A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization

Chlamydomonas cells transition through distinct Fe nutrition stages within 48 h of transfer to Fe-free medium

Low iron (Fe) bioavailability can limit the biosynthesis of Fe-containing proteins, which are especially abundant in photosynthetic organisms, thus negatively affecting global primary productivity. Understanding cellular coping mechanisms under Fe limitation is therefore of great interest. We surveyed the temporal responses of Chlamydomonas (Chlamydomonas reinhardtii) cells transitioning from an Fe-rich to an Fe-free medium to document their short and long-term adjustments. While slower growth, chlorosis and lower photosynthetic parameters are evident only after one or more days in Fe-free medium, the abundance of some transcripts, such as those for genes encoding transporters and enzymes involved in Fe assimilation, change within minutes, before changes in intracellular Fe content are noticeable, suggestive of a sensitive mechanism for sensing Fe. Promoter reporter constructs indicate a transcriptional component to this immediate primary response. With acetate provided as a source of reduced carbon, transcripts encoding respiratory components are maintained relative to transcripts encoding components of photosynthesis and tetrapyrrole biosynthesis, indicating metabolic prioritization of respiration over photosynthesis. In contrast to the loss of chlorophyll, carotenoid content is maintained under Fe limitation despite a decrease in the transcripts for carotenoid biosynthesis genes, indicating carotenoid stability. These changes occur more slowly, only after the intracellular Fe quota responds, indicating a phased response in Chlamydomonas, involving both primary and secondary responses during acclimation to poor Fe nutrition.

Cyanobacterial blooms, iron, and environmental pollutants

Iron determines the abundance and diversity of life and controls primary production in numerous aqueous environments. Over the past decades, the availability of this metal in natural waters has decreased. Iron deficiency can apply a selective pressure on microbial aquatic communities. Each aquatic organism has their individual requirements for iron and pathways for metal acquisition, despite all having access to the common pool of iron. Cyanobacteria, a photosynthesizing bacterium that can accumulate and form so-called 'algal blooms', have evolved strategies to thrive in such iron-deficient aqueous environments where they can outcompete other organisms in iron acquisition in diverse microbial communities. Metabolic pathways for iron acquisition employed by cyanobacteria allow it to compete successfully for this essential nutrient. By competing more effectively for requisite iron, cyanobacteria can displace other species and grow to dominate the microbial population in a bloom. Aquatic resources are damaged by a diverse number of environmental pollutants that can further decrease metal availability and result in a functional deficiency of available iron. Pollutants can also increase iron demand. A pollutant-exposed microbe is compelled to acquire further metal critical to its survival. Even in pollutant-impacted waters, cyanobacteria enjoy a competitive advantage and cyanobacterial dominance can be the result. We propose that cyanobacteria have a distinct competitive advantage over many other aquatic microbes in polluted, iron-poor environments.

One-third of Southern Ocean productivity is supported by dust deposition

Natural iron fertilization of the Southern Ocean by windblown dust has been suggested to enhance biological productivity and modulate the climate1-3. Yet, this process has never been quantified across the Southern Ocean and at annual timescales4,5. Here we combined 11 years of nitrate observations from autonomous biogeochemical ocean profiling floats with a Southern Hemisphere dust simulation to empirically derive the relationship between dust-iron deposition and annual net community production (ANCP) in the iron-limited Southern Ocean. Using this relationship, we determined the biological response to dust-iron in the pelagic perennially ice-free Southern Ocean at present and during the last glacial maximum (LGM). We estimate that dust-iron now supports 33% ± 15% of Southern Ocean ANCP. During the LGM, when dust deposition was 5-40-fold higher than today, the contribution of dust to Southern Ocean ANCP was much greater, estimated at 64% ± 13%. We provide quantitative evidence of basin-wide dust-iron fertilization of the Southern Ocean and the potential magnitude of its impact on glacial-interglacial timescales, supporting the idea of the important role of dust in the global carbon cycle and climate6-8.

Relationships among the climate-relevant gases during the Southern Ocean bloom season

The ocean plays an essential role in regulating the sources and sinks of climate-relevant gases, like CO2, N2O and dimethyl sulfide (DMS), thus influencing global climate change. Although the Southern Ocean is known to be a strong carbon sink, a significant DMS source and possibly a large source of N2O, our understanding of the interaction among these climate-relevant gases and their potential impacts on climate change is still insufficient in the Southern Ocean. Herein, we analyzed parameters, including surface water pCO2, dissolved inorganic carbon (DIC), alkalinity (TA), DMS and N2O in the water column, collected during the austral summer of 2015-2016 in the 32nd Chinese Antarctic Research Expedition (CHINARE) at the tip of Antarctic Peninsula. A positive correlation between DMS and pCO2 (indicated by deficit of DIC, ∆DIC, refer to values in 100 m) was observed in waters above 75 m, whereas no correlation between N2O saturation anomaly (SA) and DMS, ∆DIC was found. In the area with stable stratification with phytoplankton bloom, significant DMS source and strong CO2 uptake with weak N2O emission were observed. Conversely, strong mixing or upwelling area was shown to be a strong marine CO2 source and significant N2O release with weak DMS source.

Acetylacetone effectively controlled the secondary metabolites of Microcystis aeruginosa under simulated sunlight irradiation

Inactivation of cyanobacterial cells and simultaneous control of secondary metabolites is of significant necessity for the treatment of cyanobacteria-laden water. Acetylacetone (AcAc) has been reported a specific algicide to inactivate Microcystis aeruginosa (M. aeruginosa) and an effective light activator to degrade pollutants. This study systematically investigated the photodegradation ability of AcAc under xenon (Xe) irradiation on the secondary metabolites of M. aeruginosa, mainly algal organic matter (AOM), especially toxic microcystin-LR (MC-LR). Results showed that AcAc outperformed H2O2 in destructing the protein-like substances, humic acid-like matters, aromatic proteins and fulvic-like substances of AOM. For MC-LR (250 µg/L), 0.05 mmol/L AcAc attained the same degradation efficiency (87.0%) as 0.1 mmol/L H2O2. The degradation mechanism of Xe/AcAc might involve photo-induced energy/electron transfer and formation of carbon center radicals. Alkaline conditions (pH > 9.0) were detrimental to the photoactivity of AcAc, corresponding to the observed degradation rate constant (k1 value) of MC-LR drastically decreasing to 0.0013 min-1 as solution pH exceeded 9.0. The PO43- and HCO3- ions had obvious inhibition effects, whereas NO3- slightly improved k1 value from 0.0277 min-1 to 0.0321 min-1. The presence of AOM did not significantly inhibit MC-LR degradation in Xe/AcAc system. In addition, the biological toxicity of MC-LR was greatly reduced after photoreaction. These results demonstrated that AcAc was an alternative algicidal agent to effectively inactivate algal cells and simultaneously control the secondary metabolites after cell lysis. Nevertheless, the concentration and irradiation conditions should be further optimized in practical application.

