Oligotrophic Surface Waters Research Articles

Coral photosymbiosis on Mid-Devonian reefs.

The ability of stony corals to thrive in the oligotrophic (low-nutrient, low-productivity) surface waters of the tropical ocean is commonly attributed to their symbiotic relationship with photosynthetic dinoflagellates1,2. The evolutionary history of this symbiosis might clarify its organismal and environmental roles3, but its prevalence through time, and across taxa, morphologies and oceanic settings, is currently unclear4-6. Here we report measurements of the nitrogen isotope (15N/14N) ratio of coral-bound organic matter (CB-δ15N) in samples from Mid-Devonian reefs (Givetian, around 385 million years ago), which represent a constraint on the evolution of coral photosymbiosis. Colonial tabulate and fasciculate (dendroid) rugose corals have low CB-δ15N values (2.51 ± 0.97‰) in comparison with co-occurring solitary and (pseudo)colonial (cerioidor phaceloid) rugose corals (5.52 ± 1.63‰). The average of the isotopic difference per deposit (3.01 ± 0.58‰) is statistically indistinguishable from that observed between modern symbiont-barren and symbiont-bearing corals (3.38 ± 1.05‰). On the basis of this evidence, we infer that Mid-Devonian tabulate and some fasciculate (dendroid) rugose corals hosted active photosymbionts, while solitary and some (pseudo)colonial (cerioidor phaceloid) rugose corals did not. The low CB-δ15N values of the Devonian tabulate and fasciculate rugose corals relative to the modern range suggest that Mid-Devonian reefs formed in biogeochemical regimes analogous to the modern oligotrophic subtropical gyres. Widespread oligotrophy during the Devonian may have promoted coral photosymbiosis, the occurrence of which may explain why Devonian reefs were the most productive reef ecosystems of the Phanerozoic.

Rhizobia–diatom symbiosis fixes missing nitrogen in the ocean

Nitrogen (N2) fixation in oligotrophic surface waters is the main source of new nitrogen to the ocean1 and has a key role in fuelling the biological carbon pump2. Oceanic N2 fixation has been attributed almost exclusively to cyanobacteria, even though genes encoding nitrogenase, the enzyme that fixes N2 into ammonia, are widespread among marine bacteria and archaea3–5. Little is known about these non-cyanobacterial N2 fixers, and direct proof that they can fix nitrogen in the ocean has so far been lacking. Here we report the discovery of a non-cyanobacterial N2-fixing symbiont, ‘Candidatus Tectiglobus diatomicola’, which provides its diatom host with fixed nitrogen in return for photosynthetic carbon. The N2-fixing symbiont belongs to the order Rhizobiales and its association with a unicellular diatom expands the known hosts for this order beyond the well-known N2-fixing rhizobia–legume symbioses on land6. Our results show that the rhizobia–diatom symbioses can contribute as much fixed nitrogen as can cyanobacterial N2 fixers in the tropical North Atlantic, and that they might be responsible for N2 fixation in the vast regions of the ocean in which cyanobacteria are too rare to account for the measured rates.

Nannofossils from the Middle Eocene Sabiñánigo Sandstone Formation in the Jaca Basin (southern Pyrenees): biostratigraphy and paleoenvironmental implications

Abstract. This study presents the first detailed data on calcareous nannofossil assemblages from the Sabiñánigo Sandstone Formation in the Jaca Basin (central south Pyrenees). This formation is mainly composed of deltaic and outer-shelf sediments. These siliciclastic deposits contain nannofossil assemblages that are moderately to well-preserved, particularly in fine-grained levels. They contain a calcareous nannofossil assemblage dominated by the species Cyclicargolithus floridanus, Coccolithus pelagicus, Coccolithus formosus, Clausicoccus fenestratus, Zygrhablithus bijugatus, and several species of Sphenolithus and Chiasmolithus. The biostratigraphic results enabled the characterization of the Middle Eocene biohorizons, based on global stratigraphic scales and the improvement of the temporal correlation and lateral evolution of this basin's deposits. The sedimentary sequence of the Sabiñánigo Sandstone was deposited during the Middle Eocene, between the upper part of biozone NP16 and the base of NP17, in the Bartonian. The calcareous nannoplankton assemblage suggests warm and oligotrophic surface waters for the Bartonian interval in the Jaca Basin.

