Influence of solar radiation on the availability of dissolved organic matter to bacteria in the Southern Ocean

The influence of solar radiation on the ability of dissolved organic matter (DOM) to support bacterial growth, was studied in the eastern Atlantic sector of the Southern Ocean during the SWEDARP 1997/1998 cruise with SA Agulhas December 1997-February 1998. Vertical profiles of water samples (2-3000m) were obtained from the Spring Ice Edge (SIE, 60degreesS, high chlorophyll-a) and from the Winter Ice Edge (WIE, 56degreesS, low chlorophyll-a) areas. Filter-sterilized water from each area and depth was incubated under natural solar radiation and in the dark for comparison. Photobleaching of humic substance fluorescence occurred in all studied water samples. The bleaching was typically larger in the initially more fluorescent deep waters, compared to the low-fluorescent surface waters. Both the irradiated water and the dark controls were re-inoculated with a mixed bacterial inoculum from the initially sampled water. Bacterial growth (accumulating cells) and bacterial production (protein synthesis) were monitored during a 16-19 day incubation of these cultures at near in situ temperature (2 degreesC). Bacterial growth in cultures prepared from SIE water was largest at the surface (2-25 m), while the growth in corresponding cultures from the WIE did not vary much over depth. In contrast to the observed photobleaching, no clear effects of the irradiation on the ability of the DOM to support bacterial growth could be observed in either of the experiments. Hence, the degradation of fluorescent structures and other photochemical alterations of the organic matter did not have a major effect on the total pool of biodegradable organic substrates. The lack of effects of photoreactions on bacterial growth potential in the present study disagrees with the short-term bacterial growth response observed in other oceanic environments. This could be due to the different experimental approaches employed (short-vs. long-term incubations) or may indicate that the impact of photoreactions on bacterial growth in the Southern Ocean differ substantially from aquatic systems that are more influenced by terrestrial environments, as well as the warmer oligotrophic oceanic environments. (C) 2004 Elsevier Ltd. All rights reserved