Feasibility study of storing CO2 in the ocean by marine environmental impact assessment

Since the industrial revolution, which was accompanied with the use of fossil fuels as an energy source, the content of carbon dioxide (CO2) in the atmosphere has increased. To mitigate global warming, industries that utilize fossil fuels have continuously explored new approaches to reduce CO2 emissions and convert it to alternative fuels. The ocean is a vast source of absorbed CO2 on Earth, and various studies have been conducted on the use of the ocean to reduce global CO2. This study focused on reducing CO2 in the atmosphere by storing it as bicarbonate, a form of CO2 that exists in the ocean. The optimum condition for the conversion of CO2 into bicarbonate was investigated by considering the dissolved inorganic carbon (DIC; HCO3-, CO32-, H2CO3) concentration and pH. To confirm the biological impact of this conversion, biological impact experiments were conducted under various DIC concentrations using Skeletonema japonicum, a phytoplankton present in most areas of the sea. Based on the DIC concentration (2.09 mM) of the seawater, the DIC concentrations used in the Lab-scale experiment ranged from 2.5 mM to 18.75 mM, and the concentration with the highest conversion rate (< 6.38 mM) was applied in the pilot plant. Marine environmental impact modeling was performed to observe the effect of discharge to the ocean and its movement. The results revealed a slight growth inhibition of phytoplankton at DIC concentrations higher than the base concentration. Nevertheless, the change in the DIC concentration exerted no effect on the phytoplankton growth except at extremely high concentrations. Moreover, the high DIC concentration can be diluted by the ocean current flow rate, thus counterbalancing the growth inhibition effect. The results obtained in this study demonstrate the feasibility of CO2 storage in the form of DIC, and will be helpful for further development of CO2 mitigation.

Impact of dust deposition on the growth of marine autotrophic and heterotrophic microorganisms: Evidence from the South China Sea

Aeolian dust can provide nutrients for the ocean and affect the growth of phytoplankton. However, the impacts of dust deposition on autotrophic and heterotrophic microorganisms have rarely been studied. In this study, we conducted two microcosm experiments in the low-nutrient and low-chlorophyll environment of the South China Sea and found that dust did not stimulate the abundance of autotrophic and heterotrophic microorganisms. Our results show that dust contains most of the unreacted iron-bearing minerals, and thus provides limited bioavailable iron and nitrogen for bacterioplankton and phytoplankton growth. These results elucidate the overlooked impacts of the properties of the iron-bearing minerals in aeolian dust on microbial communities, which may play an important role in marine ecosystems and climate change.

Nanoplankton: The dominant vector for carbon export across the Atlantic Southern Ocean in spring

Across the Southern Ocean, large (≥20 μm) diatoms are generally assumed to be the primary vector for carbon export, although this assumption derives mainly from summertime observations. Here, we investigated carbon production and export potential during the Atlantic Southern Ocean's spring bloom from size-fractionated measurements of net primary production (NPP), nitrogen (nitrate, ammonium, urea) and iron (labile inorganic iron, organically complexed iron) uptake, and a high-resolution characterization of phytoplankton community composition. The nanoplankton-sized (2.7 to 20 μm) diatom, Chaetoceros spp., dominated the biomass, NPP, and nitrate uptake across the basin (40°S to 56°S), which we attribute to their low iron requirement, rapid response to increased light, and ability to escape grazing when aggregated into chains. We estimate that the spring Chaetoceros bloom accounted for >25% of annual export production across the Atlantic Southern Ocean, a finding consistent with recent observations from other regions highlighting the central role of the phytoplankton "middle class" in carbon export.

Microbial iron acquisition is influenced by spatial and temporal conditions in a glacial influenced river and estuary system

In Arctic regions, glaciers are major sources of iron to rivers and streams; however, estuaries are considered iron sinks due to the coagulation and flocculation processes that occur at higher salinities. It is unknown how iron dynamics in a glacial influenced river and estuary environment affect microbial mechanisms for iron acquisition. Microbial taxonomic and functional sequencing was performed on samples taken throughout the year from the Kenai River and the estuary, Alaska. Despite distinct iron, sodium, and other nutrient concentrations, the river and estuary did not have statistically different microbial communities nor was time of sampling significant. However, ferrous iron transport (Feo) system genes were more abundant in river environments, while siderophore genes were more abundant and diverse in estuary environments. Siderophore transport and iron storage genes were found in all samples, but gene abundance and distribution were potentially influenced by physical drivers such as discharge rates and nutrient distributions. Differences in iron metabolism between river and estuary ecosystems indicate environmental conditions drive microbial mechanisms to sequester iron. This could have implications for iron transport as the Arctic continues to warm.

A Lagrangian model for drifting ecosystems reveals heterogeneity-driven enhancement of marine plankton blooms

Marine plankton play a crucial role in carbon storage, global climate, and ecosystem function. Planktonic ecosystems are embedded in patches of water that are continuously moving, stretching, and diluting. These processes drive inhomegeneities on a range of scales, with implications for the integrated ecosystem properties, but are hard to characterize. We present a theoretical framework that accounts for all these aspects; tracking the water patch hosting a drifting ecosystem along with its physical, environmental, and biochemical features. The theory resolves patch dilution and internal physical mixing as a function of oceanic strain and diffusion. Ecological dynamics are parameterized by an idealized nutrient and phytoplankton population and we specifically capture the time evolution of the biochemical spatial variances to represent within-patch heterogeneity. We find that, depending only on the physical processes to which the water patch is subjected, the plankton biomass response to a resource perturbation can vary in size up to six times. This work indicates that we must account for these processes when interpreting and modeling marine ecosystems and provides a framework with which to do so.

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.

Ecosystem bioelement variability is associated with freshwater animal aggregations at the aquatic-terrestrial interface

The impacts of animals on the biogeochemical cycles of major bioelements like C, N, and P are well-studied across ecosystem types. However, more than 20 elements are necessary for life. The feedbacks between animals and the biogeochemical cycles of the other bioelements are an emerging research priority. We explored how much freshwater mussels (Bivalvia: Unionoida) were related to variability in ecosystem pools of 10 bioelements (Ca, Cu, Fe, K, Mn, Na, Mg, P, S and Zn) in streams containing a natural mussel density gradient in the US Interior Highlands. We studied the concentrations of these bioelements across the aquatic-terrestrial interface-in the porewater of riverine gravel bars, and the emergent macrophyte Justicia americana. Higher mussel density was associated with increased calcium in gravel bars and macrophytes. Mussel density also correlated with variability in iron and other redox-sensitive trace elements in gravel bars and macrophytes, although this relationship was mediated by sediment grain size. We found that two explanations for the patterns we observed are worthy of further research: (1) increased calcium availability in gravel bars near denser mussel aggregations may be a product of the buildup and dissolution of shells in the gravel bar, and (2) mussels may alter redox conditions, and thus elemental availability in gravel bars with fine sediments, either behaviorally or through physical structure provided by shell material. A better understanding of the physical and biogeochemical impacts of animals on a wide range of elemental cycles is thus necessary to conserve the societal value of freshwater ecosystems.

Multidimensional applications and potential health implications of nanocomposites

This study reviews the concept, classifications, and techniques involved in the synthesis of nanocomposites. The environmental and health implications of nanoparticles and composite materials were detailed, as well as the applications of nanocomposites in water remediation, antibacterial application, and printed circuit boards. The study gave insights into the challenges of water pollution treatment and provided a broad list of nanocomposites that have been explored for water remediation. Moreover, the emergence of multi-drug resistance to many antibiotics has made current antibiotics inadequate in the treatment of disease. This has engineered the development of alternative strategies in the drug industries for the production of effective therapeutic agents, comprising nanocomposites with antibacterial agents. The new therapeutic agents known as nanoantibiotics are more efficient and have paved the way to handle the challenges of antibiotic resistance. In printed circuit boards, nanocomposites have shown promising applications because of their distinct mechanical, thermal, and electrical characteristics. The uniqueness of the write-up is that it provides a broad explanation of the concept, synthesis, application, toxicity, and harmful effects of nanocomposites. Thus, it will provide all-inclusive awareness to readers to identify research gaps and motivate researchers to synthesize novel nanocomposites for use in various fields.

