Growth of marine Vibrio in oligotrophic environments is not stimulated by the addition of inorganic iron

Phytoplankton growth in many high-latitude and equatorial regions of the world ocean is limited by low iron (Fe), while regions of lower nutrient upwelling and higher dust supply such as the subtropical gyres are typically not considered Fe-limited. Recent work which showed that adding Saharan dust leachate promotes blooms of Vibrio bacteria in the North Atlantic Ocean challenges this simple paradigm, because Fe is one of the most prevalent micronutrients in the leachate. We also find that adding high quantities of dust leachate to the North Atlantic near Bermuda stimulates Vibrio growth. However, inorganic Fe alone did not stimulate the growth of Vibrio in any of eleven field experiments in the North Atlantic, central North Pacific, or the coastal Pacific, suggesting that some other growth factor besides Fe is partially or completely responsible for the Vibrio growth response with added dust leachate. We consider the possibility that organic carbon, phosphorous, or other trace-metals in dust might have stimulated Vibrio growth either directly or indirectly. Laboratory culture experiments demonstrate that dust leachate can stimulate the growth of Vibrio even when sufficient Fe is present in the medium. Global modeling suggests that dust fluxes high enough to stimulate Vibrio growth are rare.