Microbiology of the IN- and OUT-patch defined iron-fertilized areas in the Southern Ocean

In contrast to the bacterial and picophytoplankton populations, the nanophytoflagellate population increased six-fold in numbers and 23-fold in biomass. This resulted in a three-fold increase in carbon flow through the microbial food web inside the patch by the end of the experiment. The increased carbon flow resulted in a small increase in total microzooplankton biomass.

Ciliate abundances tripled and biomass, doubled; however, the ciliate population only contributed 3-10% of the microzooplankton biomass, which was dominated by the heterotrophic nanoflagellate population. The heterotrophic nanoflagellate numbers decreased three-fold by the end of the experiment; however, there was no significant change in biomass throughout the experiment. The changes in the dynamics and structure of the microbial food web during the SOIREE experiment suggest that microzooplankton grazing controlled the bacterial and possibly the picophytoplankton populations.

In contrast, the nanophytoflagellates were initially controlled by the Fe concentration, with microzooplankton having an impact on the population towards the end of the experiment. The addition of Fe to a small patch of the Southern Ocean had a considerable impact on the microbial components of the food web, even though the overall importance of the microbial pathways decreased as a result of Fe addition.