Multi-elemental consumer-driven nutrient cycling when predators feed on different prey

Predators play a fundamental role in cycling nutrients through ecosystems, by altering the amount and compositions of waste products and uneaten prey parts available to decomposers. Different prey can vary in their elemental content and the deposition of elements in predator waste can vary depending on which elements are preferentially retained versus eliminated as waste products. We tested how feeding on different prey (caterpillars, cockroaches, crickets, and flies) affected the concentrations of 23 elements in excreta deposited by wolf spider across 2 seasons (spring versus fall). Spider excreta had lower concentrations of carbon and higher concentrations of many other elements (Al, B, Ba, K, Li, P, S, Si, and Sr) compared to prey remains and whole prey carcasses. In addition, elemental concentrations in unconsumed whole prey carcasses and prey remains varied between prey species, while spider excreta had the lowest variation among prey species. Finally, the concentrations of elements deposited differed between seasons, with wolf spiders excreting greater concentrations of Fe, Mg, Mn, Mo, S, and V in the fall. However, in the spring, spiders excreted higher concentrations of Al, B, Ba, Ca, Cd, Cu, K, P, Na, Si, Sr, and Zn. These results highlight that prey identity and environmental variation can determine the role that predators play in regulating the cycling of many elements. A better understanding of these convoluted nutritional interactions is critical to disentangle specific consumer-driven effects on ecosystem function.

Iron limitation of kelp growth may prevent ocean afforestation

Carbon dioxide removal (CDR) and emissions reduction are essential to alleviate climate change. Ocean macroalgal afforestation (OMA) is a CDR method already undergoing field trials where nearshore kelps, on rafts, are purposefully grown offshore at scale. Dissolved iron (dFe) supply often limits oceanic phytoplankton growth, however this potentially rate-limiting factor is being overlooked in OMA discussions. Here, we determine the limiting dFe concentrations for growth and key physiological functions of a representative kelp species, Macrocystis pyrifera, considered as a promising candidate for OMA. dFe additions to oceanic seawater ranging 0.01-20.2 nM Fe' ‒ Fe' being the sum of dissolved inorganic Fe(III) species ‒ result in impaired physiological functions and kelp mortality. Kelp growth cannot be sustained at oceanic dFe concentrations, which are 1000-fold lower than required by M. pyrifera. OMA may require additional perturbation of offshore waters via dFe fertilisation.

Physiological responses of Humboldt current system diatoms to Fe and Cu co-limitation

Diatoms account for ∼20% of global primary production, often limited by the availability of Fe and other trace nutrients such as Cu. The present study examined the role of both metals in the physiology of two diatoms isolated from the Humboldt Currents System, the centric Chaetoceros c.f. dicipiens and the pennate Nitzschia c.f. draveillensis. Under Fe limitation, a decrease in specific growth rates and sizes of both species was observed, especially in Chaetoceros. However, regarding different photosynthetic parameters, Nitzschia was more impacted. The increase in Cu concentrations improved the physiology of both diatoms, mostly of Chaetoceros. When grown in mixed cultures and under co-limiting conditions, both species remained competive due to morphological advantages (i.e., lower cell size). These results may suggest that the increase of Cu under Fe limitation benefited C. c.f. dicipiens over N. c.f. draveillensis.

Wind-driven upwelling of iron sustains dense blooms and food webs in the eastern Weddell Gyre

The Southern Ocean is a major sink of anthropogenic CO₂ and an important foraging area for top trophic level consumers. However, iron limitation sets an upper limit to primary productivity. Here we report on a considerably dense late summer phytoplankton bloom spanning 9000 km² in the open ocean of the eastern Weddell Gyre. Over its 2.5 months duration, the bloom accumulated up to 20 g C m^-2 of organic matter, which is unusually high for Southern Ocean open waters. We show that, over 1997-2019, this open ocean bloom was likely driven by anomalies in easterly winds that push sea ice southwards and favor the upwelling of Warm Deep Water enriched in hydrothermal iron and, possibly, other iron sources. This recurring open ocean bloom likely facilitates enhanced carbon export and sustains high standing stocks of Antarctic krill, supporting feeding hot spots for marine birds and baleen whales.

Multidecadal trend of increasing iron stress in Southern Ocean phytoplankton

Southern Ocean primary productivity is principally controlled by adjustments in light and iron limitation, but the spatial and temporal determinants of iron availability, accessibility, and demand are poorly constrained, which hinders accurate long-term projections. We present a multidecadal record of phytoplankton photophysiology between 1996 and 2022 from historical in situ datasets collected by Biogeochemical Argo (BGC-Argo) floats and ship-based platforms. We find a significant multidecadal trend in irradiance-normalized nonphotochemical quenching due to increasing iron stress, with concomitant declines in regional net primary production. The observed trend of increasing iron stress results from changing Southern Ocean mixed-layer physics as well as complex biological and chemical feedback that is indicative of important ongoing changes to the Southern Ocean carbon cycle.

Giant Virus Infection Signatures Are Modulated by Euphotic Zone Depth Strata and Iron Regimes of the Subantarctic Southern Ocean

Viruses can alter the abundance, evolution, and metabolism of microorganisms in the ocean, playing a key role in water column biogeochemistry and global carbon cycles. Large efforts to measure the contribution of eukaryotic microorganisms (e.g., protists) to the marine food web have been made, yet the in situ activities of the ecologically relevant viruses that infect these organisms are not well characterized. Viruses within the phylum Nucleocytoviricota ("giant viruses") are known to infect a diverse range of ecologically relevant marine protists, yet how these viruses are influenced by environmental conditions remains under-characterized. By employing metatranscriptomic analyses of in situ microbial communities along a temporal and depth-resolved gradient, we describe the diversity of giant viruses at the Southern Ocean Time Series (SOTS), a site within the subpolar Southern Ocean. Using a phylogeny-guided taxonomic assessment of detected giant virus genomes and metagenome-assembled genomes, we observed depth-dependent structuring of divergent giant virus families mirroring dynamic physicochemical gradients in the stratified euphotic zone. Analyses of transcribed metabolic genes from giant viruses suggest viral metabolic reprogramming of hosts from the surface to a 200-m depth. Lastly, using on-deck incubations reflecting a gradient of iron availability, we show that modulating iron regimes influences the activity of giant viruses in the field. Specifically, we show enhanced infection signatures of giant viruses under both iron-replete and iron-limited conditions. Collectively, these results expand our understanding of how the water column's vertical biogeography and chemical surroundings affect an important group of viruses within the Southern Ocean. IMPORTANCE The biology and ecology of marine microbial eukaryotes is known to be constrained by oceanic conditions. In contrast, how viruses that infect this important group of organisms respond to environmental change is less well known, despite viruses being recognized as key microbial community members. Here, we address this gap in our understanding by characterizing the diversity and activity of "giant" viruses within an important region in the sub-Antarctic Southern Ocean. Giant viruses are double-stranded DNA (dsDNA) viruses of the phylum Nucleocytoviricota and are known to infect a wide range of eukaryotic hosts. By employing a metatranscriptomics approach using both in situ samples and microcosm manipulations, we illuminated both the vertical biogeography and how changing iron availability affects this primarily uncultivated group of protist-infecting viruses. These results serve as a foundation for our understanding of how the open ocean water column structures the viral community, which can be used to guide models of the viral impact on marine and global biogeochemical cycling.

