CAN HETEROTROPHIC BACTERIA BE IMPORTANT TO MARINE LIGHT-ABSORPTION

The contribution of heterotrophic bacteria to particulate light absorp tion in the ocean has been traditionally considered insignificant as c ompared to that of phytoplankton and detritus. This view has been base d on the general presumption that heterotrophic marine bacteria do not contain pigments with significant absorption in the visible spectral range. However, there exist heterotrophic bacteria that synthesize car otenoid pigments, and carotenoid-rich strains of bacteria have often b een isolated from natural seawater samples taken in the open and coast al ocean. Because carotenoids absorb strongly in the blue spectral reg ion, the heterotrophic bacteria may contribute more to marine light ab sorption than has been assumed. In order to make preliminary assessmen t of such a contribution, we measured the absorption of a strain of ca rotenoid-containing heterotrophic bacteria (CHB) grown in the laborato ry under differing conditions of light and nutrient availability. Thes e measurements showed that absorption cross-sections of CHB in the blu e could be at least twice, and possibly one order of magnitude, higher than those of non-pigmented heterotrophic bacteria (NHB). In addition , the absorption features of CHB were conserved under the differing li ght and nutrient conditions. We conclude that the role of heterotrophi c bacteria in marine light absorption needs to be re-evaluated. This w ill require further laboratory studies to quantify the absorption cros s-sections of marine bacteria with improved accuracy, as well as the d evelopment of a technique for the recognition and enumeration of CHB c ells in the ocean.