Changes in Kuroshio Current dynamics and East Asian monsoon variability during the last 26 kyr

The Kuroshio Current flows northward along the east coast of Taiwan toward the Okinawa Trough and the East China Sea, but its dynamics and trajectory were probably different during the Last Glacial Maximum (LGM) due to the globally lower sea level that could have caused a (debated) deflection of the current along the eastern edge of the Ryukyu Arc.Core MD18–3532 has been recovered in an intra-slope basin of the Ryukyu accretionary prism, currently disconnected from the Kuroshio Current, but would have been on its trajectory in case of a NE deflection. Measurements of clay mineral assemblages and illite crystallinity revealed that Taiwan has been the main sediment source at this site over the last 26 kyr. The significantly higher sedimentation rate from the Last Glacial Maximum to the Bølling–Allerød compared with the period from the Younger Dryas to the Holocene, coupled with very low δ15Nsed during LGM and Heinrich Stadial 1, provide evidence for the transport of sediments and Trichodesmium spp. cyanobacteria by the partially deflected Kuroshio Current toward the eastern edge of the Ryukyu Arc. Combined with δ13Corg, TOC, TN, and XRF analyses, an increase in primary productivity has been observed during LGM and Heinrich Stadial 1. This would have been caused by an enhanced East Asian Winter monsoon winds resulting in the deepening of the mixed layer that would have led to the upwelling of the Kuroshio Current nutrient-enriched subsurface waters to the oligotrophic surface waters, and the supply of dust-borne iron from the Chinese Loess Plateau.

Habitat and benthic fauna of the Wallaby-Cuvier escarpment, SE Indian ocean

The Wallaby-Cuvier Escarpment is a 700 km long, NW-SE trending linear geological feature that marks the southern boundary of the Cuvier Plateau. The Plateau rises from the adjacent 5500 m deep abyssal plain to a topographical high of 3000 m, approximately 450 km off the coast of Western Australian. In 2021, a 50 km long segment of the escarpment, covering an area of 1700 km2, was mapped by a high-resolution full-ocean depth multibeam echosounder. The habitat of the study area was explored during two video surveys undertaken using the crewed submersible Limiting Factor. Using Benthic Terrain Modeler (BTM), we classified the seabed into five geomorphological classes; flat plains, depressions, broad slopes, steep slopes, and crest. Video footage was used to classify habitat types and to record benthic megafauna occurrences which revealed highly heterogeneous and rapidly changing habitats. Six habitats were proposed based on the nature of geomorphology, slope, and substrate textures. A total of 202 organisms comprising 52 morphotaxa were scored during 198 min of survey that included at least eight phyla and 29 families. Despite a high number of morphotaxa, the total abundance of organisms is considered low which we attribute to oligotrophic surface waters. However, the distribution of organisms and their feeding strategies were well-aligned with habitat type. This study documents faunal distribution and habitat diversity of a rarely explored type of deep-sea geomorphological feature, and in a largely unexplored area and depth of the Indian Ocean.

Earlier sea-ice melt extends the oligotrophic summer period in the Barents Sea with low algal biomass and associated low vertical flux

The decrease in Arctic sea-ice extent and thickness as a result of global warming will impact the timing, duration, magnitude and composition of phytoplankton production with cascading effects on Arctic marine food-webs and biogeochemical cycles. Here, we elucidate the environmental drivers shaping the composition, abundance, biomass, trophic state and vertical flux of protists (unicellular eukaryotes), including phytoplankton, in the Barents Sea in late August 2018 and 2019. The two years were characterized by contrasting sea-ice conditions. In August 2018, the sea-ice edge had retreated well beyond the shelf break into the Nansen Basin (>82°N), while in 2019, extensive areas of the northwestern Barents Sea shelf (>79°N) were still ice-covered. These contrasting sea-ice conditions resulted in marked interannual differences in the pelagic protist community structure in this area. In August 2018, the protist community was in a post-bloom stage of seasonal succession characterized by oligotrophic surface waters and dominance of small-sized phytoplankton and heterotrophic protists (predominantly flagellates and ciliates) at most stations. In 2019, a higher contribution of autotrophs and large-celled phytoplankton, particularly diatoms, to total protist biomass compared to 2018 was reflected in higher chlorophyll a concentrations and suggested that the protist community was still in a late bloom stage at some stations. It is noteworthy that particularly diatoms contributed a considerably higher proportion to the protist biomass at the ice-covered stations in both years compared to the open-water stations. This pattern was also evident in the higher vertical protist biomass flux in 2019, dominated by dinoflagellates and diatoms, compared to 2018. Our results suggest that the predicted transition toward an ice-free Barents Sea will lengthen the oligotrophic summer period with low algal biomass and associated low vertical flux.

