Oligotrophic Ocean Research Articles

Stimulation of Heterotrophic and Autotrophic Metabolism in the Mixing Zone of the Kuroshio Current and Northern South China Sea: Implications for Export Production

AbstractTo evaluate the influences of the Kuroshio Current intrusion on the biogeochemistry of the northern South China Sea (NSCS), we conducted field observations of the responses of microbial metabolism to the intrusions of the Kuroshio Current into the NSCS. We used an isopycnal mixing model to quantitatively assess the extent of the Kuroshio intrusion into the NSCS and found that bacterial abundance, production, and growth efficiency were unimodal functions of the fraction of Kuroshio water. Values were maximal at ~60% Kuroshio water and decreased monotonically as the percent of Kuroshio water deviated from 60%. The patterns of gross primary production and nitrate concentration were similar, but the peaks occurred at ~50% Kuroshio water. The Kuroshio intrusion, however, had little impact on bacterial and community respiration. The observed elevation of nitrate concentrations at ~50% Kuroshio water was quantitatively consistent with estimates of the amount of inorganic nitrogen released by remineralization of dissolved organic nitrogen by bacteria. Based on these observations, we hypothesize that dissolved organic matter in the Kuroshio water stimulates bacterial activity, and catabolism of the dissolved organic matter releases inorganic nutrients that stimulate primary production in the NSCS basin. The concomitant elevation of net community production subsequently enhances export production and thereby strengthens the biological pump in the nitrogen‐limited NSCS basin. Recognition of this mechanism underscores the role of horizontal currents in regional carbon and nitrogen cycles and broadens understanding of the processes that sustain export production in the oligotrophic ocean.

Eukaryotic and cyanobacterial communities associated with marine snow particles in the oligotrophic Sargasso Sea

Marine snow aggregates represent heterogeneous agglomerates of dead and living organic matter. Composition is decisive for their sinking rates, and thereby for carbon flux to the deep sea. For oligotrophic oceans, information on aggregate composition is particularly sparse. To address this, the taxonomic composition of aggregates collected from the subtropical and oligotrophic Sargasso Sea (Atlantic Ocean) was characterized by 16S and 18S rRNA gene sequencing. Taxonomy assignment was aided by a collection of the contemporary plankton community consisting of 75 morphologically and genetically identified plankton specimens. The diverse rRNA gene reads of marine snow aggregates, not considering Trichodesmium puffs, were dominated by copepods (52%), cnidarians (21%), radiolarians (11%), and alveolates (8%), with sporadic contributions by cyanobacteria, suggesting a different aggregate composition than in eutrophic regions. Composition linked significantly with sampling location but not to any measured environmental parameters or plankton biomass composition. Nevertheless, indicator and network analyses identified key roles of a few rare taxa. This points to complex regulation of aggregate composition, conceivably affected by the environment and plankton characteristics. The extent to which this has implications for particle densities, and consequently for sinking rates and carbon sequestration in oligotrophic waters, needs further interrogation.

Potential overestimation of community respiration in the western Pacific boundary ocean: What causes the putative net heterotrophy in oligotrophic systems?

AbstractMicrobial metabolism is of great importance in affecting the efficiency of biological pump and global carbon cycles. However, the metabolic state of the oligotrophic ocean, the largest biome on Earth, remains contentious. We examined the planktonic and bacterial metabolism using in vitro incubations along the western Pacific boundary during September and October 2016. The integrated gross primary production (GPP) of the photic zone exhibited higher values in the region of 2°–8°N along 130°E and the western Luzon Strait, which is consistent with the regional variability of nutrients in the different ocean provinces. Spatially, the community respiration (CR) was less variable than the GPP and slightly exceeded the GPP at most of the sampling stations. Overall, the in vitro incubation results suggest a prevailing heterotrophic state in this region. A comparison of the metabolic rates from the in vitro incubations with recently published biogeochemical model results in the same region shows that our observed GPP values were close to those predicted by the model, but the measured CR was approximately 30% higher than the modeled values. We also found that most of the in vitro CR estimates were higher than the upper range of the empirical CR estimated from the sum of the contributions of the main trophic groups. Conversely, the estimates of the empirical CR support the rationality of the CR predicted by the biogeochemical model. In general, the results indicate that systematic net heterotrophy is more likely a result of the overestimation of CR measured by the light–dark bottle incubation experiments, although the exact cause of the methodological problem remains unknown.

