Oligotrophic Open Ocean Research Articles

Emergence of metabolic coupling to the heterotroph Alteromonas promotes dark survival in Prochlorococcus

Abstract Prochlorococcus is found throughout the euphotic zone in the oligotrophic open ocean. Deep mixing and sinking while attached to particles can, however, transport Prochlorococcus cells below this sunlit zone, depriving them of light for extended periods of time. Previous work has shown that Prochlorococcus by itself cannot survive extended periods of darkness. However, when co-cultured with a heterotrophic microbe and subjected to repeated periods of extended darkness, Prochlorococcus cells develop an epigenetically inherited dark-tolerant phenotype that can survive longer periods of darkness. Here we examine the metabolic and physiological changes underlying this adaptation using co-cultures of dark-tolerant and parental strains of Prochlorococcus, each grown with the heterotroph Alteromonas under diel light:dark conditions. The relative abundance of Alteromonas was higher in dark-tolerant than parental co-cultures, while dark-tolerant Prochlorococcus cells were larger, contained less chlorophyll, and were less synchronized to the light:dark cycle. Meta-transcriptome analysis revealed that dark-tolerant co-cultures undergo a joint change, in which Prochlorococcus undergoes a relative shift from photosynthesis to respiration, while Alteromonas shifts towards using more organic acids instead of sugars. Furthermore, the transcriptome data suggested enhanced biosynthesis of amino acids and purines in dark-tolerant Prochlorococcus and enhanced degradation of these compounds in Alteromonas. Collectively, our results demonstrate that dark adaptation involves a strengthening of the metabolic coupling between Prochlorococcus and Alteromonas, presumably mediated by an enhanced, and compositionally modified, carbon exchange between the two species.

Significance of Urea in Sustaining Nitrite Production by Ammonia Oxidizers in the Oligotrophic Ocean

AbstractNitrification, the stepwise oxidation of ammonia to nitrate via nitrite, is a key process in the marine nitrogen cycle. Reported nitrite oxidation rates frequently exceed ammonia oxidation rates below the euphotic zone, raising the fundamental question of whether the two steps are balanced and if alternative sources contribute to nitrite production in the dark ocean. Here we present vertically resolved profiles of ammonia, urea, and nitrite oxidation rates and their kinetic traits in the oligotrophic Subtropical North Pacific. Our results show active urea‐derived nitrogen oxidation (urea‐N oxidation) in the presence of experimental ammonium amendment, suggesting direct urea utilization. The depth‐integrated rates of urea‐N oxidation and ammonia oxidation are comparable, demonstrating that urea‐N oxidation is a significant source of nitrite. The additional nitrite from urea‐N oxidation helps to balance the two steps of nitrification in our study region. Nitrifiers exhibit high affinity for their substrates, and the apparent half‐saturation constants for ammonia and nitrite oxidation decrease with depth. The apparent half‐saturation constant for urea‐N oxidation is higher than that for ammonia oxidation and shows no clear vertical trend. Such kinetic traits may account for the relatively higher urea concentration than ammonium concentration in the ocean's interior. Moreover, a compilation of our results and reported data shows a trend of increased urea‐N oxidation relative to ammonia oxidation from the eutrophic coastal zone to the oligotrophic open ocean. This trend reveals a substrate‐dependent biogeographic distribution of urea‐N oxidation across marine environments and provides new information on the balance and flux of the marine nitrification process.

Quantifying the biogenic silica production of picoplankton in the oligotrophic ocean: A case study in the South China Sea

Silicon (Si) utilization is not limited to eukaryotes. Recent research has suggested that the pattern of a large contribution of picocyanobacteria to biogenic silica (bSi) stocks might be widespread in the oligotrophic open ocean. We are the first to measure the size-fractionated bSi standing stocks and production rates in the oligotrophic South China Sea (SCS), which has obvious characteristics of oligotrophic waters. The 150 m integrated bSi standing stocks in the pico-sized fractions averaged 23 % of the total; the contribution of picoplankton to the total bSi production rate was 44 %. Interestingly, our estimated contributions of Synechococcus alone to the <2 μm bSi standing stock and < 2 μm bSi production rates averaged 14 % and 66 %, respectively, indicating that the significant and persistent contribution of bSi was strongly associated with marine picocyanobacteria. Furthermore, the dynamic changes in nutrient concentrations, especially in DIN and DIP, also potentially affected the variability in picoplankton bSi stocks and production rates, while the effects of temperature and salinity were not obvious. In this study, we have provided new information on measurable bSi in the picoplankton size fraction and its production rate in the SCS. We have demonstrated that picoplankton contributes a measurable, and at times significant, proportion to both the total bSi standing stock and its production rate in the SCS. A high silicon content within picocyanobacteria has important implications for understanding both their ecology and their contribution to biogeochemistry.