Low Nitrogen Fixation Related to Shallow Nitracline Across the Eastern Indian Ocean

AbstractThe eastern Indian Ocean (EIO) is characterized by oligotrophic surface water with abundant dust supply, and thus, this environment is considered to provide a favorable niche for cyanobacterial diazotrophs. However, direct measurements have reported low nitrogen fixation with the dominance of heterotrophic diazotrophs, suggesting that the EIO has unique geophysical features and nutrient supplies controlling diazotrophy, in contrast to those in the Atlantic and Pacific Oceans. In this study, we investigated the activity and community of diazotrophs across the EIO from the Bay of Bengal to the west of Australia. Low nitrogen fixation was observed broadly in the EIO (26.1–75.6 μmol N m−2 d−1), except in the west of Australia (82–286 μmol N m−2 d−1). Although the surface nitrate concentration was low in the EIO, the upward nitrate flux was higher because of the shallower nitracline than that in other oligotrophic oceans. This created widespread conditions with low aeFe:N (supply ratio of aerosol iron to nitrogen), resulting in low nitrogen fixation rates. NifH amplicon sequencing demonstrated the dominance of heterotrophic diazotrophs widely in the EIO, with spatial segregation from cyanobacterial diazotrophs (Trichodesmium, UCYN‐A1). In general, cyanobacterial diazotrophs were abundant in the surface waters of the Bay of Bengal and west of Australia with a high aeFe:N supply, whereas heterotrophic diazotrophs were dominant in other broad regions. This finding demonstrates that the shallow nitracline and associated high upward nitrate supply are key factors for the unique diazotrophy in the EIO.

Influence of the phytoplankton community composition on the in situ fluorescence signal: Implication for an improved estimation of the chlorophyll-a concentration from BioGeoChemical-Argo profiling floats

In-situ fluorescence is a widely used method to estimate the chlorophyll-a (Chla) concentration, a proxy of the phytoplankton biomass. With the emergence of autonomous platforms such as BioGeoChemical-Argo (BGC-Argo) profiling floats, its use has expanded to global scale observations. However, the relationship between in-situ fluorescence and Chla may vary significantly, leading to major discrepancies between oceanic regions. This study aims to investigate the main sources of the natural variability in the in-situ fluorescence signal in the global open ocean, specifically the influence of the phytoplankton community composition. In this view, we analyzed a combination of three datasets comprising concomitant measurements of in-situ fluorescence, pigment concentrations and phytoplankton absorption spectra. Two datasets cover several contrasted bioregions of the global ocean whereas the third one consists of a regional time series in the northwestern Mediterranean Sea, which allows to examine the effect of phytoplankton community composition on the fluorescence signal on the global, seasonal and vertical scales. We studied the variability of the two major drivers of the natural variability of the fluorescence process, i.e. the light absorption and the fluorescence quantum yield of phytoplankton, in regards of the variability of the pigment composition of the communities. The community composition correlates substantially with the Chla-to-fluorescence ratio, with high fluorescence values associated with phytoplankton communities dominated by large cells. This trend may be explained by the combined effects of the community composition on the phytoplankton absorption coefficient and the fluorescence quantum yield, and is consistently observed globally, seasonally and vertically. Non-photosynthetic pigments also appear to play a critical role in oligotrophic surface waters, leading to a reduction of the quantum yield of fluorescence. The results indicate that the phytoplankton community composition plays a key role in the relationship between the in-situ fluorescence signal and Chla concentration. Therefore, we suggest that taking into account the composition of phytoplankton communities in the retrieval of the Chla concentration from current in-situ fluorometers, those mounted on BGC-Argo floats in particular, would lead to a better estimation of the phytoplankton biomass on a wide range of spatial and temporal scales.