Carbon and Iron Uptake by Phytoplankton in the Amundsen Sea, Antarctica

Freshwater components in the Southern Ocean, whether sea ice meltwater or meteoric water, influence the growth of phytoplankton by affecting water stability and supplying dissolved iron (DFe). In addition, melting sea ice stimulates phytoplankton blooms by providing ice algae. In this study, sea ice meltwater and meteoric water in the Amundsen Sea (AS) were differentiated by their stable oxygen isotopic compositions (δ¹⁸O), while the phytoplankton carbon fixation rate (CFR) and iron uptake rate (FeUR) values were determined using the ¹⁴C and ⁵⁵Fe tracer assays, respectively. Our results showed that FeUR exhibits a significant positive response only to sea ice meltwater, suggesting that DFe and algae provided by sea ice melting may be the main cause. In addition, the CFR had a slightly positive response to the freshwater input and a stronger correlation with the phytoplankton biomass, suggesting that the freshwater input may have enhanced the CFR through the algae released from sea ice melting. The FeUR normalized to the phytoplankton biomass was significantly positively correlated with the mixed layer depth, suggesting that water stability regulates the phytoplankton growth and the resulting Fe demand. A higher Fe demand per unit of carbon fixation during sea ice formation leads to a higher Fe/C ratio in phytoplankton. Although no significant correlations were observed between the FeUR, CFR, and meteoric water, meteoric water may have an effect on larger phytoplankton sensitive to Fe deficiencies. The results of culture experiments with DFe addition showed that the added Fe significantly enhanced the Fe uptake, carbon fixation, and Fe/C ratio of the cells, especially for micro-phytoplankton. The more pronounced response of micro-phytoplankton means that the meteoric water input may affect the efficiency of carbon export. Our study provides the first measurements of phytoplankton Fe quotas in the AS in austral late summer and early autumn, providing insights into how meteoric water and sea ice meltwater affect seasonal changes in Antarctic ecosystems.

Potential use of engineered nanoparticles in ocean fertilization for large-scale atmospheric carbon dioxide removal

Artificial ocean fertilization (AOF) aims to safely stimulate phytoplankton growth in the ocean and enhance carbon sequestration. AOF carbon sequestration efficiency appears lower than natural ocean fertilization processes due mainly to the low bioavailability of added nutrients, along with low export rates of AOF-produced biomass to the deep ocean. Here we explore the potential application of engineered nanoparticles (ENPs) to overcome these issues. Data from 123 studies show that some ENPs may enhance phytoplankton growth at concentrations below those likely to be toxic in marine ecosystems. ENPs may also increase bloom lifetime, boost phytoplankton aggregation and carbon export, and address secondary limiting factors in AOF. Life-cycle assessment and cost analyses suggest that net CO₂ capture is possible for iron, SiO₂ and Al₂O₃ ENPs with costs of 2-5 times that of conventional AOF, whereas boosting AOF efficiency by ENPs should substantially enhance net CO₂ capture and reduce these costs. Therefore, ENP-based AOF can be an important component of the mitigation strategy to limit global warming.

Recovery of carbon benefits by overharvested baleen whale populations is threatened by climate change

Despite the importance of marine megafauna on ecosystem functioning, their contribution to the oceanic carbon cycle is still poorly known. Here, we explored the role of baleen whales in the biological carbon pump across the southern hemisphere based on the historical and forecasted abundance of five baleen whale species. We modelled whale-mediated carbon sequestration through the sinking of their carcasses after natural death. We provide the first temporal dynamics of this carbon pump from 1890 to 2100, considering both the effects of exploitation and climate change on whale populations. We reveal that at their pre-exploitation abundance, the five species of southern whales could sequester 4.0 × 105 tonnes of carbon per year (tC yr-1). This estimate dropped to 0.6 × 105 tC yr-1 by 1972 following commercial whaling. However, with the projected restoration of whale populations under a RCP8.5 climate scenario, the sequestration would reach 1.7 × 105 tC yr-1 by 2100, while without climate change, recovered whale populations could sequester nearly twice as much (3.2 × 105 tC yr-1) by 2100. This highlights the persistence of whaling damages on whale populations and associated services as well as the predicted harmful impacts of climate change on whale ecosystem services.

Microbial ecology of the Southern Ocean

The Southern Ocean (SO) distributes climate signals and nutrients worldwide, playing a pivotal role in global carbon sequestration. Microbial communities are essential mediators of primary productivity and carbon sequestration, yet we lack a comprehensive understanding of microbial diversity and functionality in the SO. Here, we examine contemporary studies in this unique polar system, focusing on prokaryotic communities and their relationships with other trophic levels (i.e. phytoplankton and viruses). Strong seasonal variations and the characteristic features of this ocean are directly linked to community composition and ecosystem functions. Specifically, we discuss characteristics of SO microbial communities and emphasise differences from the Arctic Ocean microbiome. We highlight the importance of abundant bacteria in recycling photosynthetically derived organic matter. These heterotrophs appear to control carbon flux to higher trophic levels when light and iron availability favour primary production in spring and summer. Conversely, during winter, evidence suggests that chemolithoautotrophs contribute to prokaryotic production in Antarctic waters. We conclude by reviewing the effects of climate change on marine microbiota in the SO.

Gene expression changes in Epinephelus marginatus (Teleostei, Serranidae) liver reveals candidate molecular biomarker of iron ore contamination

Wastes from iron ore mining activities are potentially damaging to adjacent aquatic ecosystems. We aimed to determine biomarkers of environmental exposure to this xenobiotic in the dusky grouper Epinephelus marginatus by differential gene expression analysis. For this, fish were exposed to iron ore (15.2 mg/L) and gene expression in liver was assessed by RNA-Seq and compared to the control group. A total of 124 differentially expressed genes were identified, from which 52 were upregulated and 72 were downregulated in response to iron ore. From these, ferritin (medium subunit), cytochrome b reductase and epoxide hydrolase genes were selected for validation by RT-qPCR that confirmed the upregulation of epoxide hydrolase in fish exposed to iron ore.

Concentration, Spatial-Temporal Distribution, and Bioavailability of Dissolved Reactive Iron in Northern Coastal China Seawater

The concentrations of total dissolved iron (TdFe) and dissolved reactive iron (DrFe) in the Northern coastal China seawater (Yantai Sishili Bay) in 2018 were determined using cathodic stripping voltammetry (CSV). It was found that while the concentrations of TdFe ranged from 27.8 to 82.0 nM, DrFe concentrations changed in a much narrower range from 6.8 to 13.3 nM. The annual mean concentrations of DrFe also ranged from 7.1 to 12.6 nM at the 12 sites monitored over the 4 years of the study (2017–2020). Considering the obvious changes in temperature (T), chlorophyll a (Chl a) concentrations (Chl a contents were higher in May, July and September than in March and November), and nutrients over a year in this zone, the consumption of DrFe was expected; the supplement of DrFe observed may have resulted from the transformation of strong organically complexed iron by photoreduction and cell surface reduction. Additionally, a pre-liminary conclusion was drawn based on the theoretical calculation of Fe* that the concentration of DrFe was sufficient to meet the phytoplankton demand.

Seasonal phytoplankton and geochemical shifts in the subsurface chlorophyll maximum layer of a dimictic ferruginous lake

Subsurface chlorophyll maxima layers (SCML) are ubiquitous features of stratified aquatic systems. Availability of the micronutrient iron is known to influence marine SCML, but iron has not been explored in detail as a factor in the development of freshwater SCML. This study investigates the relationship between dissolved iron and the SCML within the dimictic, ferruginous lake Grosses Heiliges Meer in northern Germany. The occurrence of the SCML under nonferruginous conditions in the spring and ferruginous conditions in the fall are context to explore temporal changes in the phytoplankton community and indicators of primary productivity. Results indicate that despite more abundant chlorophyll in the spring, the SCML sits below a likely primary productivity maximum within the epilimnion, inferred based on colocated dissolved oxygen, δ13 CDIC , and pH maxima. The peak amount of chlorophyll in the SCML is lower in the fall than in the spring, but in the fall the SCML is colocated with elevated dissolved iron concentrations and a local δ13 CDIC maximum. Cyanobacteria and Chlorophyta have elevated abundances within the SCML in the fall. Further investigation of the relationship of iron to primary productivity within ferruginous SCML may help to understand the environmental controls on primary productivity in past ferruginous oceans.

