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

Spatiotemporal trends of aerosol provenance and trace metal concentrations in the northeast subarctic Pacific Ocean

The long-range transport of naturally occurring and anthropogenic aerosols originating from Asian deserts and megacities, respectively, can have a significant impact on the biogeochemical cycling of metals in the Fe-limited, high nutrient-low chlorophyll (HNLC) region of the northeast (NE) subarctic Pacific Ocean. These aerosols can deposit essential (e.g., Fe) and possibly toxic (e.g., Cu) metals to surface waters; thereby affecting micronutrients' bioavailability, and ultimately primary productivity in this region. In this study, we aimed to determine the provenance and spatiotemporal trends of metal inputs from Asian aerosol outflows into the NE Pacific Ocean. To do so, we collected aerosols on six research cruises along the Line P transect (GEOTRACES GPpr07), across three seasons in four years. Lead isotopic composition signatures were less radiogenic (high 208Pb/206Pb; low 206Pb/207Pb) in winter and spring compared to summer, signifying a greater anthropogenic Asian source in the cooler seasons. Furthermore, aerosol metal content also revealed seasonality. For example, aerosols collected in March 2022 (winter) contained higher concentrations of the lithogenic metals Al, Ti, Fe, Mn and Co, the anthropogenic metals Cu, Cd and Pb, as well as the mixed-source metals Ni and V, in comparison to May 2021 (spring) and August 2021 (summer). The estimated annual atmospheric flux of Fe and Zn accounted for 13-27 % and 6-10 %, respectively, of the total flux to the surface mixed layer (i.e., vertical mixing plus atmospheric flux) at Ocean Station Papa (50°N, 145°W), while that of Cu accounted for 95-99 %. This study provides the first insight into the seasonal patterns and geochemical characterizations of long-range transported Asian aerosols along the Line P transect. Anthropogenic activities will likely increase the input of aerosol-derived metals from distant Asian sources into the NE Pacific Ocean in the coming years, influencing biogeochemical cycles in this Fe-limited oceanic region.

A Phaeocystis bloom in the northern Beibu Gulf reflects nutrient-driven shifts in dominant phytoplankton taxa

Large-scale Phaeocystis globosa blooms in the Beibu Gulf have adversely affected marine ecosystems and the functioning of a nuclear power plant. To understand the formation mechanisms for such large-scale blooms, we examined the P. globosa bloom dynamics in the northern Beibu Gulf from September 2015 to March 2016 in association with changes in phytoplankton community and environmental factors. Statistical analysis methods, such as change-point method, principal component analysis (PCA), redundancy analysis (RDA) and an artificial neural network (ANN) model, were utilized to identify environmental drivers on the dynamics of phytoplankton community during the P. globosa bloom. The bloom evolution can be categorized into three stages, delineated by the fluctuation in colony abundance of P. globosa. Stage I, immediately before the Pheaocystis bloom, was characterized by the dominance of cyanobacterium Synechococcus (Syn) and the presence of the pigment 19'-hexanoyloxyfucoxanthin (Hex-fuco) in phytoplankton. Stage II, the development stage of the Phaeocystis bloom, was characterized by the appearance of P. globosa colonies and the pigment 19'-butanoyloxyfucoxanthin (But-fuco). Stage III, the decline stage of the bloom, was characterized by the increase of diatoms and pico-sized eukaryotes. Dynamics of the P. globosa bloom reflected the shift of dominant phytoplankton taxa from diatoms to haptophytes during winter, closely related to the change of turbidity, silicate limitation and rebalancing of nitrogen and phosphate. This study is the first to examine the relationship of a Phaeocystis bloom with phytoplankton structure and environmental factors in the Beibu Gulf, providing critical insights for the bloom forecasting and mitigation.

Climate change is associated with higher phytoplankton biomass and longer blooms in the West Antarctic Peninsula

The Antarctic Peninsula (West Antarctica) marine ecosystem has undergone substantial changes due to climate-induced shifts in atmospheric and oceanic temperatures since the 1950s. Using 25 years of satellite data (1998-2022), this study presents evidence that phytoplankton biomass and bloom phenology in the West Antarctic Peninsula are significantly changing as a response to anthropogenic climate change. Enhanced phytoplankton biomass was observed along the West Antarctic Peninsula, particularly in the early austral autumn, resulting in longer blooms. Long-term sea ice decline was identified as the main driver enabling phytoplankton growth in early spring and autumn, in parallel with a recent intensification of the Southern Annular Mode (2010-ongoing), which was observed to influence regional variability. Our findings contribute to the understanding of the complex interplay between environmental changes and phytoplankton responses in this climatically key region of the Southern Ocean and raise important questions regarding the far-reaching consequences that these ecological changes may have on global carbon sequestration and Antarctic food webs in the future.