Phagotrophy in the nitrogen-fixing haptophyte Braarudosphaera bigelowii.

Biological nitrogen fixation provides fixed nitrogen for microbes living in the oligotrophic open ocean. UCYN-A2, the previously known symbiont of Braarudosphaera bigelowii, now believed to be an early-stage B. bigelowii organelle that exchanges fixed nitrogen for fixed carbon, is globally distributed. Indirect evidence suggested that B. bigelowii might be a mixotrophic (phagotrophic) phototrophic flagellate. The goal of this study was to determine if B. bigelowii can graze on bacteria using several independent approaches. The results showed that B. bigelowii grazed on co-occurring bacteria at a rate of 5-7 cells/h/B. bigelowii and that the overall grazing rate was significantly higher at nighttime than at daytime. Bacterial abundance changes, assessed with 16S rRNA gene amplicon sequencing analysis, may have indicated preferential grazing by B. bigelowii on specific bacterial genotypes. In addition, Lysotracker™ staining of B. bigelowii suggested digestive activity inside B. bigelowii. Carbon and nitrogen fixation measurements revealed that the carbon demand of B. bigelowii could not be fulfilled by photosynthesis alone, implying supplementation by heterotrophy. These independent lines of evidence together revealed that B. bigelowii engages in phagotrophy, which, beyond serving as a supplementary source of carbon and energy, may also facilitate the indirect assimilation of inorganic nutrients.

Climate change in the Canary/Iberia upwelling region: the role of ocean stratification and wind

The Canary/Iberia region (CIR), part of the Canary Current Upwelling System, is well-known for its coastal productivity and crucial role in enriching the oligotrophic open ocean through the offshore transport of the upwelled coastal waters. Given its significant ecological and socio-economic importance, it is essential to assess the impact of climate change on this area. Therefore, the goal of this study is to analyze the climate change signal over the CIR using a high-resolution regional climate system model driven by the Earth system model MPI-ESM-LR under RCP8.5 scenario. This modelling system presents a regional atmosphere model coupled to a global ocean model with enough horizontal resolution at CIR to examine the role of the upwelling favourable winds and the ocean stratification as key factors in the future changes. CIR exhibits significant latitudinal and seasonal variability in response to climate change under RCP8.5 scenario, where ocean stratification and wind patterns will play both complementary and competitive roles. Ocean stratification will increase from the Strait of Gibraltar to Cape Juby by the end of the century, weakening the coastal upwelling all year long. This increase in stratification is associated with a freshening of the surface layers of the North Atlantic. However, modifications in the wind pattern will play a primary role in upwelling source water depth changes in the southernmost region of the CIR in winter and in the north of the Iberian Peninsula in summer. Wind pattern changes are related to the intensification of the Azores High in winter and to a deepening of the Iberian thermal low in summer months.

Growth and mortality of aerobic anoxygenic phototrophs in the North Pacific Subtropical Gyre.