The biological uptake of dissolved iron in the changing Daya Bay, South China Sea: Effect of pH and DO

The oceanic acidification and coastal hypoxia have potential to enhance biological uptake of dissolved iron (Fe) by phytoplankton. In this study, the Fe uptake rate (FeUR) in Daya Bay was significantly negatively correlated with pH and dissolved oxygen (DO) (r = -0.81 and -0.73, respectively, p < 0.001). In addition, binary regression (FeUR = -1.45 × pH - 0.10 × DO + 13.64) also indicated that both pH and DO played key roles in FeUR variations. As pH and DO decreased, Fe uptake by phytoplankton was promoted, and the contribution of nano-phytoplankton to Fe uptake increased significantly, while that of pico-FeUR decreased. These will result in the phytoplankton community to be miniaturized and Fe requirement of phytoplankton goes higher, thereby leading changes of phytoplankton composition and coastal ecosystem. This study helps to understand how Fe could affect the coastal ecosystem under the increasing anthropogenic influences.

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

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.

Iron and manganese play an important role in phytoplankton biomass control, but the exact effect of these elements on species composition has remained unknown. Conducting phytoplankton incubation experiments at two Drake Passage sites, we demonstrate how iron and manganese regulate phytoplankton community structure.

Australian fire nourishes ocean phytoplankton bloom

An unprecedented devastating forest fire occurred in Australia from September 2019 to March 2020. Satellite observations revealed that this rare fire event in Australia destroyed a record amount of more than 202,387 km² of forest, including 56,471 km² in eastern Australia, which is mostly composed of evergreen forest. The released aerosols contained essential nutrients for the growth of marine phytoplankton and were transported by westerly winds over the Southern Ocean, with rainfall-induced deposition to the ocean beneath. Here, we show that a prominent oceanic bloom, indicated by the rapid growth of phytoplankton, took place in the Southern Ocean along the trajectory of fire-born aerosols in response to atmospheric deposition. Calculations of carbon released during the fire versus carbon absorbed by the oceanic phytoplankton bloom suggest that they were nearly equal. This finding illustrates the critical role of the oceans in mitigating natural and anthropogenic carbon dioxide releases to the atmosphere, which are a primary driver of climate change.

Impact of Lhcx2 on Acclimation to Low Iron Conditions in the Diatom Phaeodactylum tricornutum

Iron is a cofactor of photosystems and electron carriers in the photosynthetic electron transport chain. Low concentrations of dissolved iron are, therefore, the predominant factor that limits the growth of phototrophs in large parts of the open sea like the Southern Ocean and the North Pacific, resulting in "high nutrient-low chlorophyll" (HNLC) areas. Diatoms are among the most abundant microalgae in HNLC zones. Besides efficient iron uptake mechanisms, efficient photoprotection might be one of the key traits enabling them to outcompete other algae in HNLC regions. In diatoms, Lhcx proteins play a crucial role in one of the main photoprotective mechanisms, the energy-dependent fluorescence quenching (qE). The expression of Lhcx proteins is strongly influenced by various environmental triggers. We show that Lhcx2 responds specifically and in a very sensitive manner to iron limitation in the diatom Phaeodactylum tricornutum on the same timescale as the known iron-regulated genes ISIP1 and CCHH11. By comparing Lhcx2 knockout lines with wild type cells, we reveal that a strongly increased qE under iron limitation is based on the upregulation of Lhcx2. Other observed iron acclimation phenotypes in P. tricornutum include a massively reduced chlorophyll a content/cell, a changed ratio of light harvesting and photoprotective pigments per chlorophyll a, a decreased amount of photosystem II and photosystem I cores, an increased functional photosystem II absorption cross section, and decoupled antenna complexes. H₂O₂ formation at photosystem I induced by high light is lowered in iron-limited cells, while the amount of total reactive oxygen species is rather increased. Our data indicate a possible reduction in singlet oxygen by Lhcx2-based qE, while the other iron acclimation phenotype parameters monitored are not affected by the amount of Lhcx2 and qE.

The Southern Ocean diatom Pseudo-nitzschia subcurvata flourished better under simulated glacial than interglacial ocean conditions: Combined effects of CO2 and iron

The 'Iron Hypothesis' suggests a fertilization of the Southern Ocean by increased dust deposition in glacial times. This promoted high primary productivity and contributed to lower atmospheric pCO2. In this study, the diatom Pseudo-nitzschia subcurvata, known to form prominent blooms in the Southern Ocean, was grown under simulated glacial and interglacial climatic conditions to understand how iron (Fe) availability (no Fe or Fe addition) in conjunction with different pCO2 levels (190 and 290 μatm) influences growth, particulate organic carbon (POC) production and photophysiology. Under both glacial and interglacial conditions, the diatom grew with similar rates. In comparison, glacial conditions (190 μatm pCO2 and Fe input) favored POC production by P. subcurvata while under interglacial conditions (290 μatm pCO2 and Fe deficiency) POC production was reduced, indicating a negative effect caused by higher pCO2 and low Fe availability. Under interglacial conditions, the diatom had, however, thicker silica shells. Overall, our results show that the combination of higher Fe availability with low pCO2, present during the glacial ocean, was beneficial for the diatom P. subcurvata, thus contributing more to primary production during glacial compared to interglacial times. Under the interglacial ocean conditions, on the other hand, the diatom could have contributed to higher carbon export due to its higher degree of silicification.

The Invisible Hand of the Periodic Table: How Micronutrients Shape Ecology

Beyond the better-studied carbohydrates and the macronutrients nitrogen and phosphorus, a remaining 20 or so elements are essential for life and have distinct geographical distributions, making them of keen interest to ecologists. Here, I provide a framework for understanding how shortfalls in micronutrients like iodine, copper, and zinc can regulate individual fitness, abundance, and ecosystem function. With a special focus on sodium, I show how simple experiments manipulating biogeochemistry can reveal why many of the variables that ecologists study vary so dramatically from place to place. I conclude with a discussion of how the Anthropocene's changing temperature, precipitation, and atmospheric CO2 levels are contributing to nutrient dilution (decreases in the nutrient quality at the base of food webs).

The effect of iron on Chilean Alexandrium catenella growth and paralytic shellfish toxin production as related to algal blooms

The dinoflagellate Alexandrium catenella is a well-known paralytic shellfish toxin producer that forms harmful algal blooms (HABs) worldwide. Blooms of this species have repeatedly brought severe ecological and economic impacts to Chile, especially in the southern region, where the shellfish and salmon industries are world-famous. The mechanisms of such HABs have been intensively studied but are still unclear. Nutrient overloading is one of the often-discussed drivers for HABs. The present study used the A. catenella strain isolated from southern Chile to investigate how iron conditions could affect their growth and toxin production as related to HAB. Our results showed that an optimum concentration of iron was pivotal for proper A. catenella growth. Thus, while excess iron exerted a toxic effect, low iron media led to iron insufficiency and growth inhibition. In addition, the study shows that the degree of paralytic shellfish toxin production by A. catenella varied depending on the iron concentration in the culture media. The A. catenella strain from southern Chile produced GTX1-4 exclusively in the fmol cell⁻¹ scale. Based on these findings, we suggest that including iron and paralytic shellfish toxin measurements in the fields can improve the current HAB monitoring and contribute to an understanding of A. catenella bloom dynamics in Chile.