Insights into community assembly mechanisms, biogeography, and metabolic potential of particle-associated and free-living prokaryotes in tropical oligotrophic surface oceans

Heterotrophic prokaryotes constitute the largest living biomass in the ocean and can be divided into particle-associated (PA) and free-living (FL) fractions. PA and FL prokaryotic communities play critical roles in the biogeochemical cycles of particulate and dissolved organic matter; however, their community assembly processes, biogeographical distribution patterns, and functional properties in oligotrophic surface water remain to be further elucidated. Based on 16S rRNA gene sequencing and shotgun metagenomics, we investigated the assembly mechanisms, biogeography, and functional potential of PA and FL prokaryotes in the surface waters of the West Pacific and Indian Oceans. FL prokaryotic communities were predominantly structured by deterministic processes, whereas their PA counterparts appeared to be shaped by the combined action of deterministic and stochastic processes. PA and FL prokaryotes in the tropical oligotrophic surface ocean exhibit markedly different community structures and functional potentials. Bacterial PA specialists such as Lentimonas, Alteromonas, and Pirellula as well as archaeal PA specialists Marine Group II and Marine Group III were significantly more abundant in PA assemblages, whereas lineages such as Prochlorococcus, SAR11 clade, and Candidatus Actinomarina were significantly more abundant in FL communities. The metabolic potential of the PA community was more abundant in pathways such as polyamine biosynthesis, carbohydrate metabolism, and glycosaminoglycan degradation. In contrast, the FL community was more enriched in functions related to amino acid metabolism, lipid biosynthesis, and aromatic degradation.

The Spatial Variation in Chlorophyte Community Composition From Coastal to Offshore Waters in a Subtropical Continental Shelf System

Globally, the continental shelf occupies less than 10% of the total sea surface, but supports substantial primary production and fisheries. Photosynthetic picoeukaryotes (PPE) are important primary producers in marine ecosystems, and chlorophytes make a significant contribution to PPE abundance. Although the distribution of chlorophytes has been widely studied, little is known about how their community composition varies along the coastal-offshore gradient in subtropical continental shelf waters. To better understand their spatial variations, we employed metabarcoding data of 18S rRNA V4 gene to examine chlorophyte composition within the surface and deep chlorophyll maximum (DCM) layers over the continental shelf in the East China Sea (ECS) with high-resolution sampling. Our results indicate a higher chlorophyte diversity in the surface layer than in the DCM. In addition, we found that chlorophytes in oligotrophic Kuroshio surface water were the most diverse, with a higher percentage of Chloropicophyceae, prasinophyte clade V, prasinophyte clade IX, Palmophyllophyceae and Chlorodendrophyceae. In more eutrophic waters along the coast and within the DCM layer, chlorophytes were dominated by Mamiellophyceae. A significant spatial variation in community composition occurred along the nutrient gradient in the subtropical continental shelf ecosystem, varying from Mamiellophyceae-dominant in coastal water to Chloropicophyceae-dominant in offshore water. Furthermore, at a low saline coastal station, which had the lowest chlorophyte diversity, Picochlorum (Trebouxiophyceae) was dominant. Overall, chlorophyte diversity was positively correlated with temperature, and negatively correlated with silicate concentration. This study revealed that terrestrial input into continental systems influences chlorophyte diversity in the subtropical northwestern Pacific Ocean.

Carbon fixation rates in groundwater similar to those in oligotrophic marine systems

The terrestrial subsurface contains nearly all of Earth’s freshwater reserves and harbours the majority of our planet’s total prokaryotic biomass. Although genetic surveys suggest these organisms rely on in situ carbon fixation, rather than the photosynthetically derived organic carbon transported from surface environments, direct measurements of carbon fixation in the subsurface are absent. Using an ultra-low level 14C-labelling technique, we estimate in situ carbon fixation rates in a carbonate aquifer. We find these rates are similar to those measured in oligotrophic marine surface waters and up to six-fold greater than those observed in the lower euphotic zone. Our empirical carbon fixation rates agree with nitrification rate data. Metagenomic analyses reveal abundant putative chemolithoautotrophic members of an uncharacterized order of Nitrospiria that may be behind the carbon fixation. On the basis of our determined carbon fixation rates, we conservatively extrapolate global primary production in carbonate groundwaters (10% of global reserves) to be 0.11 Pg carbon per year. These rates fall within the range found for oligotrophic marine surface waters, indicating a substantial contribution of in situ primary production to subsurface ecosystem processes. We further suggest that, just as phototrophs are for marine biogeochemical cycling, such subsurface carbon fixation is potentially foundational to subsurface trophic webs.