Basin‐Scale Variations in Labile Dissolved Phosphoric Monoesters and Diesters in the Central North Pacific Ocean

AbstractLabile dissolved phosphoric esters act as important phosphorus sources for microorganisms in the low‐phosphate oligotrophic ocean. Despite the significant role of labile dissolved phosphoric esters, previous studies have only focused on phosphoric monoesters, and the spatial distribution and dynamics of phosphoric ester species, including diesters, remain unclear. Thus, we aimed to elucidate basin‐scale variations in both monoesters and diesters and their role on microorganisms particularly in terms of diazotrophs. This is the first study to report the concentrations of these phosphoric esters in low‐ to middle‐latitudinal areas of the central North Pacific Ocean, concurrent with their hydrolysis rates. Hydrolysable monoester and diester concentrations ranged from undetectable levels to 40 nM, and their sum accounted for <11% of dissolved organic phosphorus on average. While monoester concentrations were significantly low in the low phosphate (<100 nM) region, there was no obvious trend between diester and phosphate concentrations. Monoesterase and diesterase activities showed significantly high values under low phosphate conditions when normalized by chlorophyll a concentrations. This indicated that both esters served as alternative phosphorus sources for microbial communities including phytoplankton. Diester concentrations and diesterase activities were positively correlated with nitrogen fixation activities, whereas such relationships were not observed for monoester concentrations and monoesterase activities. Thus, our results suggest that diesters have high affinity to diazotrophs compared with monoesters, which are utilized by a broad spectrum of microorganisms.

Improving Satellite Global Chlorophyll a Data Products Through Algorithm Refinement and Data Recovery

AbstractA recently developed algorithm to estimate surface ocean chlorophyll a concentrations (Chl in mg m−3), namely, the ocean color index (OCI) algorithm, has been adopted by the U.S. National Aeronautics and Space Administration to apply to all satellite ocean color sensors to produce global Chl maps. The algorithm is a hybrid between a band‐difference color index algorithm for low‐Chl waters and the traditional band‐ratio algorithms (OCx) for higher‐Chl waters. In this study, the OCI algorithm is revisited for its algorithm coefficients and for its algorithm transition between color index and OCx using a merged data set of high‐performance liquid chromatography and fluorometric Chl. Results suggest that the new OCI algorithm (OCI2) leads to lower Chl estimates than the original OCI (OCI1) for Chl < 0.05 mg m−3, but smoother algorithm transition for Chl between 0.25 and 0.40 mg m−3. Evaluation using in situ data suggests that similar to OCI1, OCI2 has significantly improved image quality and cross‐sensor consistency between SeaWiFS, MODISA, and VIIRS over the OCx algorithms for oligotrophic oceans. Mean cross‐sensor difference in monthly Chl data products over global oligotrophic oceans reduced from ~10% for OCx to 1–2% for OCI2. More importantly, data statistics suggest that the current straylight masking scheme used to generate global Chl maps can be relaxed from 7 × 5 to 3 × 3 pixels without losing data quality in either Chl or spectral remote sensing reflectance (Rrs(λ), sr−1) for not just oligotrophic oceans but also more productive waters. Such a relaxed masking scheme yields an average relative increase of 39% in data quantity for global oceans, thus making it possible to reduce data product uncertainties and fill data gaps.