Biogeographic traits of dimethyl sulfide and dimethylsulfoniopropionate cycling in polar oceans

BACKGROUND

Dimethyl sulfide (DMS) is the dominant volatile organic sulfur in global oceans. The predominant source of oceanic DMS is the cleavage of dimethylsulfoniopropionate (DMSP), which can be produced by marine bacteria and phytoplankton. Polar oceans, which represent about one fifth of Earth's surface, contribute significantly to the global oceanic DMS sea-air flux. However, a global overview of DMS and DMSP cycling in polar oceans is still lacking and the key genes and the microbial assemblages involved in DMSP/DMS transformation remain to be fully unveiled.

RESULTS

Here, we systematically investigated the biogeographic traits of 16 key microbial enzymes involved in DMS/DMSP cycling in 60 metagenomic samples from polar waters, together with 174 metagenome and 151 metatranscriptomes from non-polar Tara Ocean dataset. Our analyses suggest that intense DMS/DMSP cycling occurs in the polar oceans. DMSP demethylase (DmdA), DMSP lyases (DddD, DddP, and DddK), and trimethylamine monooxygenase (Tmm, which oxidizes DMS to dimethylsulfoxide) were the most prevalent bacterial genes involved in global DMS/DMSP cycling. Alphaproteobacteria (Pelagibacterales) and Gammaproteobacteria appear to play prominent roles in DMS/DMSP cycling in polar oceans. The phenomenon that multiple DMS/DMSP cycling genes co-occurred in the same bacterial genome was also observed in metagenome assembled genomes (MAGs) from polar oceans. The microbial assemblages from the polar oceans were significantly correlated with water depth rather than geographic distance, suggesting the differences of habitats between surface and deep waters rather than dispersal limitation are the key factors shaping microbial assemblages involved in DMS/DMSP cycling in polar oceans.

CONCLUSIONS

Overall, this study provides a global overview of the biogeographic traits of known bacterial genes involved in DMS/DMSP cycling from the Arctic and Antarctic oceans, laying a solid foundation for further studies of DMS/DMSP cycling in polar ocean microbiome at the enzymatic, metabolic, and processual levels. Video Abstract.

Release of trace elements during bioreductive dissolution of magnetite from metal mine tailings: Potential impact on marine environments

Adverse impacts of mine tailings on water and sediments quality are major worldwide environmental problems. Due to the environmental issues associated with the deposition of mine tailings on land, a controversial discussed alternative is submarine tailings disposal (STD). However, Fe(III) bioreduction of iron oxides (e.g., magnetite) in the tailings disposed might cause toxic effects on coastal environments due to the release of different trace elements (TEs) contained in the oxides. To study the extent and kinetics of magnetite bioreduction under marine conditions and the potential release of TEs, a number of batch experiments with artificial seawater (pH 8.2) and a marine microbial strain (Shewanella loihica) were performed using several magnetite ore samples from different mines and a mine tailings sample. The elemental composition of the magnetite determined in the tailings showed relatively high amounts of TEs (e.g., Mn, Zn, Co) compared with those of the magnetite ore samples (LA-ICP-MS and EMPA analyses). The experiments were conducted at 10 °C in the dark for up to 113 days. Based on the consumption of lactate and production of acetate and aqueous Fe(II) over time, the magnitude of Fe(III) bioreduction was calculated using a geochemical model including Monod kinetics. Model simulations reproduced the release of iron and TEs observed throughout the experiments, e.g., Mn (up to 203 μg L^-1), V (up to 79 μg L^-1), As (up to 17 μg L^-1) and Cu (up to 328 μg L^-1), suggesting a potential contamination of pore water by STD. Therefore, the results of this study can help to better evaluate the potential impacts of STD.

High Iron Requirements for Growth in the Nuisance Alga Gonyostomum semen (Raphidophyceae)

The bloom-forming freshwater alga Gonyostomum semen is associated with acidic, mesotrophic brown water lakes in boreal regions. However, researchers have been unable to conclusively link G. semen abundance and bloom formation to typical brown water lake traits, that is, high water color and DOC (dissolved organic carbon) concentrations. Iron is a main driver of water color in boreal lakes, and a recent study of lake monitoring data indicated a connection between lakes with high G. semen abundance and iron concentrations >200 µg · L^-1 . Thus, iron may be the missing link in explaining G. semen abundance and growth dynamics. We experimentally assessed the effects of different iron concentrations above or below 200 µg · L^-1 on the growth of G. semen batch monocultures. Iron concentrations <200 µg · L^-1 limited G. semen growth, while iron concentrations >200 µg · L^-1 did not. Moreover, the iron concentration of the medium required for growth was higher than for other common phytoplankton (Microcystis botrys and Chlamydomonas sp.) included in the experiment. These results indicate that G. semen requires high levels of iron in the lake environment. Consequently, this and previous findings using lake monitoring data support the hypothesis that high concentrations of iron favor the formation of high-density G. semen blooms in boreal brown water lakes. As lakes get browner in a changing climate, monitoring iron levels could be a potential tool to identify lakes at risk for G. semen blooms, especially among lakes that provide ecosystem services to society.

High latitude Southern Ocean phytoplankton have distinctive bio-optical properties

Studying the biogeochemistry of the Southern Ocean using remote sensing relies on accurate interpretation of ocean colour through bio-optical and biogeochemical relationships between quantities and properties of interest. During the Antarctic Circumnavigation Expedition of the 2016/2017 Austral Summer, we collected a spatially comprehensive dataset of phytoplankton pigment concentrations, particulate absorption and particle size distribution and compared simple bio-optical and particle property relationships as a function of chlorophyll a. Similar to previous studies we find that the chlorophyll-specific phytoplankton absorption coefficient is significantly lower than in other oceans at comparable chlorophyll concentrations. This appears to be driven in part by lower concentrations of accessory pigments per unit chlorophyll a as well as increased pigment packaging due to relatively larger sized phytoplankton at low chlorophyll a than is typically observed in other oceans. We find that the contribution of microphytoplankton (>20 µm size) to chlorophyll a estimates of phytoplankton biomass is significantly higher than expected for the given chlorophyll a concentration, especially in higher latitudes south of the Southern Antarctic Circumpolar Current Front. Phytoplankton pigments are more packaged in larger cells, which resulted in a flattening of phytoplankton spectra as measured in these samples when compared to other ocean regions with similar chlorophyll a concentration. Additionally, we find that at high latitude locations in the Southern Ocean, pheopigment concentrations can exceed mono-vinyl chlorophyll a concentrations. Finally, we observed very different relationships between particle volume and chlorophyll a concentrations in high and low latitude Southern Ocean waters, driven by differences in phytoplankton community composition and acclimation to environmental conditions and varying contribution of non-algal particles to the particulate matter. Our data confirm that, as previously suggested, the relationships between bio-optical properties and chlorophyll a in the Southern Ocean are different to other oceans. In addition, distinct bio-optical properties were evident between high and low latitude regions of the Southern Ocean basin. Here we provide a region-specific set of power law functions describing the phytoplankton absorption spectrum as a function of chlorophyll a.