Deep abatement of low nitrate in oligotrophic surface water based on a sustained-release carbon source and aerobic denitrifying agent

Oligotrophic surface water is characterized by low nitrogen content (mainly in nitrate-N form), low carbon content, and high levels of dissolved oxygen compared to mesotrophic and eutrophic surface water. Such waters are also slightly polluted and at risk of abnormal algae proliferation resulting from the low nitrogen levels. Thus, abatement of low nitrate-N levels ( ∼ 1.5 mg/L) is urgent, but also a massive challenge, especially when organic carbon is typically limited. In this study, a novel strategy for deep abatement of low nitrate in oligotrophic surface waters was proposed, mainly based on the use of an aerobic denitrifying agent. The aerobic denitrifying agent, termed PHB(AOB-7), was developed using an aerobic denitrifying bacterial strain, Pseudomonas sp. AOB-7, which attaches and grows on polyhydroxy butyrate (PHB) granules serving as a sustained-release carbon source. Two oligotrophic surface water samples, from the South-to-North Water Diversion Project and the urban stormwater storage project, were treated with PHB(AOB-7) in the laboratory. The results indicated that this aerobic denitrifying agent was fairly efficient in abatement of nitrate-N (<1.5 mg/L) in both South-to-North water and stormwater, in both conical flask and bioreactor experiments. Nitrate-N was reduced from ∼ 1.5 to ∼ 0.3 mg/L. Meanwhile, ammonia and nitrite were maintained at very low levels. All N-contents satisfy level II of the Environmental Quality Standards for Surface Water. This study shows the great potential of the addition of low-N-abatement technology based on aerobic denitrification and a sustained-release carbon source to surface water. • A novel strategy for deep abatement of low nitrate in surface water was proposed. • It was based on aerobic denitrifying strain and sustained-release C-source. • Oligotrophic surface water samples were treated efficiently with this strategy. • NO 3 - -N was reduced from ∼ 1.5 to ∼ 0.3 mg/L, accompanied by lower NH 4 + -N and NO 2 - -N.

Algae control in oligotrophic surface water under the joint effect of nutritional competition and microbial algae-lytic substances

A novel algae control strategy in oligotrophic surface water was proposed. Under the combined effect of nutritional competition and bacilysin lysis, a 97% decline in chlorophyll-a content was achieved in South-to-North water and 61.5% in stormwater.

Calcareous plankton biostratigraphic fidelity and species richness during the last 10 m.y. of the Cretaceous at Blake Plateau, subtropical North Atlantic

Species distributions of well-preserved and diverse assemblages of planktonic foraminifera and calcareous nannofossils spanning the last 10 m.y. of the Cretaceous (middle Campanian through Maastrichtian) are analyzed from samples taken across a 1400 m depth transect at Blake Nose in the western subtropical North Atlantic (Ocean Drilling Program Sites 1049, 1050 and 1052). Age models constructed by integrating foraminiferal, calcareous nannofossil, and magnetic polarity datum events provide a reliable framework for temporal correlation of the sites. This framework enables comparisons of species richness and abundance among sites and evaluation of the reliability of first and last appearance datums for regional and global correlation. Among the standard primary zonal marker datums, six of nine planktonic foraminifer and six of seven calcareous nannofossil events are considered reliable for constraining the age-depth models. Secondary datum ages calculated for 17 planktonic foraminiferal events suggest correlation offsets among the three sites of <0.1 m.y. for four species (including Pseudoguembelina praehariaensis Tur and Huber n. sp.), 0.1–0.5 m.y. for six events, and 0.6–1.0 m.y. for eight species. Secondary calcareous nannofossil datum ages calculated for six species show less reliability, with offsets of <0.1 m.y. for one species, 0.7 m.y. for one species, and 1.0–1.5 m.y. for the remaining four species. The distinctly identifiable new species Trinitella suturis Tur and Huber has lowest occurrences that are diachronous by as much as 1.73 m.y. among the Blake Nose sites. Occurrence rarity is the most likely explanation for the age offsets of this and other diachronous species.Planktonic foraminiferal assemblages show no significant differences in species composition and relative abundance among the three sites, suggesting the sites were all located below a single oligotrophic surface water mass. Species richness counts pooled in 200 kyr bins for the Blake Nose sites reveals high species origination rates from the late Campanian through early Maastrichtian and highest species richness (51–63 species) during the Maastrichtian. The only significant extinction pulse during the mid-Campanian through Maastrichtian occurs between 66.2 and 66.4 Ma with loss of five species representing ∼9% of the assemblage. These extinctions occur at the same time as a globally recognized warming event correlated with a pulse of eruptions at the Deccan Traps in India.Calcareous nannofossil assemblages show no significant change in relative abundance among the three sites. Two significant extinction events are documented: one from 75.20 to 75.40 Ma with a loss of six species representing ∼6% of the assemblage and one from 66.2 to 66.4 Ma with a loss of nine species representing ∼9% of the assemblage. The former event is associated with a hiatus at the base of Zone CC24, and the latter corresponds to Deccan Trap warming.Hiatuses are identified at all three Blake Nose sites near the base of the Maastrichtian (∼71.5 Ma; lowermost Pseudoguembelina palpebra/CC24 Zone) and only at the deeper Sites 1049 and 1050 in the mid-Maastrichtian (∼67.2 Ma; Pseudoguembelina hariaensis/CC26b Zone) and the mid-Campanian (∼75.9 Ma; base of Radotruncana calcarata/CC22 Zone). Slumping from across the shelf and slope could have caused the early Maastrichtian hiatus while changes in the pattern and strength of deep-water circulation may have been responsible for the mid-Maastrichtian and mid-Campanian hiatuses.

