Deep Chlorophyll Maximum Layer Research Articles

Enhanced nutrient supply promotes mutualistic interactions between cyanobacteria and bacteria in oligotrophic ocean.

Cyanobacteria can form complex interactions with heterotrophic microorganisms, but this relationship is susceptible to nutrient concentrations. Disentangling the cyanobacteria-bacteria interactions in relation to nutrient supply is essential to understanding their roles in geochemical cycles under global change. We hypothesize that enhanced nutrient supply in oligotrophic oceans can promote interactions among cyanobacteria and bacteria. Therefore, we investigated the planktonic bacteria and their interactions with cyanobacteria in relation to elevated nutrients caused by enhanced upwelling around a shallow and a deep seamount in the tropical western Pacific Ocean. We found obviously higher complexity of network occurred with significantly more cyanobacteria in the deep chlorophyll maximum layer ofthe shallow seamount when compared with that ofthe deep seamount. Cyanobacteria can shape bacterial interaction and community evenness in response to relatively high nutrient concentrations. The effects of the nutrients on cyanobacteria-related networks were further estimated based on the Tara Oceans data. Statistical analyses further showed a facilitative effect of nitrate concentrations on cyanobacteria-bacteria mutualistic interactions in the global oligotrophic ocean. By analysing the Tara Ocean macrogenomic data, we detected functional genes related to cyanobacteria-bacteria interactions in all samples, indicating the existence of a mutualistic relationship. Our results reveal cyanobacteria-bacteria interaction in response to nutrient elevation in oligotrophic ocean and highlight the potentially negative effects of global change on the bacterial community from the view of the bio-interaction.

Prokaryotic community structure and key taxa in the Arabian Sea’s oxygen minimum zone

Microbial communities within oxygen minimum zones (OMZs) play crucial roles in the marine biogeochemical cycling. Arabian Sea (AS) has one of the largest OMZs among the global oceans, however, knowledge about the microbial ecology of the AS OMZ remained limited. In the present study, 44 water samples collected from six stations across the AS, spanning from the deep chlorophyll maximum (DCM) layer to 4000m depth were analyzed. High-throughput sequencing of 16S rRNA genes revealed the structural diversity of bacterial and archaeal communities, influenced primarily by depth and dissolved oxygen (DO) levels. Distinct community compositions were observed across different oxygen gradients, with shifts in the relative abundance of key taxa. Notably, Desulfosarcinaceae, UBA10353, Nitrospina, SUP05, Sva0996_marine_group, Microtrichaceae, and Nitrosopumilus emerged as bioindicator taxa in the AS hypoxic zones. Co-occurrence network analysis identified SAR324, Alteromonadaceae, and Sphingomonadaceae as keystone taxa. The spatial and depth-wise distribution patterns revealed that Desulfosarcinaceae was predominantly found in the hypoxic zones of the Arabian Sea, whereas UBA10353, Nitrospina, SUP05, Microtrichaceae and SAR324 were ubiquitous across AS, Bay of Bengal (BOB), and Eastern Tropical North Pacific (ETNP) OMZs, with OTU-level niche differentiation observed for the latter two. Functional profiling using FAPROTAX predicted higher metabolic potential for nitrogen and sulfur in the OMZ compared to other layers of the AS. Our findings provide valuable insights into the distribution, structure, and diversity of microbial communities in the AS OMZ, highlighting the ecological roles of key taxa in hypoxic environments. The established sequence database offers a foundation for further research into the complex interactions within these microbial ecosystems.

Stimulation of small phytoplankton drives enhanced sinking particle formation in a subtropical ocean eddy

AbstractNutrient transfer into the sunlit surface ocean by cyclonic eddies is potentially crucial for sustaining primary productivity in the stratified subtropical gyres. However, the nature of productivity enhancements, including the flow of matter to higher trophic levels and its impact on carbon fluxes, remain poorly resolved. Here, we report a detailed assessment of the biogeochemical response to a cyclonic eddy in the subtropical Northwest Pacific via a combination of ship‐based and autonomous platforms. Primary production was enhanced twofold within the eddy core relative to reference sites outside, whereas phytoplankton biomass even decreased. Pico‐phytoplankton (< 2 μm) dominated (> 80%) both phytoplankton biomass and primary production inside and outside the eddy. The stimulated primary production in the eddy core was accompanied by an approximately twofold increase in mesozooplankton abundance, an approximately threefold increase in particle formation in the deep chlorophyll maximum layer, as well as significantly enhanced surface oceanic CO2 uptake and net community production. We suggest these observations carry important implications for understanding carbon export in the subtropical ocean and highlight the need to include such subtropical eddy features in ocean carbon budget analyses.