Zonally asymmetric phytoplankton response to the Southern annular mode in the marginal sea of the Southern ocean

Antarctic marine biological variability modulates climate systems via the biological pump. However, the knowledge of biological response in the Southern Ocean to climate variability still has been lack of understanding owing to limited ocean color data in the high latitude region. We investigated the surface chlorophyll concentration responses to the Southern annular mode (SAM) in the marginal sea of the Southern ocean using satellite observation and reanalysis data focusing on the austral summer. The positive phase of SAM is associated with enhanced and poleward-shifted westerly winds, leading to physical and biogeochemical responses over the Southern ocean. Our result indicates that chlorophyll has strong zonally asymmetric responses to SAM owing to different limiting factors of phytoplankton growth per region. For the positive SAM phase, chlorophyll tends to increase in the western Amundsen-Ross Sea but decreases in the D'Urville Sea. It is suggested that the distinct limiting factors are associated with the seasonal variability of sea ice and upwelling per region.

Metabolic pathways inferred from a bacterial marker gene illuminate ecological changes across South Pacific frontal boundaries

Global oceanographic monitoring initiatives originally measured abiotic essential ocean variables but are currently incorporating biological and metagenomic sampling programs. There is, however, a large knowledge gap on how to infer bacterial functions, the information sought by biogeochemists, ecologists, and modelers, from the bacterial taxonomic information (produced by bacterial marker gene surveys). Here, we provide a correlative understanding of how a bacterial marker gene (16S rRNA) can be used to infer latitudinal trends for metabolic pathways in global monitoring campaigns. From a transect spanning 7000 km in the South Pacific Ocean we infer ten metabolic pathways from 16S rRNA gene sequences and 11 corresponding metagenome samples, which relate to metabolic processes of primary productivity, temperature-regulated thermodynamic effects, coping strategies for nutrient limitation, energy metabolism, and organic matter degradation. This study demonstrates that low-cost, high-throughput bacterial marker gene data, can be used to infer shifts in the metabolic strategies at the community scale.

Monthly Variation in the Macromolecular Composition of Phytoplankton Communities at Jang Bogo Station, Terra Nova Bay, Ross Sea

Organic carbon fixed by photosynthesis of phytoplankton during the polar growing period could be important for their survival and consumers during the long polar night. Differences in biochemical traits of phytoplankton between ice-free and polar night periods were investigated in biweekly water samples obtained at the Korean “Jang Bogo Station” located in Terra Nova Bay, Antarctica. The average concentration of total Chl-a from phytoplankton dominated by micro-sized species from the entire sampling period was 0.32 μg L^–1 (SD = ± 0.88 μg L^–1), with the highest concentration of 4.29 μg L^–1 in February and the lowest concentration of 0.01 μg L^–1 during the ice-covered polar night (April–October) in 2015. The highest protein concentration coincided with the peak Chl-a concentration in February and decreased rapidly relative to the carbohydrate and lipid concentrations in the early part of polar night. Among the different biochemical components, carbohydrates were the predominant constituent, accounting for 69% (SD = ± 14%) of the total particulate organic matter (POM) during the entire study period. The carbohydrate contributions to the total POM markedly increased from 39 ± 8% during the ice-free period to 73 ± 9% during the polar night period. In comparison, while we found a significant negative correlation (r² = 0.92, p < 0.01) between protein contributions and carbohydrate contributions, lipid contributions did not show any particular trend with relatively small temporal variations during the entire observation period. The substantial decrease in the average weight ratio of proteins to carbohydrates from the ice-free period (mean ± SD = 1.0 ± 0.3) to the ice-covered period (mean ± SD = 0.1 ± 0.1) indicates a preferential loss of nitrogen-based proteins compared to carbohydrates during the polar night period. Overall, the average food material (FM) concentration and calorific contents of FM in this study were within the range reported previously from the Southern Ocean. The results from this study may serve as important background data for long-term monitoring of the regional and interannual variations in the physiological state and biochemical compositions of phytoplankton resulting from future climate change in Antarctica.

Weathering Intensity and Presence of Vegetation Are Key Controls on Soil Phosphorus Concentrations: Implications for Past and Future Terrestrial Ecosystems

Phosphorus (P) is an essential limiting nutrient in marine and terrestrial ecosystems. Understanding the natural and anthropogenic influence on P concentration in soils is critical for predicting how its distribution in soils may shift as climate changes. While it is known that P is sourced from bedrock weathering, relationships between weathering, P, and other soil-forming factors have not been quantified at continental scales, limiting our ability to predict large-scale changes in P concentrations. Additionally, while we know that Fe oxide-associated P is an important P phase in terrestrial environments, the range in and controls on soil Fe concentrations and species (e.g., Fe in oxides, labile Fe) are poorly constrained. Here, we explore the relationships between soil P and Fe concentrations, soil order, climate, and vegetation in over 5000 soils, and Fe speciation in ca. 400 soils. Weathering intensity has a nuanced control on P concentrations in soils, with P concentrations peaking at intermediate weathering intensities (Chemical Index of Alteration, CIA~60). The presence of vegetation (but not plant functional types) affected soils’ ability to accumulate P. Contrary to expectations, P was not more strongly associated with Fe in oxides than other Fe phases. These results are useful both for predicting changes in potential P fluxes from soils to rivers under climate change and for reconstructing changes in terrestrial nutrient limitations in Earth’s past. In particular, soils’ tendency to accumulate more P with the presence of vegetation suggests that biogeochemical models invoking the evolution and spread of land plants as a driver for increased P fluxes in the geological record may need to be revisited.

Exopolymeric Substances Control Microbial Community Structure and Function by Contributing to both C and Fe Nutrition in Fe-Limited Southern Ocean Provinces

Organic ligands such as exopolymeric substances (EPS) are known to form complexes with iron (Fe) and modulate phytoplankton growth. However, the effect of organic ligands on bacterial and viral communities remains largely unknown. Here, we assessed how Fe associated with organic ligands influences phytoplankton, microbial, and viral abundances and their diversity in the Southern Ocean. While the particulate organic carbon (POC) was modulated by Fe chemistry and bioavailability in the Drake Passage, the abundance and diversity of microbes and viruses were not governed by Fe bioavailability. Only following amendments with bacterial EPS did bacterial abundances increase, while phenotypic alpha diversity of bacterial and viral communities decreased. The latter was accompanied by significantly enhanced POC, pointing toward the relief of C limitation or other drivers of the microbial loop. Based on the literature and our findings, we propose a conceptual framework by which EPS may affect phytoplankton, bacteria, and viruses. Given the importance of the Southern Ocean for Earth's climate as well as the prevalence of viruses and their increasingly recognized impact on marine biogeochemistry and C cycling; the role of microbe-virus interactions on primary productivity in the Southern Ocean needs urgent attention.

A case study of Chlorophyll a response to tropical cyclone Wind Pump considering Kuroshio invasion and air-sea heat exchange

New evidences provided that the tropical cyclone (TC) Linfa in 2015 induced looping path of Kuroshio invasion into the northeastern South China Sea (NESCS) through the northwestern Luzon Strait (LS), based on the in-situ measurements, satellite data and model output data. This TC-enhanced Kuroshio invasion with low nutrients and denser waters suppressed the TC "Wind Pump" induced upwelling and nutrients uptake, and therefore inhibited the Chlorophyll a concentration (Chl-a) increase from surface to ~50 m in the open ocean of the NESCS. The TC-induced Kuroshio invasion promoted the generation of the strong cyclonic eddy to its left side where weak Ekman Pumping Velocity was observed. This enhancing cyclonic eddy then dominated the nutrients uplift and increased the surface and subsurface (0-50 m) Chl-a through eddy pumping rather than Ekman Pumping. The TC-declined anti-cyclonic eddy, which shoaled the Mixed Layer Depth (MLD), benefited to the nutrient uptake through TC-induced upwelling and thereby increased the surface Chl-a and raised the Chl-a Maximum Layer (CML) to ~20 m over the southwestern LS. The temporal Chl-a variations were also influenced by TC intensities and biochemical processes. The air-sea heat budget analysis indicated that, the air-sea heat exchange contributed to nearly 80% of the sea surface cooling (SST cooling) over the northwestern LS with Kuroshio invasion, while eddy-induced upwelling dominated the SST cooling over the western LS, and the wind-driven upwelling (and mixing) controlled the SST cooling over the southwestern LS. These different formations of SST cooling then played important role in Chl-a variations. This study is the first case of TC "Wind Pump" induced Chl-a variations considering air-sea heat exchange, Kuroshio invasion and mesoscale eddies over LS, which would help to evaluate the influence of TCs over the other major heat transport arteries of the world ocean: The Gulf Stream area.

