Resource co-limitation drives competition between phytoplankton and bacteria in the Southern Ocean

Lavenia Ratnarajah, University of Liverpool, Liverpool, United Kingdom, Ingrid Obernosterer, Université Pierre et Marie Curie-Paris 6, Banyuls-sur-mer, France, Philip W Boyd, University of Tasmania, Institute for Marine and Antarctic Studies, Antarctic Climate Ecosystems Cooperative Research Centre, Hobart, Australia, Stephane Blain, Marion Fourquez, Mediterranean Institute of Oceanography, Marseille Cedex 09, France and Alessandro Tagliabue, University of Liverpool, Earth, Ocean and Ecological Sciences, Liverpool, United Kingdom
In large regions of the Southern Ocean, phytoplankton growth is regulated by the micronutrient iron (Fe) and can simultaneously be co-limited by light due to the deep seasonal mixed layers. In parallel, heterotrophic bacterial growth can be co-limited by the low availability of both Fe and labile dissolved organic carbon (DOC). The latter being due to low rates of primary production caused by Fe-deplete conditions. Most studies have investigated how Fe influences phytoplankton growth, but little research has been done to understand how heterotrophic bacteria compete with phytoplankton for Fe. In this study, we develop a model to quantitatively examine the poorly constrained mechanisms behind these competitive interactions in response to Fe, light and DOC co-limitation and compared it to simulations undertaken in the absence of competition.

We find that competition alters the physiological response of phytoplankton and heterotrophic bacteria. Phytoplankton growth rate and chlorophyll synthesis decreases due to competition for Fe by heterotrophic bacteria. In contrast, the increase in DOC supplied via phytoplankton exudation under Fe limited conditions leads to an increase in heterotrophic bacterial growth rate. Although our model suggests that heterotrophic bacteria are a major consumer of Fe, ultimately the magnitude of phytoplankton and heterotrophic bacterial biomass and dominance is dependent on light and DOC respectively. Applying our model to the Southern Ocean, we find that the seasonal irradiance cycle determines the dominance of phytoplankton or heterotrophic bacteria in the pelagic zone as DOC exudation in insufficient to stimulate significant heterotrophic bacterial production to such an extent that phytoplankton are outcompeted. However, sloppy feeding and faecal pellet production by predators, viral lysis and phytoplankton cell death following the initial spring bloom could add to the labile DOC inventory in the upper ocean thereby strengthening competitive interactions between phytoplankton and bacteria.