Iron and manganese co-limit the growth of two phytoplankton groups dominant at two locations of the Drake Passage

While it has been recently demonstrated that both iron (Fe) and manganese (Mn) control Southern Ocean (SO) plankton biomass, how in particular Mn governs phytoplankton species composition remains yet unclear. This study, for the first time, highlights the importance of Mn next to Fe for growth of two key SO phytoplankton groups at two locations in the Drake Passage (West and East). Even though the bulk parameter chlorophyll a indicated Fe availability as main driver of both phytoplankton assemblages, the flow cytometric and microscopic analysis revealed FeMn co-limitation of a key phytoplankton group at each location: at West the dominant diatom Fragilariopsis and one subgroup of picoeukaryotes, which numerically dominated the East community. Hence, the limitation by both Fe and Mn and their divergent requirements among phytoplankton species and groups can be a key factor for shaping SO phytoplankton community structure. Iron and manganese play an important role in phytoplankton biomass control, but the exact effect of these elements on species composition has remained unknown. Conducting phytoplankton incubation experiments at two Drake Passage sites, we demonstrate how iron and manganese regulate phytoplankton community structure.