Genomic and metatranscriptomic analyses of carbon remineralization in an Antarctic polynya

BackgroundPolynyas in the Southern Ocean are regions of intense primary production, mainly by Phaeocystis antarctica. Carbon fixed by phytoplankton in the water column is transferred to higher trophic levels, and finally, to the deep ocean. However, in the Amundsen Sea, most of this organic carbon does not reach the sediment but is degraded in the water column due to high bacterial heterotrophic activity.ResultsWe reconstructed 12 key bacterial genomes from different phases of bloom and analyzed the expression of genes involved in organic carbon remineralization. A high correlation of gene expression between the peak and decline phases was observed in an individual genome bin-based pairwise comparison of gene expression. Polaribacter belonging to Bacteroidetes was found to be dominant in the peak phase, and its transcriptional activity was high (48.9% of the total mRNA reads). Two dominant Polaribacter bins had the potential to utilize major polymers in P. antarctica, chrysolaminarin and xylan, with a distinct set of glycosyl hydrolases. In the decline phase, Gammaproteobacteria (Ant4D3, SUP05, and SAR92), with the potential to utilize low molecular weight-dissolved organic matter (LMW-DOM) including compatible solutes, was increased. The versatility of Gammaproteobacteria may contribute to their abundance in organic carbon-rich polynya waters, while the SAR11 clade was found to be predominant in the sea ice-covered oligotrophic ocean. SAR92 clade showed transcriptional activity for utilization of both polysaccharides and LMW-DOM; this may account for their abundance both in the peak and decline phases. Ant4D3 clade was dominant in all phases of the polynya bloom, implicating the crucial roles of this clade in LMW-DOM remineralization in the Antarctic polynyas.ConclusionsGenomic reconstruction and in situ gene expression analyses revealed the unique metabolic potential of dominant bacteria of the Antarctic polynya at a finer taxonomic level. The information can be used to predict temporal community succession linked to the availability of substrates derived from the P. antarctica bloom. Global warming has resulted in compositional changes in phytoplankton from P. antarctica to diatoms, and thus, repeated parallel studies in various polynyas are required to predict global warming-related changes in carbon remineralization.

Tracing nitrogen sources fueling coastal green tides off a volcanic island using radon and nitrogen isotopic tracers

The main sources of nutrients fueling coastal green tides off a volcanic island surrounded by an oligotrophic ocean are obscure, although they result in many societal and ecosystem problems. In this study, we attempted to trace the source inputs of nutrients in coastal waters off a volcanic island, Jeju, Korea, where the formation of green tides is perennial, using a radioisotope (222Rn) and stable isotopes (δ15N and δ18O) as tracers. Sampling of groundwater, seawater, fish-farm water, and Ulva spp. was performed during April and July 2015. The contribution of submarine fresh groundwater discharge (SFGD) to the dissolved inorganic nitrogen input was >70%, with additional inputs from aqua-cultural activities and bottom sediments. The δ15N-NO3 and δ18O-NO3 values in the coastal seawater and groundwater indicate that the main source of NO3− is fertilizer, rather than other potential sources, such as aquacultural wastewater, sewage/manure contamination, or precipitation, in this region. The δ15N value (+7.3–+7.7‰) in Ulva spp. also indicates the same source. Thus, our results suggest that the rapid infiltration of land N-fertilizer and subsequent leakage into the coastal ocean through submarine groundwater discharge (SGD) results in green tide massive occurrence in coastal waters off a high-permeability volcanic island.

