fraction. Depth profiles of DNA damage in the bacterioplankton size fraction appear to be dependent on surface water mixing. Damage was greatest in surface waters, decreased with depth and could be detected to 10 m in calm seas. No net accumulation of damage was observed in moderate seas, even at the surface. Solar radiation was found to inhibit significantly both 3H‐thymidine and 14C‐leucine incorporation. Ultraviolet B was responsible for approximately half of the total inhibition of 3H‐thymidine incorporation, UVA contributing the other half of the inhibition. The vast majority of 14C‐leucine incorporation inhibition was due to UVB, suggesting that protein synthesis is less affected by UVA. The results demonstrate that direct measures of DNA damage can be made of indigenous planktonic communities and that bacterioplankton are highly susceptible to UVB damage and may serve as a more sensitive indicator of UVR stress than other microorganisms.