Bacterial growth and primary production along a north–south transect of the Atlantic Ocean

The oceanic carbon cycle is mainly determined by the combined activities of bacteria and phytoplankton 1 , 2 , but the interdependence of climate, the carbon cycle and the microbes is not well understood. To elucidate this interdependence, we performed high-frequency sampling of sea water along a north–south transect of the Atlantic Ocean. Here we report that the interaction of bacteria and phytoplankton is closely related to the meridional profile of water temperature, a variable directly dependent on climate. Water temperature was positively correlated with the ratio of bacterial production to primary production, and, more strongly, with the ratio of bacterial carbon demand to primary production. In warm latitudes (25° N to 30° S), we observed alternating patches of predominantly heterotrophic and autotrophic community metabolism. The calculated regression lines (for data north and south of the Equator) between temperature and the ratio of bacterial production to primary production give a maximum value for this ratio of 40% in the oligotrophic equatorial regions. Taking into account a bacterial growth efficiency 3 , 4 of 30%, the resulting area of net heterotrophy (where the bacterial carbon demand for growth plus respiration exceeds phytoplankton carbon fixation 4 , 5 , 6 ) expands from 8° N (27 °C) to 20° S (23 °C). This suggests an output of CO 2 from parts of the ocean to the atmosphere 6 , 7 .