Co-culture and biogeography of Prochlorococcus and SAR11

Prochlorococcus and SAR11 are among the smallest and most abundant organisms on Earth. With a combined global population of about 2.7 × 1028 cells, they numerically dominate bacterioplankton communities in oligotrophic ocean gyres and yet they have never been grown together in vitro. Here we describe co-cultures of Prochlorococcus and SAR11 isolates representing both high- and low-light adapted clades. We examined: (1) the influence of Prochlorococcus on the growth of SAR11 and vice-versa, (2) whether Prochlorococcus can meet specific nutrient requirements of SAR11, and (3) how co-culture dynamics vary when Prochlorococcus is grown with SAR11 compared with sympatric copiotrophic bacteria. SAR11 grew 15–70% faster in co-culture with Prochlorococcus, while the growth of the latter was unaffected. When Prochlorococcus populations entered stationary phase, this commensal relationship rapidly became amensal, as SAR11 abundances decreased dramatically. In parallel experiments with copiotrophic bacteria; however, the heterotrophic partner increased in abundance as Prochlorococcus densities leveled off. The presence of Prochlorococcus was able to meet SAR11’s central requirement for organic carbon, but not reduced sulfur. Prochlorococcus strain MIT9313, but not MED4, could meet the unique glycine requirement of SAR11, which could be due to the production and release of glycine betaine by MIT9313, as supported by comparative genomic evidence. Our findings also suggest, but do not confirm, that Prochlorococcus MIT9313 may compete with SAR11 for the uptake of 3-dimethylsulfoniopropionate (DMSP). To give our results an ecological context, we assessed the relative contribution of Prochlorococcus and SAR11 genome equivalents to those of identifiable bacteria and archaea in over 800 marine metagenomes. At many locations, more than half of the identifiable genome equivalents in the euphotic zone belonged to Prochlorococcus and SAR11 – highlighting the biogeochemical potential of these two groups.

Quantitative estimation of submarine groundwater discharge using airborne thermal infrared data acquired at two different tidal heights

AbstractSubmarine groundwater discharge (SGD) plays an important role in coastal biogeochemical processes and hydrological cycles, particularly off volcanic islands in oligotrophic oceans. However, the spatial and temporal variations of SGD are still poorly understood owing to difficulty in taking rapid SGD measurements over a large scale. In this study, we used four airborne thermal infrared surveys (twice each during high and low tides) to quantify the spatiotemporal variations of SGD over the entire coast of Jeju Island, Korea. On the basis of an analytical model, we found a linear positive correlation between the thermal anomaly and squares of the groundwater discharge velocity and a negative exponential correlation between the anomaly and water depth (including tide height and bathymetry). We then derived a new equation for quantitatively estimating the SGD flow rates from thermal anomalies acquired at two different tide heights. The proposed method was validated with the measured SGD flow rates using a current meter at Gongcheonpo Beach. We believe that the method can be effectively applied for rapid estimation of SGD over coastal areas, where fresh groundwater discharge is significant, using airborne thermal infrared surveys.

Effects of Asian dust input on eukaryotic phytoplankton community structure in the open areas in the Northwestern Pacific Ocean

ABSTRACTTo study the effects of aerosol particulates originated from Asian dust on the growth of eukaryotic phytoplankton in the oligotrophic open ocean, we conducted deck-board incubation experiments in the oligotrophic region of Northwestern Pacific Ocean (NWPO). Our results showed that when dust was added at a concentration of 2 mg/L (Dust-2), the NO3–N concentration increased by 3.2 fold, and chlorophyll a (chl a) concentration of nano- and micro-eukaryotic phytoplankton increased, while that of pico-eukaryotic phytoplankton did not change significantly. The microphytoplankton abundance increased but the species number decreased in Dust-2. Community structure of eukaryotic microphytoplankton also changed after dust addition. The abundance of diatoms in Dust-2 (23,072 cells/L) was 4.0 fold of that in the control (5750 cells/L), and 2.4 fold of that in Dust-1 (0.2 mg/L of dust addition) (9425 cells/L) at the 7th day of incubation. Abundance of dinoflagellates decreased in Dust-2, being 42.2% that of the control at the 7th day of the incubation. Effects of dust addition on the growth of phytoplankton differed among the dominant genera: growth of Pseudo-nitzschia and Chaetoceros were promoted while that of Prorocentrum was inhibited, and growth of Thalassiosira, Heterocapsa and Scrippsiella was not influenced significantly. The growth of nano- and pico-eukaryotic phytoplankton was promoted, with the cell abundance in Dust-2 2.4 fold of that in control. Our results indicated that Asian dust-originated aerosol particulates could provide nutrients to the oligotrophic NWPO, increase the marine productivity in the area, and alter the eukaryotic phytoplankton community structure.

