Marine bacteria in surface waters must cope daily with the damaging effects of exposure to solar radiation (containing both UV-A and UV-B wavelengths), which produces lesions in their DNA. As the stratospheric ozone layer is depleted, these coping mechanisms are likely to play an even more important role in the viability of marine bacterial communities. The recA gene is ubiquitous among eubacteria and is highly conserved both in nucleotide and amino acid sequence. Besides its role in generalized recombination, the gene's translational product, RecA, is the regulator of 'dark repair' activity (DNA-repair mechanisms that do not require visible light as a cofactor). We have taken advantage of this function and used recA gene expression as a barometer of the DNAdamage repair capacity of bacterial assemblages in the Southern Ocean. Studies were conducted in the Gerlache Strait, Antarctica, in the austral springs of 1995 and 1996. Analysis of both recA mRNA and RecA protein extracted from natural communities indicated that the level of expression of this gene varied in a diel fashion, suggesting an increased repair capacity in these organisms. These included an early morning rise in RecA levels followed by a plateau or even a reduction in RecA concentration during the remainder of the day. A much greater increase in RecA was consistently observed after sunset, followed by a constant decrease during the night. Microcosm experiments with a RecA + Gerlache Strait γ-proteobacteria isolate, RM11001, demonstrated a similar diel pattern of expression. These studies demonstrate the usefulness of RecA as a biological indicator of DNA repair capacity in natural bacterial assemblages. They indicate that 'dark repair' of DNA damage is an important coping mechanism for bacteria in the marine environment of Antarctica.KEY WORDS: Southern Ocean · Antarctica · Solar UV radiation · DNA repair · Marine bacterioplankton communities · recA gene · RecA protein Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 24: [51][52][53][54][55][56][57][58][59] 2001 Solar UV radiation consists of UV-C (100 to 280 nm), UV-B (280 to 320 nm), and UV-A (320 to 400 nm) (Peak & Peak 1983). UV-C is environmentally irrelevant because it does not penetrate the earth's atmosphere. UV-B is selectively absorbed by ozone and increases significantly during ozone depletion events (Stolarski 1988). Longer UV-A wavelengths are unaffected by changes in column ozone (Stolarski 1988).Recent evidence suggests that UV-B may have significant effects on marine microbial communities. Phytoplankton and primary production have been the focus of the majority of previous studies (Bidigare 1989, Marchant et al. 1991, Holm-Hansen et al. 1993, Schick et al. 1995, Forster & Lüning 1996, Herrmann et al. 1997, Holm-Hansen 1997, Zagarese et al. 1997, while significantly less is known about UV radiation effects on bacterioplankton in the Southern Ocean. Helbling et al. (1995) reported that viability, based on colony-forming uni...