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

Abstract

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.

Manganese Limitation of Phytoplankton Physiology and Productivity in the Southern Ocean

“…This indirect effect can be accounted for by defining a state of “Mn deficiency,” which instead normalizes Q Mn to the amount of Mn required to support growth rates in the absence of Fe limitation (i.e., Q Mn,Req,max ). Conceptually, our definition of Mn deficiency is similar to the additive responses observed in bottle incubations where simultaneous addition of both Mn and Fe increase biomass more than addition of Fe alone (Balaguer et al., 2022 ; Browning et al., 2021 ). In the standard version of the model, over half of the Southern Ocean experiences “Mn deficiency” at some point during the seasonal cycle (62% of waters poleward of 40°S; Figure 3f ).…”

“…Relative to other Fe-limited regions where Mn supply is greater (e.g., the Subarctic Pacific), the first reports of dMn in the Southern Ocean emphasized unusually low concentrations, proposing the potential for Mn co-limitation alongside Fe . More recent surveys have confirmed that dMn in both the Antarctic and Subantarctic zones can be <0.05 nM, the lowest measured globally, especially in remote waters away from continental margin and islands (Balaguer et al, 2022;Browning et al, 2014;Latour et al, 2021;Middag et al, 2011Middag et al, , 2013. Surface waters south of the Polar Front also feature Zn 2+ concentrations that are 100-1,000-fold higher than temperate and tropical regions, which should depress algal Mn uptake via competition for membrane transporters (Baars & Croot, 2011;Baars et al, 2014).…”

Process controlling iron–manganese regulation of the Southern Ocean biological carbon pump

“…Experimental work using nutrient additions has shown that the phytoplankton community in the Southern Ocean can also respond to addition of volcanic ash (containing both Mn and Fe) [ 22 ]. Co-limitation between Mn and Fe has been detected in Drake Passage, both in the western [ 23 ] and central part [ 7 ] of the passage, and in McMurdo Sound in the Ross Sea [ 8 ]. Additionally, the species Chaetoceros debilis has been shown to be Mn deficient in the Atlantic sector of the Southern Ocean [ 19 ].…”

Biogeochemistry of climate driven shifts in Southern Ocean primary producers

“…Other micronutrients beyond iron, predominantly manganese, have been shown to play an important role in controlling oxidative stress by catalysing antioxidant production in some diatom species (McCain et al, 2021), explaining the observed phenomena of iron-manganese co-limitation in the Southern Ocean (Browning et al, 2021, Pausch et al, 2019, Balaguer et al, 2022. Despite the importance of micronutrients such as manganese in controlling phytoplankton growth, particularly during seasonal transitions (Browning et al, 2021)…”