Phaeocystis: A Global Enigma

The genus Phaeocystis is globally distributed, with blooms commonly occurring on continental shelves. This unusual phytoplankter has two major morphologies: solitary cells and cells embedded in a gelatinous matrix. Only colonies form blooms. Their large size (commonly 2 mm but up to 3 cm) and mucilaginous envelope allow the colonies to escape predation, but data are inconsistent as to whether colonies are grazed. Cultured Phaeocystis can also inhibit the growth of co-occurring phytoplankton or the feeding of potential grazers. Colonies and solitary cells use nitrate as a nitrogen source, although solitary cells can also grow on ammonium. Phaeocystis colonies might be a major contributor to carbon flux to depth, but in most cases, colonies are rapidly remineralized in the upper 300 m. The occurrence of large Phaeocystis blooms is often associated with environments with low and highly variable light and high nitrate levels, with Phaeocystis antarctica blooms being linked additionally to high iron availability. Emerging results indicate that different clones of Phaeocystis have substantial genetic plasticity, which may explain its appearance in a variety of environments. Given the evidence of Phaeocystis appearing in new systems, this trend will likely continue in the near future.

Iron and manganese availability drives primary production and carbon export in the Weddell Sea

Next to iron (Fe), recent phytoplankton-enrichment experiments identified manganese (Mn) to (co-)limit Southern Ocean phytoplankton biomass and species composition. Since taxonomic diversity affects aggregation time and sinking rate, the efficiency of the biological carbon pump is directly affected by community structure. However, the impact of FeMn co-limitation on Antarctic primary production, community composition, and the subsequent export of carbon to depth requires more investigation. In situ samplings of 6 stations in the understudied southern Weddell Sea revealed that surface Fe and Mn concentrations, primary production, and carbon export rates were all low, suggesting a FeMn co-limited phytoplankton community. An Fe and Mn addition experiment examined how changes in the species composition drive the aggregation capability of a natural phytoplankton community. Primary production rates were highest when Fe and Mn were added together, due to an increased abundance of the colonial prymnesiophyte Phaeocystis antarctica. Although the community remained diatom dominated, the increase in Phaeocystis abundance led to highly carbon-enriched aggregates and a 4-fold increase in the carbon export potential compared to the control, whereas it only doubled in the Fe treatment. Based on the outcome of the FeMn-enrichment experiment, this region may suffer from FeMn co-limitation. As the Weddell Sea represents one of the most productive Antarctic marginal ice zones, our findings highlight that in response to greater Fe and Mn supply, changes in plankton community composition and primary production can have a disproportionally larger effect on the carbon export potential.

Global analysis of ocean phytoplankton nutrient limitation reveals high prevalence of co-limitation

Nutrient availability limits phytoplankton growth throughout much of the global ocean. Here we synthesize available experimental data to identify three dominant nutrient limitation regimes: nitrogen is limiting in the stratified subtropical gyres and in the summertime Arctic Ocean, iron is most commonly limiting in upwelling regions, and both nutrients are frequently co-limiting in regions in between the nitrogen and iron limited systems. Manganese can be co-limiting with iron in parts of the Southern Ocean, whilst phosphate and cobalt can be co-/serially limiting in some settings. Overall, an analysis of experimental responses showed that phytoplankton net growth can be significantly enhanced through increasing the number of different nutrients supplied, regardless of latitude, temperature, or trophic status, implying surface seawaters are often approaching nutrient co-limitation. Assessments of nutrient deficiency based on seawater nutrient concentrations and nutrient stress diagnosed via molecular biomarkers showed good agreement with experimentally-assessed nutrient limitation, validating conceptual and theoretical links between nutrient stoichiometry and microbial ecophysiology.

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

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'.

Manganese Limitation of Phytoplankton Physiology and Productivity in the Southern Ocean

Although iron and light are understood to regulate the Southern Ocean biological carbon pump, observations have also indicated a possible role for manganese. Low concentrations in Southern Ocean surface waters suggest manganese limitation is possible, but its spatial extent remains poorly constrained and direct manganese limitation of the marine carbon cycle has been neglected by ocean models. Here, using available observations, we develop a new global biogeochemical model and find that phytoplankton in over half of the Southern Ocean cannot attain maximal growth rates because of manganese deficiency. Manganese limitation is most extensive in austral spring and depends on phytoplankton traits related to the size of photosynthetic antennae and the inhibition of manganese uptake by high zinc concentrations in Antarctic waters. Importantly, manganese limitation expands under the increased iron supply of past glacial periods, reducing the response of the biological carbon pump. Overall, these model experiments describe a mosaic of controls on Southern Ocean productivity that emerge from the interplay of light, iron, manganese and zinc, shaping the evolution of Antarctic phytoplankton since the opening of the Drake Passage.