Manganese biogeochemistry in the Southern Ocean

Manganese (Mn) is a redox-active metal essential for most life on Earth. In photosynthetic microalgae (phytoplankton), Mn is used in the oxygen evolving complex of photosystem II, and in the superoxide dismutase enzyme to detoxify reactive oxygen species. Thus, phytoplankton have a strict Mn requirement for growth. In the Southern Ocean, the largest High-Nutrient Low-Chlorophyll (HNLC) region, phytoplankton growth is strongly limited by the micronutrient iron (Fe), but recent evidence shows Mn can (co-)limit phytoplankton growth in both coastal and open ocean regions. These results conflict with earlier studies that found Mn levels in Southern Ocean waters were sufficient to support phytoplankton growth. Hence, there is a need to constrain the distribution of Mn in Southern Ocean waters, to describe its sources and sinks and to identify potential regions of limitation. Here, we aim to tackle this problem by firstly describing Mn distribution along a meridional transect between Tasmania and Antarctica. Secondly, we study the zonal distribution of Mn near major Antarctic coastal sources with a focus on the northward supply of fertilised waters into HNLC waters. Finally, we use ship-based bioassays to test Mn limitation of Southern Ocean phytoplankton in subantarctic and polar waters. Manganese concentrations were measured in the Australian sector of the Southern Ocean, following the GEOTRACES-SR3 meridional transect, from Tasmania (Australia) to Antarctica. Manganese distribution was related to two external sources: sediment and hydrothermal inputs. We found both dissolved and particulate Mn concentrations were extremely low along this transect, despite strong inputs from Tasmanian and Antarctic shelf sediments, and hydrothermal vents. The presence of a cold-core eddy induced upward movement of Mn enriched waters. However, this enrichment did not reach surface waters where it could fertilize Mn depleted waters. At the southern end of the SR3 section, we studied the potential export of Mn-enriched shelf waters to Southern Ocean open waters. This was done in the context of the complex oceanography near the shelf break. We found that despite high Mn concentrations present on the shelf (> 0.25 nM), export toward depleted open waters was limited. This was due to three processes: biological uptake decreased dissolved Mn concentrations in surface waters while dilution of Mn-rich Antarctic Bottom Waters with Mn-depleted Low Circumpolar Deep Water and scavenging processes decreased concentrations in bottom waters. The latter finding was unexpected considering elevated Mn concentrations are commonly observed near the seafloor, implying constant sediment inputs and increase of Mn concentrations in bottom waters. However, additional bottom water measurements remain necessary to evaluate Mn oxidation rates. As very low surface dissolved Mn concentrations were observed in this region, we performed repeated field bioassays in subantarctic waters to study the seasonality of Fe- and Mn (co-)limitation. To the best of our knowledge, only one other study has looked at Fe- and Mn (co-)limitation in subantarctic waters, and no other study has looked at the potential seasonality of this limitation. To evaluate this, surface seawater was incubated with additions of Fe and Mn in austral spring, summer, and autumn. After eight days of incubation, we collected samples for macronutrient concentrations, photophysiology, phytoplankton community composition measured by flow cytometry, and Fe/carbon uptake. We found no clear signal of Mn limitation at any season. However, we observed strong seasonality in Fe and silicic acid limitation of phytoplankton growth. Iron limited phytoplankton growth in summer while silicic acid levels limited diatom growth in autumn. In spring, neither Fe nor silicic acid limited phytoplankton growth. Carbon uptake measurements suggested a slight stimulation by Mn in both spring and summer. In spring, the combined addition of Fe and Mn resulted in significant carbon uptake stimulation in the medium size class (composed of multiple species: small diatoms, cyanobacteria and prymnesiophytes). Similarly, during the summer, only the addition of both Fe and Mn led to significantly higher carbon uptake in the large size class (comprised primarily of large diatoms and/or dinoflagellates), indicating that only part of the community may have been (co-)limited by Mn. This latter result suggests Mn limitation may be missed during conventional field bioassays. We repeated the subantarctic field experiment south of the Polar Front to study the response of the phytoplankton community collected from a deep chlorophyll maximum to increases in light, Fe and Mn conditions. To the best of our knowledge, no other study has looked at Mn limitation of phytoplankton growth in deep chlorophyll maxima. We tested the hypothesis that phytoplankton Mn requirement may vary with changing light or Fe conditions. Seawater was collected from a diatom-dominated deep chlorophyll maximum and incubated at ambient and elevated irradiances (1 and 12% of incident irradiance, respectively). We observed that the community was primarily light limited. Once light limitation was alleviated Fe became the limiting factor and adding Fe primarily stimulated the growth of large diatoms. We did not observe evidence of Mn limitation, suggesting natural Mn levels (0.33 nM) were sufficient to support phytoplankton growth. However, we observed a small shift in phytoplankton community composition when both Fe and Mn were added, indicating that some phytoplankton species, within the nanoeukaryote size class, may have benefited from Mn additions. In conclusion, this work described the first set of dissolved and particulate Mn concentrations along a full depth meridional transect in the Australian sector of the Southern Ocean where trace metal (especially Mn) datasets are limited. Low Mn levels observed across the transect contrasted with potential inputs from the Antarctic shelf. However, we conclude that the export of Mn from the East Antarctic shelf adjacent to open waters was limited by biological uptake, water masses mixing and scavenging processes. We tested the hypothesis that low Mn concentrations limit phytoplankton growth in subantarctic waters with a potential seasonal variability. No seasonal signal of Mn (co-)limitation was observed at the subantarctic site. However, we did observe some responses to Mn addition: stimulation of carbon uptake and phytoplankton communities shifts that indicated Mn may control the primary productivity of a sub-set of phytoplankton taxa. In addition, we investigated the hypothesis that Mn may limit phytoplankton growth from a polar deep chlorophyll maximum. Again, no clear signal of Mn limitation was observed but subtle responses suggested some population benefited from Mn additions. Overall, identifying limiting parameters of phytoplankton growth remains essential to predict the future evolution of the ocean carbon cycle. Our results suggest Mn (co-)limitation is nuanced and may be hard to capture, particularly when Fe limits much of the phytoplankton community. This points to the need for further study on the Mn requirements of Southern Ocean phytoplankton and their interaction with other variables such as light and Fe.