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

Abstract

Iron (Fe) is a key limiting nutrient driving the biological carbon pump and is routinely represented in global ocean biogeochemical models. However, in the Southern Ocean, the potential role for other micronutrients has not received the same attention. For example, although manganese (Mn) is essential to photosynthetic oxygen production and combating oxidative stress, it is not included in ocean models and a clear understanding of its interaction with Fe in the region is lacking. This is especially important for the Southern Ocean because both Mn and Fe are strongly depleted. We use a hierarchical modelling approach to explore how the physiological traits associated with Fe and Mn contribute to driving the footprint of micronutrient stress across different phytoplankton functional types (PFTs). We find that PFT responses are driven by physiological traits associated with their physiological requirements and acclimation to environmental conditions. Southern Ocean-specific adaptations to prevailing low Fe, such as large photosynthetic antenna sizes, are of major significance for the regional biological carbon pump. Other traits more strongly linked to Mn, such as dealing with oxidative stress, may become more important under a changing Fe supply regime. This article is part of a discussion meeting issue ‘Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities’.

Characterization of a Southern Ocean deep chlorophyll maximum: Response of phytoplankton to light, iron, and manganese enrichment

“…However, initial dMn concentrations were elevated ($ 0.3 nmol L À1 ), resulting in a high Mn* of 0.23, compared to the central Drake Passage where Mn limitation was previously observed (0.08-0.2 nmol L À1 and Mn* < 0.16; Browning et al 2021;Balaguer et al 2022). Moreover, the phytoplankton community had low growth rates under DCM conditions (0.02-0.08 d À1 ; Table 2) and exhibited a photoacclimatory strategy consistent with an increase in photosynthetic antenna size rather than increasing Mn-containing PSII reaction centers, both of which would decrease cellular Mn requirements (Hawco et al 2022;Anugerahanti and Tagliabue 2023). We surmise that this combination of photoacclimation strategy and low (lightlimited) growth rates act in concert to minimize Mn limitation in the DCM.…”

Response of Southern Ocean Resource Stress in a Changing Climate

“…We used the PISCES‐QUOTA global ocean biogeochemical ocean model (Aumont et al., 2015; Kwiatkowski et al., 2018), with the inclusion of the biogeochemical Mn cycle and phytoplankton Mn limitation (Anugerahanti & Tagliabue, 2023; Hawco et al., 2022). The model has three phytoplankton functional types (PFTs); diatoms, nanophytoplankton, and picophytoplankton, two zooplankton and multiple limiting nutrients (nitrogen, phosphorus, iron, manganese, and silica for diatoms).…”

“…Our control experiment uses the default Fe‐Mn limitation parameterization scheme, as described in Anugerahanti and Tagliabue (2023); where growth is limited Mn, and the Mn uptake can be affected by zinc (Zn), as it has higher affinity for the Mn transporter (Sunda & Huntsman, 1996, 1998). We conducted experiments using offline physical fields from IPSL‐CM5A climate model under the high emissions RCP8.5 scenario.…”

“…Moreover, Southern Ocean phytoplankton are known to employ a unique set of physiological adaptations that allow more efficient use of Fe, relative to temperate groups, by both increasing their light‐harvesting antenna size (Strzepek et al., 2019) and exploiting organically complexed Fe pools more efficiently (Strzepek et al., 2011). Adjustments to photophysiology are thought to affect both Fe and Mn demands (Hawco et al., 2022), and which affect NPP and export production (Anugerahanti & Tagliabue, 2023). However, we lack an assessment of how Fe and Mn impact regional NPP and export production in a CC scenario and the potential role played by distinct Antarctic and temperate phytoplankton traits.…”

Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities

“…The emerging complexity of the biological component of the Southern Ocean carbon cycle is addressed in Ahugerahanti & Tagliabue [ 8 ] who revisit the paradigm of micronutrient limited primary productivity and explore the processes controlling iron and manganese co-limitation. As in the physical and biogeochemical circulation systems, autonomous instrumentation has provided new insights into the fine scale structures of the biological carbon pump, and Thomalla et al .…”