Aerobic anoxygenic phototrophic (AAP) bacteria harvest light energy using bacteriochlorophyll-containing reaction centers to supplement their mostly heterotrophic metabolism. While their abundance and growth have been intensively studied in coastal environments, much less is known about their activity in oligotrophic open ocean regions. Therefore, we combined in situ sampling in the North Pacific Subtropical Gyre, north of O'ahu island, Hawaii, with two manipulation experiments. Infra-red epifluorescence microscopy documented that AAP bacteria represented approximately 2% of total bacteria in the euphotic zone with the maximum abundance in the upper 50 m. They conducted active photosynthetic electron transport with maximum rates up to 50 electrons per reaction center per second. The in situ decline of bacteriochlorophyll concentration over the daylight period, an estimate of loss rates due to predation, indicated that the AAP bacteria in the upper 50 m of the water column turned over at rates of 0.75-0.90 d-1. This corresponded well with the specific growth rate determined in dilution experiments where AAP bacteria grew at a rate 1.05 ± 0.09 d-1. An amendment of inorganic nitrogen to obtain N:P = 32 resulted in a more than 10 times increase in AAP abundance over 6 days. The presented data document that AAP bacteria are an active part of the bacterioplankton community in the oligotrophic North Pacific Subtropical Gyre and that their growth was mostly controlled by nitrogen availability and grazing pressure.IMPORTANCEMarine bacteria represent a complex assembly of species with different physiology, metabolism, and substrate preferences. We focus on a specific functional group of marine bacteria called aerobic anoxygenic phototrophs. These photoheterotrophic organisms require organic carbon substrates for growth, but they can also supplement their metabolic needs with light energy captured by bacteriochlorophyll. These bacteria have been intensively studied in coastal regions, but rather less is known about their distribution, growth, and mortality in the oligotrophic open ocean. Therefore, we conducted a suite of measurements in the North Pacific Subtropical Gyre to determine the distribution of these organisms in the water column and their growth and mortality rates. A nutrient amendment experiment showed that aerobic anoxygenic phototrophs were limited by inorganic nitrogen. Despite this, they grew more rapidly than average heterotrophic bacteria, but their growth was balanced by intense grazing pressure.

Planktonic carbon metabolism of an underwater coral atoll in the oligotrophic sea: a case study of Zhongsha Atoll, Central South China Sea

Coral atolls are widely recognized as oases of remarkable biodiversity and productivity within the oligotrophic open ocean. However, considerable debate surrounds the net carbon metabolism of planktonic communities in oceanic coral atolls and their contribution to the overall carbon budget of the ocean. Zhongsha Atoll, situated in the central South China Sea, represents one of the largest submerged reef atolls globally. Despite its significance, the ecological environment and characteristics of community production and metabolism at Zhongsha Atoll have been scarcely studied, and it remains uncertain whether these features differ from those observed in barrier-type reefs. In this study, we examined the gross primary production (GPP), community respiration (CR), and net community production (NCP) of the planktonic community in Zhongsha Atoll and its surrounding waters from 22 June to 6 July 2020. We also analyzed the potential influences of their distribution patterns. Our findings revealed that CR did not vary significantly with depth and it was considerably higher than GPP. As a result, the waters at the euphotic depth of Zhongsha Atoll were found to be heterotrophic, with negative NCP. Additionally, the correlation analysis demonstrated a negative relationship between NCP and CR. The NCP values were -35.28 ± 26.73 and -53.18 ± 31.77 mg C m-3 d-1 for the surface and chlorophyll maximum (DCM) layers, respectively. The NCP for the upper water column was -3023 mg C m-2 d-1. The waters above the reef flat (FL) exhibited higher primary productivity, with GPP in the surface and DCM layer being 1.61 and 2.71 times greater, respectively, than in the surrounding oceanic regions. However, the FL displayed a greater level of heterotrophy due to its stronger CR. In conclusion, the carbon metabolism of the planktonic community in submerged Zhongsha Atoll acts as a source of atmospheric CO2, and the distribution of coral reefs considerably contributes to the efficiency of carbon cycling within the atoll.

Dinoflagellate cyst and pollen assemblages as tracers for marine productivity and river input in the northern Gulf of Mexico

