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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We evaluated marine aggregates as environmental reservoirs for a thraustochytrid pathogen, Quahog Parasite Unknown (QPX), of the northern quahog or hard clam, Mercenaria mercenaria. Positive results from in situ hybridization and denaturing gradient gel electrophoresis confirm the presence of QPX in marine aggregates collected from coastal embayments in Cape Cod, Massachusetts, where QPX outbreaks have occurred. In laboratory experiments, aggregates were observed and recorded by entering a quahog's pallial cavity, thereby delivering embedded particles from the water column to its benthic bivalve host. The occurrence of pathogen-laden aggregates in coastal areas experiencing repeated disease outbreaks suggests a means for the spread and survival of pathogens between epidemics and provides a specific target for environmental monitoring of those pathogens.