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.

The proportion of viruses in natural marine communities that are potentially infectious was inferred from the relationship between DNA damage and the loss of infectivity in marine viral isolates and measurements of the DNA damage in natural viral communities. Several viral isolates which infect marine Vibrio spp. were exposed to UV-C radiation and the concentration of cyclobutane pyrirnidine dimers in the viral DNA was measured with a highly sensitive radioimmunoassay. The loss of infectivity in the UV-exposed isolates was also determined under conditions which either activated or repressed the blue light dependent photolyase enzyme in host cells in order to examine the damagedependent response of this bacterial repair system. In addition, the accumulation of DNA photodamage during the solar day was measured in DNA isolated from natural viral communities collected along a transect in the western Gulf of Mexico. Using the correlation between DNA damage and infectivity for one of the viral isolates, we estimated the proportion of the natural viral community which was infective. The results imply that, due to light-mediated repair of damaged viral DNA by host-cell mechanisms (photoreactivation), greater than 50 % of the viruses in natural communities are infective despite high rates of DNA damage. Furthermore, the accumulation of cyclobutane pyrirnidine dirners was highest at the station where the surface mixed layer was shallowest, emphasizing the importance of mixing depth in relation to the accumulation of DNA damage. These experiments demonstrate that physical parameters such as mixing depth are critically interwoven with light penetration in influencing the infectivity of marine viral communities.

An extremely thermophilic coccoid bacterium, designated strain EM, was isolated from a flange (a newly described geological structure associated with massive sulphide mounds) at the Juan de Fuca Ridge hydrothermal vent system. The organism is a salt-requiring obligately anaerobic chemoorganotroph which reduces elemental sulphur but can grow very poorly in the absence of sulphur. Organic substrates utilized include yeast extract, peptone and amino acid hydrolysate as well as individual amino acids, the peptide Leu-Ser and starch. When growing on a defined medium, ES4 requires a supplement of vitamins and 20 amino acids at trace concentrations.Growth is most rapid in the temperature range 90-99 "C, with a maximum growth temperature of about 110 "C. In the optimal temperature range for growth, EM has a doubling time of about 1 h and can reach densities of lo9 cells ml-l . Insensitivity to the antibiotics vancomycin, streptomycin and chloramphenicol, and the presence of di-and tetra-ether phytanyl lipids, indicate that ES4 is an archaeobacterium.