Vertical variations of bacterial growth, mortality loss to nanoflagellates, and viruses in the subtropical northwestern Pacific Ocean

Nanoflagellate grazing and viral lysis are the two main causes of mortality losses of marine bacterioplankton. Deciphering the mortality losses across the water column helps us understand their ecological and biogeochemical consequences. In this study, we implemented the two-point modified dilution method consisting of treatment of undiluted and 25% nanoflagellates and/or virus density at the surface (3 m), deep chlorophyll maximum (DCM), and the mesopelagic zone (300 m and 500 m deep) in the subtropical northwestern Pacific Ocean in summer. We found that the bacterial per capita growth (2.3 ± 0.6 d−1) and production (12.4 ± 6.9 μgC L−1 d−1) were significantly higher at the DCM layer than at the mesopelagic zone, possibly because of tight bacteria-phytoplankton coupling and trophic interactions between bacteria, nanoflagellates, and viruses. Further, we found that ∼70% of the bacterial mortality loss can be attributed to nanoflagellate grazing in the DCM layer, while most of the mortality loss in the surface and the mesopelagic zone can be attributed to viral lysis. We argue that while bacterial production is more efficiently transferred to higher trophic levels at the DCM layer, it is predominately recycled in the viral loop on the surface and the mesopelagic zone. Our results reveal the vertical variation of bacterial growth, production, mortality loss to nanoflagellate grazing, and viral lysis, from which we could deduct their depth-dependent impacts on carbon flux in the water column. Our study facilitates the understanding of the impacts of nanoflagellates and viruses on bacterioplankton and the bacteria-mediated biogeochemical cycling.

Effects of environmental variability on phytoplankton structure, diversity and biomass at the Brazil-Malvinas Confluence (BMC).

The Brazil-Malvinas Confluence (BMC) is a significant biological frontier where distinct currents meet, fostering optimal conditions for phytoplankton development. In this study we tested the hypothesis that eddys promote an increase in phytoplankton biomass at the Brazil-Malvinas Confluence (BMC), altering species diversity. Phytoplankton were collected with Niskin bottles and nutrient concentrations assessed at two depths (Surface and Deep Chlorophyll Maximum Layer - DCML) in areas outside and under the influence of Cold-Core (CCE) and Warm-Core (WCE) Eddies. Environmental variables were determined in situ using a CTD profiler. Four regions were separated based on environmental variables and phytoplankton species, namely, the Brazil Current (BC), Malvinas Current (MC), CCE, and WCE. Species diversity was higher in the eddies. The conditions of the WCE were different from those of the CCE, with low temperature and salinity and high cell density values in the latter. The phylum Bacillariophyta was predominant in terms of species richness in all regions and was responsible for the higher cell density in the MC, while dinoflagellates were dominant in the BC and eddies. Therefore, eddy activity alters the structure, diversity and biomass of the phytoplankton community in the BMC.

Vertical distribution of nitrogen sources for new and regenerated production in the western Bay of Bengal

The study investigated the new and regenerated production using 15N tracers in the western Bay of Bengal, with observations were made in a vertical water column and sunlight gradients under stratified oligotrophic conditions, which are the major changes that distinguish the hydrobiological parameters associated with the deep chlorophyll maximum (DCM). In this study, new and regenerated production were measured in at different zones such as the surface, the DCM region and the euphotic depth (below 1% of light intensity) during the high stratification period. The results showed that new and regenerated productivity at the surface was low, while higher light intensity decreased productivity at the surface and productivity in the DCM layer increased significantly. Productivity at different location within regions is primarily controlled by the supply of nitrogen nutrients from deep within the region. In contrast, productivity at a given location with the available nutrient pool, at different depths is highly influenced by light availability. The study region exhibited potential for moderate export production during spring inter-monsoon, as is the case in other parts of the Indian Ocean. The f-ratio was estimated to be 0.2, which is a very low value in deep water and plays an important role in the Earth’s carbon cycle, as carbon transported to the deep ocean is absorbed from the atmosphere, and the ocean thus acts as a carbon reservoir. The results also found that the availability of nitrogen nutrients is capable of exporting a significant part of the total production to the deep ocean when conditions are favorable. Our results suggest that the Bay of Bengal may be equally important in its efficiency to export productivity to the deep sea and the DCM layer. Statistical analysis revealed a positive correlation between nitrogen productivity and chlorophyll-a, dissolved inorganic nitrogen and total nitrogen: phosphorous ratio.

