Sunlight-induced growth delay of planktonic marine bacteria in filtered seawater

The growth of natural populations of planktonic manne bacter~a was observed upon exposure to natural sunhght In Narragansett Bay seawater with most of the phototrophic and bactenagrazing nanoflagellates removed by filtrabon, a shght increase in bactenal populahons began after 24 to 36 h in the dark In populabons exposed to sunlight, growth was delayed further Dilutmg the bactena-dom~nated size frachon (< 0 6 pm) with 0 2 pm filtered seawater resulted in a shorter lag penod and more rapid growth than unrllluted filtrates Sunlight caused a s ~ g n ~ f ~ c a n t le gthening of the lag penod but did not affect the exponenbal growth rate suggesbng that the bacteria were susceptible to sunhght whlle undergo~ng protein synthesis pnor to euponenhal growth Use of spectral fllters indicated that the growth delay was pnmanly due to UV-A wavelengths (320 to 400 nm) These results are analogous to sunl~ght effects on bacteria observed in pure culture experiments Use of neutral density fdters ~ndicated that the effect was pronounced at 50 and 25"L of surface sunlight and detectable at 16 O/O Photo~nhib~tion of bactenal activities could be a signif~cant factor in the d ~ e l cychng of organic matter nutr~ents and reactlve gases m the euphot~c zone of the oceans