Dynamics of Prochlorococcus Diversity and Photoacclimation During Short-Term Shifts in Water Column Stratification at Station ALOHA

The cyanobacterium Prochlorococcus is the dominant phototroph in surface waters of the vast oligotrophic oceans, the foundation of marine food webs, and an important component of global biogeochemical cycles. The prominence of Prochlorococcus across the environmental gradients of the open ocean is attributed to its extensive genetic diversity and flexible chlorophyll physiology, enabling light capture over a wide range of intensities. What remains unknown is the balance between temporal dynamics of genetic diversity and chlorophyll physiology in the ability of Prochlorococcus to respond to a variety of short (approximately one day) and longer (months to year) changes in the environment. Previous field research established depth-dependent Prochlorococcus single cell chlorophyll distributions in the North Pacific Subtropical Gyre. Here, we examined whether the shifts in chlorophyll distributions correspond to changes in Prochlorococcus genetic diversity (i.e. ecotype-level community structure) or photoacclimation of stable communities over short time intervals. We report that community structure was relatively stable despite abrupt shifts in Prochlorococcus chlorophyll physiology, due to unexpected physiological plasticity of high-light adapted Prochlorococcus ecotypes. Through comparison with seasonal-scale changes, our data suggest that variability on daily scales triggers shifts in Prochlorococcus photoacclimation, while seasonal-scale dynamics trigger shifts in community structure. Together, these data highlight the importance of incorporating the process of photoacclimation and chlorophyll dynamics into interpretations of phytoplankton population dynamics from chlorophyll data as well as the importance of daily-scale variability to Prochlorococcus ecology.

Phenology of Trichodesmium spp. blooms in the Great Barrier Reef lagoon, Australia, from the ESA-MERIS 10-year mission.

Trichodesmium, a filamentous bloom-forming marine cyanobacterium, plays a key role in the biogeochemistry of oligotrophic ocean regions because of the ability to fix nitrogen. Naturally occurring in the Great Barrier Reef (GBR), the contribution of Trichodesmium to the nutrient budget may be of the same order as that entering the system via catchment runoff. However, the cyclicity of Trichodesmium in the GBR is poorly understood and sparsely documented because of the lack of sufficient observations. This study provides the first systematic analysis of Trichodesmium spatial and temporal occurrences in the GBR over the decade-long MERIS ocean color mission (2002–2012). Trichodesmium surface expressions were detected using the Maximum Chlorophyll Index (MCI) applied to MERIS satellite imagery of the GBR lagoonal waters. The MCI performed well (76%), albeit tested on a limited set of images (N = 25) coincident with field measurements. A north (Cape York) to south (Fitzroy) increase in the extent, frequency and timing of the surface expressions characterized the GBR, with surface expressions extending over several hundreds of kilometers. The two southernmost subregions Mackay and Fitzroy accounted for the most (70%) bloom events. The bloom timing of Trichodesmium varied from May in the north to November in the south, with wet season conditions less favorable to Trichodesmium aggregations. MODIS-Aqua Sea Surface Temperature (SST) datasets, wind speed and field measurements of nutrient concentrations were used in combination with MCI positive instances to assess the blooms’ driving factors. Low wind speed (<6 m.s-1) and SST > 24°C were associated with the largest surface aggregations. Generalized additive models (GAM) indicated an increase in bloom occurrences over the 10-year period with seasonal bloom patterns regionally distinct. Interannual variability in SST partially (14%) explained bloom occurrences, and other drivers, such as the subregion and the nutrient budget, likely regulate Trichodesmium surface aggregations in the GBR.