Abstract. Both marine dinoflagellate cysts and terrestrially derived pollen and spores are abundant in coastal sediments close to river mouths, making sediment records from such settings ideal to simultaneously study land–ocean climate interactions, marine productivity patterns and freshwater input over time. However, few studies consider the combined calibration of these palynological proxies in modern coastal sediments offshore from rivers, which is needed to strengthen the interpretation of paleoreconstructions. Here, we analyze the palynological content of marine surface sediments along land–sea transects off the Mississippi and Atchafalaya river mouths in the northern Gulf of Mexico (GoM) and test three palynological indices which are often employed in the paleo-domain: (i) the abundance of cysts of heterotrophic and autotrophic dinoflagellates (dinocysts) as a tracer for primary productivity (H/A ratio) and (ii) the ratio between non-bisaccate pollen and bisaccates (P/B) as well as (iii) the ratio between pollen (excluding bisaccates) and dinocysts (P/D), which are both tracers for river input and distance to the coast. Our results show that dinoflagellate cysts are most abundant on the shelf, where heterotrophic dinocyst species dominate coastal assemblages in reach of the river plume, while autotrophic taxa are more present in the oligotrophic open ocean. This is clearly reflected in decreasing H/A values further offshore. Individual dinocyst taxa also seem to inhabit specific niches along an onshore–offshore transect, linked to nutrient availability and proximity to the turbid river plume. The highest pollen concentrations are found close to the Mississippi river mouth and mostly represent a mixture of local coastal and upstream vegetation, whereas bisaccate pollen was most abundant further offshore of the Mississippi river. Multivariate redundancy analysis (RDA) performed on both pollen and dinocyst assemblages, a set of environmental parameters, and the three palynological ratios showed that net primary productivity was the most important variable influencing the dinocyst assemblages, likely as the result of nutrient input. Additionally, the RDA confirmed that the H/A ratio indeed seems to track primary productivity, while the P/B ratio results in a robust indicator for distance to the coast, and the P/D ratio better reflects river input. Together, our data confirm and further specify the suitability of these three palynological ratios in river-dominated coastal margins as proxies for (past) marine productivity and distance to the coast and river.

Relative importance of bacterivorous mixotrophs in an estuary‐coast environment

AbstractMixotrophic eukaryotes are important bacterivores in oligotrophic open oceans, but their significance as grazers in more nutrient‐rich waters is less clear. Here, we investigated the bacterivory partition between mixotrophs and heterotrophs in a productive, estuary‐influenced coastal region in the East China Sea. We found ubiquitous, actively feeding phytoplankton populations and taxa with mixotrophic potential by identifying ingestion of fluorescent prey surrogate and analyzing community 18S rRNA gene amplicons. Potential and active mixotrophs accounted for 10–63% of the total eukaryotic community and 17–69% of bacterivores observed, respectively, contributing 6–48% of estimated in situ bacterivory. The much higher mixotroph fitness outside of the turbid plume was potentially driven by increased light and decreased nutrient availability. Our results suggest that, although heterotrophs dominated overall in situ bacterivory, mixotrophs were abundant and important bacterivores in this low‐latitude mesotrophic coastal region.

Identification of hidden N4-like viruses and their interactions with hosts.

The N4-like viruses, which were recently assigned to the novel viral family Schitoviridae in 2021, belong to a podoviral-like viral lineage and possess conserved genomic characteristics and a unique replication mechanism. Despite their significance, our understanding of N4-like viruses is primarily based on viral isolates. To address this knowledge gap, this study has established a comprehensive N4-like viral data sets comprising 342 high-quality N4-like viruses/proviruses (144 viral isolates, 158 uncultured viruses, and 40 integrated N4-like proviruses). These viruses were classified into 97 subfamilies (89 of which are newly identified), 148 genera (100 of which are newly identified), and 253 species (177 of which are newly identified). The study reveals that N4-like viruses inhibit the polar region, oligotrophic open oceans, and the human gut, where they infect various bacterial lineages, such as Alpha/Beta/Gamma/Epsilon-proteobacteria in the Proteobacteria phylum. Although N4-like viral endogenization appears to be prevalent in Proteobacteria, it has also been observed in Firmicutes. Additionally, the phylogenetic analysis has identified evolutionary divergence within the hallmark genes of N4-like viruses, indicating a complex origin of the different conserved parts of viral genomes. Moreover, 1,101 putative auxiliary metabolic genes (AMGs) were identified in the N4-like viral pan-proteome, which mainly participate in nucleotide and cofactor/vitamin metabolisms. Of these AMGs, 27 were found to be associated with virulence, suggesting their potential involvement in the spread of bacterial pathogenicity. IMPORTANCE The findings of this study are significant, as N4-like viruses represent a unique viral lineage with a distinct replication mechanism and a conserved core genome. This work has resulted in a comprehensive global map of the entire N4-like viral lineage, including information on their distribution in different biomes, evolutionary divergence, genomic diversity, and the potential for viral-mediated host metabolic reprogramming. As such, this work significantly contributes to our understanding of the ecological function and viral-host interactions of bacteriophages.