An along-track Biogeochemical Argo modelling framework: a case study of model improvements for the Nordic seas

Abstract. We present a framework that links in situ observations from the Biogeochemical Argo (BGC-Argo) array to biogeochemical models. The framework minimizes the technical effort required to construct a Lagrangian-type 1D modelling experiment along BGC-Argo tracks. We utilize the Argo data in two ways: (1) to drive the model physics and (2) to evaluate the model biogeochemistry. BGC-Argo physics data are used to nudge the model physics closer to observations to reduce the errors in the biogeochemistry stemming from physics errors. This allows us to target the model biogeochemistry and, by using the Argo biogeochemical dataset, we identify potential sources of model errors, introduce changes to the model formulation, and validate model configurations. We present experiments for the Nordic seas and showcase how we identify potential BGC-Argo buoys to model, prepare forcing, design experiments, and approach model improvement and validation. We use the ECOSMO II(CHL) model as the biogeochemical component and focus on chlorophyll a. The experiments reveal that ECOSMO II(CHL) requires improvements during low-light conditions, as the comparison to BGC-Argo reveals that ECOSMO II(CHL) simulates a late spring bloom and does not represent the deep chlorophyll maximum layer formation in summer periods. We modified the productivity and chlorophyll a relationship and statistically documented decreased bias and error in the revised model when using BGC-Argo data. Our results reveal that nudging the model temperature and salinity closer to BGC-Argo data reduces errors in biogeochemistry, and we suggest a relaxation time period of 1–10 d. The BGC-Argo data coverage is ever-growing and the framework is a valuable asset, as it improves biogeochemical models by performing efficient 1D model configurations and evaluation and then transferring the configurations to a 3D model with a wide range of use cases at the operational, regional/global and climate scales.

Phytoplankton carbon biomass: Insights from the eastern Indian Ocean

The study of phytoplankton carbon biomass is limited but has the potential to provide important insights into several oceanographic processes, such as the vertical carbon export and yield from the oceanic food web. In this study, we estimated the carbon biomass of the whole phytoplankton community in the eastern Indian Ocean (EIO) during spring of 2021 using flow cytometry and microscopy. We also examined the effect of physicochemical parameters on the spatial distribution of phytoplankton carbon biomass (phytoplankton-C). In the EIO, the range of phytoplankton-C was between 0.03 and 33.96 μg C L-1, with picophytoplankton (≤2 μm) accounting for 92.41 ± 8.95% and 89.06 ± 13.21% of the total phytoplankton-C in the surface and deep chlorophyll maximum (DCM) layer, respectively. Among various phytoplankton groups, Prochlorococcus and picoeukaryotes accounted for a major part of the phytoplankton-C in the EIO, contributing (57 ± 12%) and (25 ± 7%) to the depth-integrated carbon biomass, respectively. This was followed by Synechococcus (6 ± 4%), diatoms (6 ± 4%) and dinoflagellates (3 ± 1%). Phytoplankton-C exhibited significant variations in both horizontal and vertical distribution within the study area. Horizontally, the distribution of phytoplankton-C was markedly influenced by physical events in the EIO, such as freshwater inputs in the Bay of Bengal, Wyrtki Jet at the Equator and Southern Equatorial Current in the southern EIO. Vertically, different phytoplankton groups exhibited varying distribution patterns in carbon biomass with increasing depths, primarily due to their diverse responses to the vertical variation of irradiance and nutrients in the water column. The total phytoplankton-C in the surface layer (9.56 ± 3.01 μg C L-1) and the DCM layer (9.07 ± 3.75 μg C L-1) did not show a significant difference between each other (t-test, p > 0.1, n = 42), indicating that DCM didn't correspond to a phytoplankton-C maximum but rather a physiological adaptation of C: Chl a ratios of phytoplankton cells in the EIO. Therefore, vertical profiles of Chl a should be interpreted with caution when establishing their ecological significance in the study area. The carbon biomass of the entire phytoplankton community in the EIO was reported in this study, which could enhance our understanding of the contribution of different phytoplankton groups to the carbon pool and their role in the biogeochemical cycle in the EIO.