Modelling the biogeochemical effects of heterotrophic and autotrophic N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fixation in the Gulf of Aqaba (Israel), Red Sea

Abstract. Recent studies demonstrate that marine N2 fixation can be carried out without light by heterotrophic N2 fixers (diazotrophs). However, direct measurements of N2 fixation in aphotic environments are relatively scarce. Heterotrophic as well as unicellular and colonial photoautotrophic diazotrophs are present in the oligotrophic Gulf of Aqaba (northern Red Sea). This study evaluates the relative importance of these different diazotrophs by combining biogeochemical models with time series measurements at a 700 m deep monitoring station in the Gulf of Aqaba. At this location, an excess of nitrate, relative to phosphate, is present throughout most of the water column and especially in deep waters during stratified conditions. A relative excess of phosphate occurs only at the water surface during nutrient-starved conditions in summer. We show that a model without N2 fixation can replicate the observed surface chlorophyll but fails to accurately simulate inorganic nutrient concentrations throughout the water column. Models with N2 fixation improve simulated deep nitrate by enriching sinking organic matter in nitrogen, suggesting that N2 fixation is necessary to explain the observations. The observed vertical structure of nutrient ratios and oxygen is reproduced best with a model that includes heterotrophic as well as colonial and unicellular autotrophic diazotrophs. These results suggest that heterotrophic N2 fixation contributes to the observed excess nitrogen in deep water at this location. If heterotrophic diazotrophs are generally present in oligotrophic ocean regions, their consideration would increase current estimates of global N2 fixation and may require explicit representation in large-scale models.

Climate sensitivities and uncertainties in food-web pathways supporting larval bluefin tuna in subtropical oligotrophic oceans

Abstract Compared with high-latitude seas, the ecological implications of climate change for top consumers in subtropical regions are poorly understood. One critical area of knowledge deficiency is the nature of food-web connections to larvae during their vulnerable time in the plankton. Bluefin tuna (BFT) are highly migratory temperate species whose early life stages are spent in ultra-oligotrophic subtropical waters. Dietary studies of BFT larvae provide evidence of prey-limited growth coupled with strong selection for specific prey types—cladocerans and poecilostomatoid copepods—whose paradoxical or poorly resolved trophic characteristics do not fit the conventional understanding of open-ocean food-web structure and flows. Current knowledge consequently leaves many uncertainties in climate change effects, including the possibility that increased nitrogen fixation by Trichodesmium spp. might enhance resiliency of BFT larvae, despite a projected overall decline in system productivity. To advance understanding and future predictions, the complementary perspectives of oceanographers and fisheries researchers need to come together in studies that focus on the trophic pathways most relevant to fish larvae, the factors that drive variability in spawning regions, and their effects on larval feeding, growth, and survival.

Isotopic Constraints on Nitrogen Transformation Rates in the Deep Sedimentary Marine Biosphere

AbstractLittle is known about the nature of microbial community activity contributing to the cycling of nitrogen in organic‐poor sediments underlying the expansive oligotrophic ocean gyres. Here we use pore water concentrations and stable N and O isotope measurements of nitrate and nitrite to constrain rates of nitrogen cycling processes over a 34‐m profile from the deep North Atlantic spanning fully oxic to anoxic conditions. Using a 1‐D reaction‐diffusion model to predict the distribution of nitrogen cycling rates, results converge on two distinct scenarios: (1) an exceptionally high degree of coupling between nitrite oxidation and nitrate reduction near the top of the anoxic zone or (2) an unusually large N isotope effect (~60‰) for nitrate reduction that is decoupled from the corresponding O isotope effect, which is possibly explained by enzyme‐level interconversion between nitrite and nitrate.

