MICROBIAL ECOLOGY OF THE CARIACO BASIN'S REDOXCLINE: THE U.S.-VENEZUELA CARIACO TIMES SERIES PROGRAM

Abstract

The cooperative U.S.-Venezuela CARIACO program (CArbon Retention I nA Colored Ocean) has begun to elucidate the microbial ecology of the Cariaco Basin's redoxcline. This anoxic water column supports highly stratified microbial assemblages of prokaryotes, protozoa and viruses, exhibiting abundance and activity maxima near the O2/H2S interface. In the oxic layer, abundance and activity of microheterotrophs vary annually to the same extent (16 to 20-fold) as primary producers in the upper 75-100 m, but out of phase. In the redoxcline and anoxic layer, relationships of these same variables to surface production are not readily apparent. Heterotrophic carbon demands within the redoxcline exceed delivery of sinking organic matter from the mixed layer. The Cariaco's redoxcline appears to be inhabited by microaerophilic and anaerobic chemoautotrophs, such as e-proteobacteria, whose metabolism is controlled by inorganic chemical gradients and transport. Time series data demonstrate that distribution and activity profiles of prokaryotes, protozoa and viruses vary in response to one another and to fluctuations in the interface's position. Rapid turnover of prokaryotic biomass in the redoxcline is deduced from the perennial presence of bacterivorous protozoan and viral communities. Chemoautotrophic production is sufficient to support heterotrophic demand for reduced carbon within the redoxcline and yields reasonable specific growth rates for total prokaryotic communities, averaging between 0.4 and 0.6 d −1 . However, reconciliation of microbial demand for energy and oxidants with supply is not possible applying the classic 1-D vertical model to the Cariaco and remains one of the greatest challenges to understanding the microbial ecology of anoxic water columns in general.