Seasonality of coastal upwelling trends in the Mauritania-Senegalese region under RCP8.5 climate change scenario

The Mauritania-Senegalese upwelling region (MSUR), the southernmost region of the Canary current upwelling system, is well-known for its coastal productivity and the key role it plays in enriching the oligotrophic open ocean through the offshore transport of the upwelled coastal waters. The great ecological and socio-economic importance makes it necessary to evaluate the impact of climate change on this region. Hence, our main objective is to examine the climate change signal over the MSUR with a high resolution regional climate system model (RCSM) forced by the Earth system model MPI-ESM-LR under RCP8.5 scenario. This RCSM has a regional atmosphere model (REMO) coupled to a global ocean model (MPIOM) with high-resolution in the MSUR, which allows us to evaluate the wind pattern, the ocean stratification, as well as the upwelling source water depth, while maintaining an ocean global domain. Under RCP8.5 scenario, our results show that the upwelling favourable winds of the northern MSUR are year-round intensified, while the southern MSUR presents a strengthening in winter and a weakening in March–April. Along with changes in the wind pattern, we found increased ocean stratification in the spring months. In those months southern MSUR presents a shallowing of the upwelling source water depth associated to changes in both mechanisms. However, in winter the whole MSUR shows a deepening of the upwelling source water depth due to the intensification of the upwelling favourable winds, with the increased ocean stratification playing a secondary role. Our results demonstrate the need to evaluate the future evolution of coastal upwelling systems taking into account their latitudinal and seasonal variability and the joint contribution of both mechanisms.

Particle-associated bacteria differentially influence the aggregation of the marine diatom Minutocellus polymorphus

The aggregation of phytoplankton leads to the settling of particulate organic carbon in the form of marine snow, making it an important process in marine biogeochemical cycles. Diatoms >20 µm in size are considered to contribute appreciably to sinking particle fluxes due to aggregation and the production of transparent exopolymeric particles (TEP), the matrix for marine snow aggregates; however, it is not known whether nano-sized (2–20 µm) diatoms are able to aggregate and produce TEP. Here, we tested the aggregation and production of TEP by the nano-diatom Minutocellus polymorphus and investigated if interactions with bacteria influence aggregation by comparing axenic M. polymorphus cultures with co-cultures of the diatom with bacterial taxa known to colonize marine snow particles. We found that M. polymorphus form sinking aggregates and produce TEP comparably to other phytoplankton groups and that aggregation and TEP production were influenced depending on the species of bacteria added. Aggregation was enhanced in the presence of Marinobacter adhaerens HP15, but not in the presence of Pseudoalteromonas carrageenovora or Vibrio thalassae. Cell aggregation mediated by interactions with specific bacterial species are possible mechanisms behind the export of nano-sized diatoms, such as M. polymorphus, especially in oligotrophic open ocean regions where small phytoplankton dominate.

Underestimation of biogenic silica sinking flux due to dissolution in sediment traps: A case study in the South China Sea

The dissolved silicate concentrations in the supernatant of sediment trap sampling bottles retrieved from deep water (1000-3000 m) at four stations in the South China Sea (SCS) were measured to calculate the underestimated flux of biogenic silica (bSi) in sinking particles due to bSi dissolution. High dissolved silicate concentrations in the supernatant, ranging from 122.6 to 1365.8 μmol/L, indicated significant dissolution of particulate bSi in the sampling bottles. Underestimation of the bSi flux in the SCS by ~2% to ~34% (average: ~10%) due to bSi dissolution was revealed, and the degree of underestimation increased with decreasing total bSi flux. The amount of bSi dissolved within the sampling series at each station was generally positively correlated with dissolution time and to a certain extent influenced by the bSi amount collected by the sampling bottles under a low bSi sinking flux. Apparently stronger bSi dissolution was found at two of the four stations due to relatively high bSi dissolution rates, which were possibly related to a higher reactive surface area of the bSi or bacterial activity. Overall, our results demonstrated that the considerable bSi dissolution in sediment trap sampling bottles should not be ignored, especially in the low-productivity oligotrophic ocean, and bSi flux calibration via measurement of the dissolved silicate in sampling bottles is necessary. To reduce bSi flux underestimation due to the deployment of time-series sediment traps, larger-volume sampling bottles should be avoided in the oligotrophic open ocean, and sinking particle samples should be analyzed as soon as the sediment traps are recovered.