Plastic, nutrition and pollution; relationships between ingested plastic and metal concentrations in the livers of two Pachyptila seabirds

Naturally occurring metals and metalloids [metal(loid)s] are essential for the physiological functioning of wildlife; however, environmental contamination by metal(loid) and plastic pollutants is a health hazard. Metal(loid)s may interact with plastic in the environment and there is mixed evidence about whether plastic ingested by wildlife affects metal(loid) absorption/assimilation and concentration in the body. We examined ingested plastic and liver concentration of eleven metal(loid)s in two seabird species: fairy (Pachyptila turtur) and slender-billed prions (P. belcheri). We found significant relationships between ingested plastic and the concentrations of aluminium (Al), manganese (Mn), iron (Fe), cobalt (Co), copper (Cu) and zinc (Zn) in the liver of prions. We investigated whether the pattern of significant relationships reflected plastic-metal(loid) associations predicted in the scientific literature, including by transfer of metals from ingested plastics or malnutrition due to dietary dilution by plastics in the gut. We found some support for both associations, suggesting that ingested plastic may be connected with dietary dilution / lack of essential nutrients, especially iron, and potential transfer of zinc. We did not find a relationship between plastic and non-essential metal(loid)s, including lead. The effect of plastic was minor compared to that of dietary exposure to metal(oid)s, and small plastic loads (< 3 items) had no discernible link with metal(loid)s. This new evidence shows a relationship between plastic ingestion and liver metal(loid) concentrations in free-living wildlife.

Dynamics of phytoplankton and nutrient uptake following dust additions in the northwest Pacific

Dust deposition can supply nutrients that affect marine phytoplankton, but changing trophic statuses of the surface ocean increase the complexity of interpreting the process. In this study, four onboard incubation experiments amended with various nutrients and dust were performed in the Kuroshio Extension (KE) and Kuroshio-Oyashio transition (TR) of the northwest Pacific (NWP), which are characterised by lower and higher trophic statuses, respectively. According to the nutrient-addition experiments, phytoplankton were limited by nitrogen (N) in the KE, and limited by iron (Fe) or co-limited by Fe and phosphorus (P) in the TR. Dust additions supplied a considerable amount of N and Fe but negligible amount of P to stimulate phytoplankton growth, as indicated by chlorophyll a (Chl a) concentration. In the KE incubations, dust additions enhanced the shift of phytoplankton size structure towards larger cells from dominantly pico-sized (0.2-2 μm) Chl a to comparable contributions from each size class (i.e. pico-, nano-: 2-20 μm, micro-: >20 μm). On the basis of the large shift of size structure towards nano- or micro-phytoplankton in the unamended control treatments in the TR, dust additions furtherly promoted the shift towards micro-phytoplankton becoming the dominant contributor to the total Chl a. The collective analysis of the data from experiments in both regions revealed that, the extent of phytoplankton growth stimulation and the shift towards larger cells were enhanced gradually with increasing amounts of nutrient uptake (including N, P, and silicon). The nutrient uptake ratios of phytoplankton converged towards the Redfield ratio in comparison to the wider range of nutrient ratios in the dust-amended seawater. This study suggested consistencies in the dynamic of phytoplankton growth, shift of size structure, and nutrient uptake following dust additions in the KE and TR, although the trophic status and limiting nutrient varied between these two regions.

Influence of pH on the Kinetics and Mechanism of Photoreductive Dissolution of Amorphous Iron Oxyhydroxide in the Presence of Natural Organic Matter: Implications to Iron Bioavailability in Surface Waters

In this study, we investigate the influence of pH on the kinetics and mechanism of photoreductive dissolution of amorphous iron oxyhydroxide (AFO) in view of the recognition that the light-mediated dissolution of iron oxides controls Fe availability in many natural waters. Our results show that both ligand-to-metal charge transfer (LMCT) and photogenerated superoxide (O₂^•-) play an important role in AFO photoreductive dissolution in the presence of the chosen surrogate of natural organic matter, Suwannee river fulvic acid (SRFA). The pH dependence of LMCT-mediated AFO photoreductive dissolution is mainly controlled by the influence of pH on AFO solubility. A decrease in pH increases the concentration of the dissolved and more photolabile Fe(III)-SRFA complex present in equilibrium with AFO, a complex in which Fe(III) is readily reduced by LMCT. The pH dependence of superoxide-mediated Fe(III) reduction (SMIR) is also controlled by the influence of pH on AFO solubility with an increase in the dissolved inorganic Fe(III) concentration with the decrease in pH resulting in an increased rate of SMIR. No influence of pH was observed on the steady-state O₂^•- concentration generated on SRFA irradiation as well as the O₂^•- decay rate in the presence of SRFA, suggesting that the concentration and lifetime of O₂^•- are not important factors in controlling the pH dependence of O₂^•--mediated AFO dissolution. Overall, the results of this study show that the impact of acidification of natural waters on Fe availability will be much more pronounced when Fe is present as iron oxyhydroxide compared to that observed when organically bound Fe dominates with this effect because of the strong dependency of iron oxyhydroxide solubility on pH. The increased rate and extent of dissolution of iron oxyhydroxides on the acidification of natural waters will also have implications to the fate of other contaminants (such as heavy metals and organic compounds) that may be present on the iron oxyhydroxide surface.

Responses of Two Coastal Algae (Skeletonema costatum and Chlorella vulgaris) to Changes in Light and Iron Levels1

Iron (Fe) is essential for phytoplankton growth and photosynthesis, and is proposed to be an important factor regulating algal blooms under replete major nutrients in coastal environments. Here, Skeletonema costatum, a typical red-tide diatom species, and Chlorella vulgaris, a widely distributed Chlorella, were chosen to examine carbon fixation and Fe uptake by coastal algae under dark and light conditions with different Fe levels. The cellular carbon fixation and intracellular Fe uptake were measured via ¹⁴ C and ⁵⁵ Fe tracer assay, respectively. Cell growth, cell size, and chlorophyll-α concentration were measured to investigate the algal physiological variation in different treatments. Our results showed that cellular Fe uptake proceeds under dark and the uptake rates were comparable to or even higher than those in the light for both algal species. Fe requirements per unit carbon fixation were also higher in the dark resulting in higher Fe: C ratios. During the experimental period, high Fe addition significantly enhanced cellular carbon fixation and Fe uptake. Compared to C. vulgaris, S. costatum was the common dominant bloom species because of its lower Fe demand but higher Fe uptake rate. This study provides some of the first measurements of Fe quotas in coastal phytoplankton cells, and implies that light and Fe concentrations may influence the phytoplankton community succession when blooms occur in coastal ecosystems.