Can heterotrophic bacteria be important to marine light absorption?

The contribution of heterotrophic bacteria to particulate light absorption in the ocean has been traditionally considered insignificant as compared to that of phytoplankton and detritus. This view has been based on the general preswnption that heterotrophic marine bacteria do not contain pigments with significant absorption in the visible spectral range. However, there exist heterotrophic bacteria that synthesize carotenoid pigments, and carotenoid-rich strains of bacteria have often been isolated from natural seawater samples taken in the open and coastal ocean. Because carotenoids absorb strongly in the blue spectral region, the heterotrophic bacteria may contribute more to marine light absorption than has been assumed. In order to make preliminary assessment of such a contribution, we measured the absorption of a strain of carotenoid-containing heterotrophic bacteria (CHB) grown in the laboratory under differing conditions of light and nutrient availability. These measurements showed that absorption cross-sections of CHB in the blue 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 light and nutrient conditions. We conclude that the role of heterotrophic bacteria in marine light absorption needs to be re-evaluated. This will require further laboratory studies to quantify the absorption cross-sections of marine bacteria with improved accuracy, as well as the development of a technique for the recognition and enumeration of CHB cells in the ocean.