Symbiont Chloroplasts Remain Active During Bleaching-Like Response Induced by Thermal Stress in Collozoum pelagicum (Collodaria, Retaria)

Collodaria (Retaria) are important contributors to planktonic communities and biogeochemical processes (e.g. the biologic pump) in oligotrophic oceans. Similarly to corals, Collodaria live in symbiosis with dinoflagellate algae, a relationship that is thought to explain partly their ecological success. In the context of global change, the robustness of the symbiotic interaction and potential subsequent bleaching events are of primary interest for oceanic ecosystems functioning. In the present study, we compared the ultrastructure, morphology, symbiont density, photosynthetic capacities and respiration rates of colonial Collodaria exposed to a range of temperatures corresponding to natural conditions (21°C), moderate (25°C) and high (28°C) thermal stress. We showed that symbiont density immediately decreased when temperature rose to 25°C, while the overall Collodaria holobiont metabolic activity increased. When temperature reached 28°C, the holobiont respiration nearly stopped and the host morphological structure was largely damaged, as if the host tolerance threshold has been crossed. Over the course of the experiment, the photosynthetic capacities of remaining algal symbionts were stable, chloroplasts being the last degraded organelles in the microalgae. These results contribute to a better characterization and understanding of temperature-induced bleaching processes in planktonic photosymbioses.

&amp;lt;i&amp;gt;Trichodesmium&amp;lt;/i&amp;gt; physiological ecology and phosphate reduction in the western tropical South Pacific

Abstract. N2 fixation by the genus Trichodesmium is predicted to support a large proportion of the primary productivity across the oligotrophic oceans, regions that are considered among the largest biomes on Earth. Many of these environments remain poorly sampled, limiting our understanding of Trichodesmium physiological ecology in these critical oligotrophic regions. Trichodesmium colonies, communities that consist of the Trichodesmium host and their associated microbiome, were collected across the oligotrophic western tropical South Pacific (WTSP). These samples were used to assess host clade distribution, host and microbiome metabolic potential, and functional gene expression, with a focus on identifying Trichodesmium physiological ecology in this region. Genes sets related to phosphorus, iron, and phosphorus–iron co-limitation were dynamically expressed across the WTSP transect, suggestive of the importance of these resources in driving Trichodesmium physiological ecology in this region. A gene cassette for phosphonate biosynthesis was detected in Trichodesmium, the expression of which co-varied with the abundance of Trichodesmium Clade III, which was unusually abundant relative to Clade I in this environment. Coincident with the expression of the gene cassette, phosphate reduction to phosphite and low-molecular-weight phosphonate compounds was measured in Trichodesmium colonies. The expression of genes that enable use of such reduced-phosphorus compounds were also measured in both Trichodesmium and the microbiome. Overall, these results highlight physiological strategies employed by consortia in an undersampled region of the oligotrophic WTSP and reveal the molecular mechanisms underlying previously observed high rates of phosphorus reduction in Trichodesmium colonies.

Prochlorococcus viruses—From biodiversity to biogeochemical cycles

As the dominant primary producer in oligotrophic oceans, the unicellular picocyanobacterium Prochlorococcus is the smallest and most abundant photosynthetic phytoplankton in the world and plays an important role in marine carbon cycling. Cyanophages that infect Prochlorococcus influence the growth, carbon fixation, diversity, evolution, and environmental adaptation of their hosts. Here, we review studies on the isolation, genomics, and phylogenetic diversity of Prochlorococcus viruses and their interactions with Prochlorococcus. We also review the potential effects of Prochlorococcus viruses on biogeochemical cycling in the ocean.