Anammox bacteria drive fixed nitrogen loss in hadal trench sediments

Benthic N2 production by microbial denitrification and anammox is the largest sink for fixed nitrogen in the oceans. Most N2 production occurs on the continental shelves, where a high flux of reactive organic matter fuels the depletion of nitrate close to the sediment surface. By contrast, N2 production rates in abyssal sediments are low due to low inputs of reactive organics, and nitrogen transformations are dominated by aerobic nitrification and the release of nitrate to the bottom water. Here, we demonstrate that this trend is reversed in the deepest parts of the oceans, the hadal trenches, where focusing of reactive organic matter enhances benthic microbial activity. Thus, at ∼8-km depth in the Atacama Trench, underlying productive surface waters, nitrate is depleted within a few centimeters of the sediment surface, N2 production rates reach those reported from some continental margin sites, and fixed nitrogen loss is mainly conveyed by anammox bacteria. These bacteria are closely related to those known from shallow oxygen minimum zone waters, and comparison of activities measured in the laboratory and in situ suggest they are piezotolerant. Even the Kermadec Trench, underlying oligotrophic surface waters, exhibits substantial fixed N removal. Our results underline the role of hadal sediments as hot spots of deep-sea biological activity, revealing a fully functional benthic nitrogen cycle at high hydrostatic pressure and pointing to hadal sediments as a previously unexplored niche for anaerobic microbial ecology and diagenesis.

Effect of Intensity and Mode of Artificial Upwelling on Particle Flux and Carbon Export

Reduction of anthropogenic CO2 emissions alone will not sufficiently restrict global warming and enable the 1.5°C goal of the Paris agreement to be met. To effectively counteract climate change, measures to actively remove carbon dioxide from the atmosphere are required. Artificial upwelling has been proposed as one such carbon dioxide removal technique. By fueling primary productivity in the surface ocean with nutrient-rich deep water, it could potentially enhance downward fluxes of particulate organic carbon (POC) and carbon sequestration. In this study we investigated the effect of different intensities of artificial upwelling combined with two upwelling modes (recurring additions vs. one singular addition) on POC export, sinking matter stoichiometry and remineralization depth. We carried out a 39 day-long mesocosm experiment in the subtropical North Atlantic, where we fertilized oligotrophic surface waters with different amounts of deep water. The total nutrient inputs ranged from 1.6 to 11.0 μmol NO3– L–1. We found that on the one hand POC export under artificial upwelling more than doubled, and the molar C:N ratios of sinking organic matter increased from values around Redfield (6.6) to ∼8–13, which is beneficial for potential carbon dioxide removal. On the other hand, sinking matter was remineralized at faster rates and showed lower sinking velocities, which led to shallower remineralization depths. Particle properties were more favorable for deep carbon export in the recurring upwelling mode, while in the singular mode the C:N increase of sinking matter was more pronounced. In both upwelling modes roughly half of the produced organic carbon was retained in the water column until the end of the experiment. This suggests that the plankton communities were still in the process of adjustment, possibly due to the different response times of producers and consumers. There is thus a need for studies with longer experimental durations to quantify the responses of fully adjusted communities. Finally, our results revealed that artificial upwelling affects a variety of sinking particle properties, and that the intensity and mode with which it is applied control the strength of the effects.