Diverse Genomic Traits Differentiate Sinking-Particle-Associated versus Free-Living Microbes throughout the Oligotrophic Open Ocean Water Column.

ABSTRACTBacteria and archaea are central to the production, consumption, and remineralization of dissolved and particulate organic matter and contribute critically to carbon delivery, nutrient availability, and energy transformations in the deep ocean. To explore environmentally relevant genomic traits of sinking-particle-associated versus free-living microbes, we compared habitat-specific metagenome-assembled genomes recovered throughout the water column in the North Pacific Subtropical Gyre. The genomic traits of sinking-particle-associated versus free-living prokaryotes were compositionally, functionally, and phylogenetically distinct. Substrate-specific transporters and extracellular peptidases and carbohydrate-active enzymes were more enriched and diverse in particle-associated microbes at all depths than in free-living counterparts. These data indicate specific roles for particle-attached microbes in particle substrate hydrolysis, uptake, and remineralization. Shallow-water particle-associated microbes had elevated genomic GC content and proteome nitrogen content and reduced proteome carbon content in comparison to abyssal particle-associated microbes. An inverse trend was observed for their sympatric free-living counterparts. These different properties of attached microbes are postulated to arise in part due to elevated organic and inorganic nitrogen availability inside sinking particles. Particle-attached microbes also were enriched in genes for environmental sensing via two-component regulatory systems, and cell-cell interactions via extracellular secretion systems, reflecting their surface-adapted lifestyles. Finally, particle-attached bacteria had greater predicted maximal growth efficiencies than free-living bacterioplankton at all depths. All of these particle-associated specific genomic and proteomic features appear to be driven by microhabitat-specific elevated nutrient and energy availability as well as surface-associated competitive and synergistic ecological interactions. Although some of these characteristics have been previously postulated or observed individually, we report them together here in aggregate via direct comparisons of cooccurring free-living and sinking-particle-attached microbial genomes from the open ocean.

Stratification governs the plankton community structure and trophic interaction in the Southwestern tropical Indian Ocean during boreal summer

Seychelles–Chagosthermocline ridge is one of the major upwelling area in the tropical Indian Ocean, where planktons are known to have large impacts on ocean ecosystem and biogeochemical cycles. Time series assessments of surface zooplankton abundance, biovolume, and community composition were carried out in the Indian Ocean during June 2014. In this paper, we explore the impact of phytoplankton size class biomass on trophic interaction in the thermocline ridge of the southwestern tropical Indian Ocean. Relatively, nitrate was a dominant contributor than the nitrite in the dissolved inorganic nitrogen pool. Size fractionated chlorophyll-a concentration clearly showed that the study area was a typical oligotrophic open ocean, in which picophytoplankton biomass was dominated, accounted for approximately 72% of total Chl a. We assumed that picophytoplankton biomass was most likely influenced by dissolved inorganic nutrients (chiefly nitrite), that showed a strong linear relationship with picophytoplankton biomass (r2=0.41). These results support the key role of nitrite is not only supplying and promoting the growth of smaller phytoplankton biomass, but also controls the structure of zooplankton communities. As a result surface waters were dominated by picophytoplankton biomass, in this condition result in zooplankton being dominated by Poecilostomatoida, constituting approximately 68% of the total zooplankton count. These organisms were widely distributed and occurred in large numbers and displayed a strong linear relationship with picophytoplankton biomass (r2=0.87). These empirical results suggest that abundance of poecilostomatoids presumably supports tertiary trophic levels, suggesting they might play a key role as carbon drivers in the vicinity of the thermocline ridge of the Indian Ocean.

Untangling local and remote influences in two major petrel habitats in the oligotrophic Southern Ocean.