Proportion of dead cells in phytoplankton modulates community structure

The growth and death of phytoplankton are two key processes that affect structure and function of marine ecosystem. However, current understanding is elaborated from the perspective of growth, and relatively little is known about the environment-driven death of phytoplankton. Our study is aimed to elucidate the differences in group-specific proportion of dead cells and their effects on community structure. Based on flow cytometry analysis, the proportion of dead cells among eukaryotic phytoplankton groups was analyzed in the oligotrophic South China Sea. The eukaryotic phytoplankton were divided into five groups (eukaryotic group 1, eukaryotic group 2, eukaryotic group 3, cryptophytes, and eukaryotic group 4) with successively larger cell size from pico to micro. The larger cells (from nano to micro size) had a higher proportion of dead cells and a lower relative abundance. Only the group of large cells showed a complete correspondence between the proportion of dead cells and abundance in the vertical profile. Above the deep chlorophyll maximum layer, death induced by nutrient limitation was apparent in most groups, and the larger the cell size, the greater the limitation. Below the deep chlorophyll maximum layer, all groups were light-limited, but differences in the degree of light limitation among groups were not significant. These results suggested that differences in cell size contribute to some extent to differences in the proportion of dead cells among groups, making the proportion of dead cells important in regulating community structure and thus further influencing the structure and function of pelagic marine ecosystems.

Responses of Phytoplankton Communities to Internal Waves in Oligotrophic Oceans

AbstractUnderstanding the potential impacts of internal waves on phytoplankton communities in oligotrophic oceans remains an important research challenge. In this study, we elucidated the impact of internal waves on phytoplankton communities through a comprehensive 154‐hr time‐series of observations in the South China Sea (SCS). We identified distinctive variations in phytoplankton pigment biomass and composition across the upper, middle, and lower layers of the euphotic zone, which we attributed to the perturbations triggered by internal waves. Phytoplankton other than Prochlorococcus in the lower, nutrient‐replete layer likely benefitted from allochthonous nutrients introduced by internal waves, but their growth rates were constrained by light limitation, and their pigment biomass was held in check by microzooplankton grazing. In contrast, in the upper, nutrient‐depleted layer, the relative abundance of Prochlorococcus increased, likely because of the ammonium regenerated by zooplankton. The middle layer, characterized as the deep chlorophyll maximum layer, exhibited a dynamic equilibrium characterized by nutrient and light co‐limitation. This equilibrium resulted in high nitrate assimilation and growth by phytoplankton. The balancing of those rates by significant grazing losses maintained total chlorophyll a concentrations at a high level. Based on these findings, we proposed a three‐layer euphotic zone structure characterized by distinct physiological conditions, nutrient‐light dynamics, grazing pressure, and phytoplankton responses to internal waves. This three‐layer paradigm elucidated the intricate interplay between internal waves and phytoplankton communities and provided insights into the mechanisms that govern primary production and carbon cycling in oligotrophic oceanic ecosystems.

Phytoplankton Diversity and Co-Dependency in a Stratified Oligotrophic Ecosystem in the South Adriatic Sea

The oligotrophy of the southern Adriatic Sea is characterized by seasonal stratification which enables nutrient supply to the euphotic layer. A set of interdisciplinary methods was used to elucidate the diversity and co-dependency of bacterio- and phytoplankton of the water column during the stratification period of July 2021. A total of 95 taxa were determined by microscopy: 58 diatoms, 27 dinoflagellates, 6 coccolithophores, and 4 other autotrophs, which included Chlorophyceae, Chrysophyceae, and Cryptophytes. Nanophytoplankton abundances were higher in comparison to microphytoplankton. The prokaryotic plankton community as revealed by HTS was dominated by Proteobacteria (41–73%), Bacteroidota (9.5–27%), and cyanobacteria (1–10%), while the eukaryotic plankton community was composed of parasitic Syndiniales (45–80%), Ochrophyta (2–18%), Ciliophora (2–21%), Chlorophytes (2–4%), Haptophytes (1–4%), Bacillariophyta (1–13%), Pelagophyta (0.5–12%) and Chrysophyta (0.5–3%). Flow cytometry analysis has recorded Prochlorococcus and photosynthetic picoeukaryotes as more abundant in deep chlorophyll maximum (DCM), and Synechococcus and heterotrophic bacteria as most abundant in surface and thermocline layers. Surface, thermocline, and DCM layers were distinct considering community diversity, temperature, and nutrient correlations, while extreme nutrient values at the beginning of the investigating period indicated a possible nutrient flux. Nutrient and temperature were recognized as the main environmental drivers of phytoplankton and bacterioplankton community abundance.