Linking diatom-diazotroph symbioses to nitrogen cycle perturbations and deep-water anoxia: Insights from Mediterranean sapropel events

Elevated organic matter (OM) export flux promotes marine anoxia, thus increasing carbon sequestration efficiency and decreasing atmospheric carbon dioxide levels. However, the mechanisms that trigger and sustain anoxic events—particularly those associated with nutrient-poor, oligotrophic surface waters—remain poorly constrained. Mediterranean Sea sapropels are well-preserved sediments deposited during episodic anoxic events throughout the Plio-Pleistocene; as such, they may provide unique insight into the biogeochemical and ecological drivers of—and responses to—marine anoxia. Using biomarker distributions, we demonstrate that anaerobic ammonium oxidizing (anammox) bacteria and diazotrophic endosymbionts of mat- and/or raft-forming diatoms were both abundant during sapropel events, particularly in the Ionian and Libyan seas. In these sapropels, the carbon isotope compositions of anammox biomarkers directly capture progressive 13C-depletion in deep-water dissolved inorganic carbon, indicating sustained carbon sequestration. To explain these observations, we propose a reinforcing feedback whereby initial nutrient and/or circulation perturbations promote fixed nitrogen loss via intensified anammox and heterotrophic denitrification, which in turn favors proliferation of rapidly sinking diatom-diazotroph symbiotic consortia, increases OM burial flux, and sustains anoxia. This mechanism resolves the long-standing conundrum that small and buoyant diazotrophs are apparently associated with high OM export during periods of marine anoxia and oligotrophy.

Atlantic‐Origin Cold Saline Water Intrusion and Shoaling of the Nutricline in the Pacific Arctic

AbstractAtlantic‐origin cold saline water has previously not been considered an important contributor to the nutrient supply in the Pacific Arctic due to the effective insulation by the overlying Pacific‐origin waters that separate the surface mixed layer from the deeper Atlantic Water. Based on hydrographic observations in the northwestern Chukchi Sea from 2015 to 2017, we demonstrate that the intrusion of Atlantic‐origin cold saline water into the halocline boundary between Pacific and Atlantic‐origin waters in 2017 lifted Pacific‐origin nutrients up to the surface layer. We find that the cyclonic atmospheric circulation in 2017 was considerably strengthened, leading to lateral intrusions of two bodies of cold halocline water from the Eurasian marginal seas into the northwestern Chukchi Sea. Our results reveal that the intrusions of cold halocline waters caused unprecedented shoaling of the nutricline and anomalously high surface phytoplankton blooms in typically highly oligotrophic surface waters in the region during summer.

Metagenomic Analysis Reveals Microbial Community Structure and Metabolic Potential for Nitrogen Acquisition in the Oligotrophic Surface Water of the Indian Ocean.

Despite being the world’s third largest ocean, the Indian Ocean is one of the least studied and understood with respect to microbial diversity as well as biogeochemical and ecological functions. In this study, we investigated the microbial community and its metabolic potential for nitrogen (N) acquisition in the oligotrophic surface waters of the Indian Ocean using a metagenomic approach. Proteobacteria and Cyanobacteria dominated the microbial community with an average 37.85 and 23.56% of relative abundance, respectively, followed by Bacteroidetes (3.73%), Actinobacteria (1.69%), Firmicutes (0.76%), Verrucomicrobia (0.36%), and Planctomycetes (0.31%). Overall, only 24.3% of functional genes were common among all sampling stations indicating a high level of gene diversity. However, the presence of 82.6% common KEGG Orthology (KOs) in all samples showed high functional redundancy across the Indian Ocean. Temperature, phosphate, silicate and pH were important environmental factors regulating the microbial distribution in the Indian Ocean. The cyanobacterial genus Prochlorococcus was abundant with an average 17.4% of relative abundance in the surface waters, and while 54 Prochlorococcus genomes were detected, 53 were grouped mainly within HLII clade. In total, 179 of 234 Prochlorococcus sequences extracted from the global ocean dataset were clustered into HL clades and exhibited less divergence, but 55 sequences of LL clades presented more divergence exhibiting different branch length. The genes encoding enzymes related to ammonia metabolism, such as urease, glutamate dehydrogenase, ammonia transporter, and nitrilase presented higher abundances than the genes involved in inorganic N assimilation in both microbial community and metagenomic Prochlorococcus population. Furthermore, genes associated with dissimilatory nitrate reduction, denitrification, nitrogen fixation, nitrification and anammox were absent in metagenome Prochlorococcus population, i.e., nitrogenase and nitrate reductase. Notably, the de novo biosynthesis pathways of six different amino acids were incomplete in the metagenomic Prochlorococcus population and Prochlorococcus genomes, suggesting compensatory uptake of these amino acids from the environment. These results reveal the features of the taxonomic and functional structure of the Indian Ocean microbiome and their adaptive strategies to ambient N deficiency in the oligotrophic ocean.

