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.

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.

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ABSTRACT. The influence of sunlight on bacterioplankton production [I4C-leucine (Leu) and 3H-thyrnidine (TdR) incorporation; changes in cell abundances] and O2 consumption was investigated in a shallow subtropical coral reef located near Key Largo, Florida, USA. Quartz (light) and opaque (dark) glass biological oxygen demand (BOD) bottles containing 0.8 pm filtered reef water amended wlth C, N and P were incubated in situ and exposed to natural variations in solar radiation over a 48 h period. Photoinhibition of Leu and TdR incorporation was observed at all depths during both daylight periods. Photoinhibition of bacterial production decreased with depth and was significantly higher during the first day of exposure. Bacterial abundances also decreased during daylight periods particularly during the second day of exposure. Leu and TdR incorporation rates and bacterial abundances exhibited recovery during periods of darkness. Light treatment bacterial O2 consumption was inhibited at all depths during Day 1 but enhanced relative to dark treatments at all depths during Day 2. Estimates of light treatment bacterial gross growth efficiencies (GGE) determined during the evening of Day 1 were similar to dark treatment estimates. Light treatment GGE determined during Day 2, however, were lower than dark treatments but increased with depth. Recovery of bacterial production and respiration during the second day of exposure suggested photoinduced selection for light tolerant cells and/or physiological adaptation to ambient light reglmes occurred over the duration of exposure. The results of this experiment suggested that solar radiation may have a significant effect on bacterial metabol~sm in this shallow euphotic marine ecosystem.