The Collapse of Prochlorococcus Populations in the Transition Between the Subtropical and Subpolar Gyres

Traditional ecological theory suggests small phytoplankton should be found ubiquitously, with larger size classes only co-existing in regions of higher Chl-a and productivity. In situ observations in general support these theoretical predictions, and the clear correlation between size fractionation and Chl-a concentration has been used in diagnostic models and satellite algorithms to estimate size-structure from Chl-a alone. But there is also evidence that this ubiquity of small classes is not true for the smallest phytoplankton, Prochlorococcus, whose cell counts drop dramatically in the transition between subtropical and subpolar gyres. Here we provide theoretical and computer model results along with field observations to explore this “collapse” of Prochlorococcus. We find including the relationship with heterotrophic bacteria (which are of similar size to Prochlorococcus) is necessary to understand this phenomenon. In the oligotrophic subtropical gyres there is a “friendly” relationship between Prochlorococcus and bacteria – with Prochlorococcus supplying organic matter to the bacteria, and in turn the bacteria remineralize organic matter supplying inorganic nutrients to the Prochlorococcus. In the transition to subpolar waters with higher supply of new nutrients from below, there is increase productivity and larger size classes can be supported. The larger productivity subsequently leads to greater supply of organic matter and heterotrophic bacteria cell counts increase in response, as is evident in field data. Grazing pressure by species that prey on both Prochlorococcus and bacteria increase until Prochlorococcus are no longer able to sustain a population and “collapse”. In this transition region increased bacteria populations negatively affect Procholorococcus through shared grazing. This study provides an explanation of the clearly defined biogeography of Prochlorococcus that does not fit into classical description of size-fractionation of phytoplankton.