Interactions between marine cyanobacteria and heterotrophic bacteria: A case study of &amp;lt;italic&amp;gt;Prochlorococcus&amp;lt;/italic&amp;gt;

Cyanobacteria are the key primary producers in the global ocean and account for 25% of its net productivity. They play a vital role in the ocean by driving the circulation of matter and the flow of energy via the absorption of considerable amounts of carbon dioxide and the release of dissolved organic matter. This dissolved organic matter is converted into biomass by marine heterotrophs, which keeps the cycling of elements in oligotrophic waters at equilibrium. Prochlorococcus, an ecologically important genus of cyanobacteria, is the smallest and most abundant oxygenic phototroph in the global ocean. Prochlorococcus has highly streamlined genomes, which can provide a competitive advantage in the oligotrophic surface waters; however, the streamlined genomes also force Prochlorococcus to adapt to a narrow range of environmental parameters. Owing to these genomic characteristics, Prochlorococcus often establishes interactions with heterotrophic bacteria to survive in the challenging marine environment. Because cyanobacteria are abundant in the ocean, the cyanobacteria-heterotrophic bacteria relationship can significantly affect marine carbon sequestration and storage, which in turn has important ecological implications. Previous studies have reported on the relationships between Prochlorococcus and various heterotrophic bacteria. For example, Prochlorococcus establishes positive interactions with Alteromonas sp. that enhance its growth rate and environmental adaptability . In this review, we discuss the underlying mechanisms of the interaction patterns between Prochlorococcus and heterotrophic bacteria, as well as their genetic, physiological, and ecological significance. Prochlorococcus and heterotrophic bacteria tend to form mutualistic relationships in which they directly or indirectly exchange metabolites or recycle organic carbon through complementary excretion and crosstalk between pathways, such as citrate, glycolate, and malate pathways. Prochlorococcus and heterotrophic bacteria also commonly establish commensal relationships. For instance, heterotrophic bacteria protect Prochlorococcus by scavenging hydrogen peroxide, which Prochlorococcus is sensitive to. Although many heterotrophic bacteria cannot synthesize vitamin B12, they require it as a cofactor for essential functions. Their genetic characteristics suggest that most cyanobacteria have the pathways for synthesizing vitamin B12, but that they may provide vitamin B12 to heterotrophic bacteria to promote the growth of “helper” in the oligotrophic ocean. Therefore, the interactions between cyanobacteria and heterotrophic bacteria help maintain the stability and diversity of marine ecosystems. Global climate change may influence cyanobacteria-heterotrophic bacteria interactions, which could directly affect the structure and dynamics of the microbial community in the oligotrophic ocean, and therefore, primary productivity and element cycling in the ocean. Here, we suggest future research priorities and potential applications based on newfound knowledge of the subject. First, using the known “heterotrophic bacteria-cyanobacteria” relationships, the cultivation of marine heterotrophic bacteria, cyanobacteria and their viruses should be promoted to build a more comprehensive “cyanobacteria-heterotrophic bacteria-virus” ecological model for marine environments. Second, the interaction mechanisms between cyanobacteria and heterotrophic bacteria should be revealed by new biotechnologies, such as (meta) genomics, (meta) transcriptomics, (meta) proteomics, and (meta) metabolomics.Third, cyanobacteria-heterotrophic bacteria interactions should be studied against the backdrop of climate change using both ex situ (i.e., laboratory) and in situ investigations to predict changes in marine microbial community and its potential impact on biogeochemical cycling in oceans.