Ocean circulation connects geographically distinct ecosystems across a wide range of spatial and temporal scales via exchanges of physical and biogeochemical properties. Remote oceanographic processes can be especially important for ecosystems in the Southern Ocean, where the Antarctic Circumpolar Current transports properties across ocean basins through both advection and mixing. Recent tracking studies have indicated the existence of two large-scale, open ocean habitats in the Southern Ocean used by grey petrels (Procellaria cinerea) from two populations (i.e., Kerguelen and Antipodes islands) during their nonbreeding season for extended periods during austral summer (i.e., October to February). In this work, we use a novel combination of large-scale oceanographic observations, surface drifter data, satellite-derived primary productivity, numerical adjoint sensitivity experiments, and output from a biogeochemical state estimate to examine local and remote influences on these grey petrel habitats. Our aim is to understand the oceanographic features that control these isolated foraging areas and to evaluate their ecological value as oligotrophic open ocean habitats. We estimate the minimum local primary productivity required to support these populations to be much <1% of the estimated local primary productivity. The region in the southeast Indian Ocean used by the birds from Kerguelen is connected by circulation to the productive Kerguelen shelf. In contrast, the region in the south-central Pacific Ocean used by seabirds from the Antipodes is relatively isolated suggesting it is more influenced by local factors or the cumulative effects of many seasonal cycles. This work exemplifies the potential use of predator distributions and oceanographic data to highlight areas of the open ocean that may be more dynamic and productive than previously thought. Our results highlight the need to consider advective connections between ecosystems in the Southern Ocean and to re-evaluate the ecological relevance of oligotrophic Southern Ocean regions from a conservation perspective.

Late Summer Peak and Scavenging-Dominant Metal Fluxes in Particulate Export Near a Seamount in the Western North Pacific Subtropical Gyre

We measured sedimentation fluxes around JA06 Seamount (Xufu Guyot; 19°30′N, 158°00′E) as part of an environmental baseline survey in Japan’s exploration area for cobalt-rich crusts in the subtropical Northwest Pacific. Sinking particles were collected at the flat top (sediment trap depths: 900 and 1000 m) and northeastern base (sediment trap depths: 1000 and 4720 m) of the seamount from June 2016 to April 2017. Total mass fluxes were very low, with average values of 4.3–4.9 mg m–2 d–1 and 9.3 mg m–2 d–1 in the shallow traps at the flat top and base, respectively, which is consistent with an oligotrophic system. The lower fluxes at the flat top probably reflect lower productivity of siliceous microplankton, such as diatoms. However, we were unable to substantiate any potential mechanisms for this difference in productivity and cannot evaluate whether this is representative of typical conditions. When combined with previous observations at two adjacent seamounts, our results indicate widespread seasonality in sediment fluxes with a peak in late summer (August–September). However, satellite data indicate that summer is the season with the lowest primary production. This discrepancy could be explained either by phytoplankton blooms fueled by symbiotic nitrogen fixation that only cause minor increases in surface-ocean chlorophyll or short-lived blooms induced by passing typhoons under thick cloud cover. At the base site, we also analyzed material and element transport rates from shallow to deep waters. Half of the organic matter and >80% of the carbonate in sinking particles was not degraded in the water column, suggesting that most of the regeneration of these materials occurs near or on the sediment surface. Furthermore, four major processes appeared to control elemental fluxes in the area: lithogenic (Al, Ti, Fe), carbonate (Mg, Ca, Sr), biogenic (+scavenging) (Ni, Zn, Cd, Pb), and scavenging (V, Mn, Co, Cu, rare earth elements) processes. The estimation of excess flux based on the composition of upper continental crust demonstrated that >85% of total Mn, Co, Ni, Cu, Zn, Cd, and Pb fluxes were attributable to scavenging (+biogenic uptake). Scavenging-dominant metal fluxes are likely ubiquitous in the oligotrophic open ocean.

