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.

Highlights d A vast icefish breeding colony has been discovered in the southern Weddell Sea d Extremely high benthic biomass provides food for predators and scavengers d Oceanographic conditions influence brooding habitat suitability for icefish d Unique ecosystem supports establishment of a marine protected area (MPA)

Iron (Fe) and manganese (Mn) availability and the divergent requirements of phytoplankton species were recently shown to be potential important drivers of Southern Ocean community composition. Knowledge about Antarctic phytoplankton species requirements for Fe and Mn remains, however, scarce. By performing laboratory experiments and additional calculations of the photosynthetic electron transport, we investigated the response of the ecologically important species Phaeocystis antarctica under a combination of different Fe and Mn concentrations. Fe deprivation alone provoked typical physiological characteristics of Fe limitation in P. antarctica (e.g., lowered growth and photosynthetic efficiency). In comparison, under Mn deprivation alone, the growth and carbon production of P. antarctica were not impacted. Its tolerance to cope with low Mn concentrations resulted from an efficient photoacclimation strategy, including a higher number of active photosystems II through which fewer electrons were transported. This strategy allowed us to maintain similar high growth and carbon production rates as FeMn‐enriched cells. Due to its low Mn requirement, P. antarctica performed physiologically as Fe‐deprived cells under the combined depletion of Fe and Mn. Hence, our study reveals that different from other Southern Ocean phytoplankton species, P. antarctica possesses a high capacity to cope with natural low Mn concentrations, which can facilitate its dominance over others, potentially explaining its ecological success across the Southern Ocean.

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