Response of Subtropical Phytoplankton Communities to Ocean Acidification Under Oligotrophic Conditions and During Nutrient Fertilization

The subtropical oceans are home to the largest phytoplankton biome on the planet. Yet, little is known about potential impacts of ocean acidification (OA) on phytoplankton community composition in the vast oligotrophic ecosystems of the subtropical gyres. To address this question, we conducted an experiment with 9 in situ mesocosms (~35 m3) off the coast of Gran Canaria in the eastern subtropical North Atlantic over a period of 9 weeks. By establishing a gradient of pCO2 ranging from ~350 to 1025 µatm, we simulated carbonate chemistry conditions as projected until the end of the 21st century. Furthermore, we injected nutrient-rich deep water into the mesocosms halfway through the experiment to simulate a natural upwelling event, which regularly leads to patchy nutrient fertilization in the study region. The temporal developments of major taxonomic groups of phytoplankton were analyzed by flow cytometry, pigment composition and microscopy. We observed distinct shifts in phytoplankton community structure in response to high CO2, with markedly different patterns depending on nutrient status of the system. Phytoplankton biomass during the oligotrophic phase was dominated by picocyanobacteria (Synechococcus), which constituted 60-80% of biomass and displayed significantly higher cell abundances at elevated pCO2. The addition of deep water triggered a substantial bloom of large, chain-forming diatoms (mainly Guinardia striata and Leptocylindrus danicus) that dominated the phytoplankton community during the bloom phase (70-80% of biomass) and until the end of the experiment. A CO2 effect on bulk diatom biomass became apparent only in the highest CO2 treatments (>800 µatm), displaying elevated concentrations especially in the stationary phase after nutrient depletion. Notably, these responses were tightly linked to distinct interspecific shifts within the diatom assemblage, particularly favoring the largest species Guinardia striata. Other taxonomic groups contributed less to total phytoplankton biomass, but also displayed distinct responses to OA treatments. For instance, higher CO2 favored the occurrence of prymnesiophyceae (Phaeocystis globosa) and dictyochophyceae, whereas dinoflagellates were negatively affected by increasing CO2. Altogether, our findings revealed considerable shifts in species composition in response to elevated CO2 and indicated that phytoplankton communities in the subtropical oligotrophic oceans might be profoundly altered by ocean acidification.

Impact of cyclonic eddies and typhoons on biogeochemistry in the oligotrophic ocean based on biogeochemical/physical/meteorological time-series at station KEO

In 2014, a sediment trap mooring was deployed adjacent to station Kuroshio Extension Observatory (KEO)’s National Ocean and Atmosphere Administration (NOAA) surface mooring. These data, from July 2014 to July 2016, are used here to investigate nutrient supply mechanisms that support ocean productivity in this oligotrophic region of the subtropical western North Pacific. Both years of sediment trap data show that biogenic material fluxes at ~ 5000 m increased between late winter (March) and late spring (June). Based on sea surface temperature and upper ocean water temperature profiles, from the NOAA surface buoy, and satellite-based surface chlorophyll-a , this increase was likely due to an increase of ocean productivity in early spring (March) that was supported by nutrients supplied by winter mixing. On the other hand, biogenic material fluxes also increased in October 2014, and between late December 2014 and January 2015 when concentrations of nutrients near the surface typically are low. Sea surface height anomalies and vertical profiles of water temperature in the upper 500 m showed cyclonic eddies passing station KEO and causing upwelling in late July–early August 2014 and November 2014. It appears that these events supplied nutrients to the upper layer, which then caused ocean productivity in the subsurface layer to temporally increase, resulting in increased deep biogenic material fluxes in autumn and winter. This interpretation of the data is consistent with a simple 3D physical-biological model simulation that shows meso-scale cyclonic eddies can supply nutrient to support new production at KEO. During the 2-year-long sediment trap deployments, several typhoons also passed near station KEO and near-inertial internal waves were observed near the nitracline depth after the typhoons passed. Although turbulent mixing caused by near-inertial internal wave could have possibly supplied nutrient to upper oligotrophic euphotic layer, numerical simulations of the turbulent nutrient supply indicate that enhanced turbulent diffusion across the nutrient concentration gradient did not supply enough nitrate to support the increase in biogenic material flux in autumn.