Starvation-Dependent Inhibition of the Hydrocarbon Degrader Marinobacter sp. TT1 by a Chemical Dispersant

During marine oil spills, chemical dispersants are used routinely to disperse surface slicks, transferring the hydrocarbon constituents of oil into the aqueous phase. Nonetheless, a comprehensive understanding of how dispersants affect natural populations of hydrocarbon-degrading bacteria, particularly under environmentally relevant conditions, is lacking. We investigated the impacts of the dispersant Corexit EC9500A on the marine hydrocarbon degrader Marinobacter sp. TT1 when pre-adapted to either low n-hexadecane concentrations (starved culture) or high n-hexadecane concentrations (well-fed culture). The growth of previously starved cells was inhibited when exposed to the dispersant, as evidenced by 55% lower cell numbers and 30% lower n-hexadecane biodegradation efficiency compared to cells grown on n-hexadecane alone. Cultures that were well-fed did not exhibit dispersant-induced inhibition of growth or n-hexadecane degradation. In addition, fluorescence microscopy revealed amorphous cell aggregate structures when the starved culture was exposed to dispersants, suggesting that Corexit affected the biofilm formation behavior of starved cells. Our findings indicate that (previous) substrate limitation, resembling oligotrophic open ocean conditions, can impact the response and hydrocarbon-degrading activities of oil-degrading organisms when exposed to Corexit, and highlight the need for further work to better understand the implications of environmental stressors on oil biodegradation and microbial community dynamics.

Diel variability of bulk optical properties associated with the growth and division of small phytoplankton in the North Pacific Subtropical Gyre.

Cross-platform observing systems are requisite to capturing the temporal and spatial dynamics of particles in the ocean. We present simultaneous observations of bulk optical properties, including the particulate beam attenuation (cp) and backscattering (bbp) coefficients, and particle size distributions collected in the North Pacific Subtropical Gyre. Clear and coherent diel cycles are observed in all bulk and size-fractionated optical proxies for particle biomass. We show evidence linking diurnal increases in cp and bbp to daytime particle growth and division of cells, with particles <7µm driving the daily cycle of particle production and loss within the mixed layer. Flow cytometry data reveal the nitrogen-fixing cyanobacterium Crocosphaera (∼4-7µm) to be an important driver of cp at the time of sampling, whereas Prochlorococcus dynamics (∼0.5µm) were essential to reproducing temporal variability in bbp. This study is a step towards improved characterization of the particle size range represented by in situ bulk optical properties and a better understanding of the mechanisms that drive variability in particle production in the oligotrophic open ocean.

Atmospheric wet and dry deposition of dissolved inorganic nitrogen to the South China Sea

At the global scale, atmospheric inputs of nitrogen are an important source of the new nitrogen that supports new marine production, especially in oligotrophic open oceans and marginal seas. This study reports quantities of atmospheric deposition of dissolved inorganic nitrogen (DIN) to the largest marginal sea in the North Pacific (the oligotrophic South China Sea, SCS) based primarily on rainwater sampling in the open northwestern region (Yongxing Island) from 2013 to 2015, and aerosol sampling from the SCS basin in June 2017. Atmospheric wet and dry deposition of DIN and their potential contributions to productivity were estimated. The volume-weighted mean rainwater concentrations during the wet and dry seasons were 4.9 and 18.1 μmol L−1 for N+N ( NO 3 − + NO 2 − ), and 5.7 and 4.0 μmol L−1 for NH 4 + , respectively. Rainwater concentrations of DIN were lower in the marginal seas than in the open ocean. The aerosol NO 3 − concentration was 1.15 ± 1.18 μg m−3 during the wet season, which is slightly lower than reported for the East China Sea and East Sea, but higher than in the Arabian Sea. Monthly wet and dry deposition rates ranged from 0.4–3.9 and 0.4–1.2 mmol m−2 mon−1 for NO 3 − , and 0.2–1.3 and 0.01–0.02 mmol m−2 mon−1 for NH 4 + , respectively. The annual wet and dry deposition fluxes of DIN were estimated to be 16.8 and 10.1 mmol m−2 yr−1, respectively. Compared to other marginal seas, the SCS receives less atmospheric NO 3 − inputs than the Yellow Sea, East China Sea, East Sea, and northeastern Mediterranean Sea. The total atmospheric DIN deposition may account for 1.8–11.1% of the nitrogen supporting new production and 0.7–1.8% of the nitrogen supporting primary production.