Dilution solves the recalcitrance question The deep ocean is full of dissolved organic carbon, some of which has remained unchanged for thousands of years. What makes these compounds so resistant to microbial degradation? Perhaps their chemical structures make them intrinsically difficult to metabolize? In contrast, Arrieta et al. show that they are simply too dilute to be viable sources of energy for microorganisms (see the Perspective by Middleburg). Further experiments show that if these seemingly recalcitrant organic molecules are concentrated, the ambient microbes can consume them. Science , this issue p. 331 ; see also p. 290

The atmosphere plays a fundamental role in the transport of microbes across the planet but it is often neglected as a microbial habitat. Although the ocean represents two thirds of the Earth’s surface, there is little information on the atmospheric microbial load over the open ocean. Here we provide a global estimate of microbial loads and air-sea exchanges over the tropical and subtropical oceans based on the data collected along the Malaspina 2010 Circumnavigation Expedition. Total loads of airborne prokaryotes and eukaryotes were estimated at 2.2 × 1021 and 2.1 × 1021 cells, respectively. Overall 33–68% of these microorganisms could be traced to a marine origin, being transported thousands of kilometres before re-entering the ocean. Moreover, our results show a substantial load of terrestrial microbes transported over the oceans, with abundances declining exponentially with distance from land and indicate that islands may act as stepping stones facilitating the transoceanic transport of terrestrial microbes.