Hydrography and deep chlorophyll maximum patterns of the Athos Basin and the Thracian Sea continental shelf (North Aegean Sea) combining glider and satellite observations

This work focuses on the North Aegean Sea deep chlorophyll maximum (DCM) dynamics, as revealed during the first glider mission over the continental shelf and slope of the Thracian Sea and along the Athos basin, and presents a conceptual model on the response of the DCM in relation to the prevailing stratification/mixing of the water column. Water temperature, conductivity, salinity, chlorophyll-a, CDOM, DO and particle backscatter data (bb700), were recorded during this underwater glider mission in summer 2019. Daily satellite data from Sentinel 3 complemented the glider collected data to validate the satellite-derived estimates of surface Chlorophyll-a levels. The high-resolution physical and chemical observations were analyzed emphasizing on thermal stratification, pycnocline strength, CDOM and dissolved oxygen vertical distribution and the presence of deep chlorophyll maximum (DCM). DCM statistical characteristics were derived by fitting a Gaussian model on the chlorophyll-a profiles. The mean DCM was observed at 76 ± 14.5 m depth, located below the euphotic layer, estimated at 64 ± 2.1 m, with the water column dynamics affecting directly its depth, thickness and peak prominence. The satellite-derived chlorophyll-a, integrated within the euphotic layer, was found in fair agreement to the integrated chlorophyll-a values from surface to optical depth recorded by the glider. Chl-a at the DCM were found up to 0.60 mg m−3, the vertically-integrated pigment content was estimated at 45 ± 14 mg m−2 and the values of Optical Community Index (OCI) illustrate the dominance of small-sized plankton. The increased variability in the particulate backscattering coefficient bb700 in Thracian Sea shelf and slope deep waters seems related to sediment resuspension. CDOM levels increased from 0.76 ppb at the sea surface to 1.11 ppb at 40 m depth, being positively correlated to Chl-a within the DCM. CDOM concentration increased steadily at intermediate and deeper waters up to 1.34 ± 0.07 ppb. DO profiles revealed the presence of an oxycline in the DCM layer, capable to oxygenate the intermediate and deeper waters (+3 × 10−3 ml/L/m). Bottom DO in Athos basin was measured at 5.0 ml L−1, indicating that the apparent oxygen utilization in Athos Basin is approximately 50 μmol L−1.

Reinterpreting radiocarbon records in bamboo corals – New insights from the tropical North Atlantic

Deep-sea bamboo corals (family Isididae) have been used as archives for reconstructing changes in the past ocean. However, uncertainties remain regarding the interpretation of geochemical signals from their organic nodes, specifically the water depth of the signals recorded by the coral. Here we explore this question by measuring radiocarbon (14C) and nitrogen (15N) isotopic compositions of the organic nodes in six bamboo corals collected from the central and eastern tropical Atlantic between 700 m and 2000 m water depth. By comparing coral 14C to measured seawater data, regional shallow-water coral records and climate-model outputs, we find contrasting results between the two regions. Our bamboo coral 14C results from the eastern tropical Atlantic support previous studies that suggest organic node carbon is sourced primarily from the mixed layer of the ocean. By contrast, the 14C of bamboo coral organic nodes from the oligotrophic central Atlantic better correlates with the 14C content of the subsurface deep chlorophyll maximum layer rather than the surface mixed layer. Combined with nitrogen isotope data, this observation suggests that sinking and/or ambient zooplankton feeding on phytoplankton from the deep chlorophyll maximum layer can contribute a significant proportion of the diet of bamboo corals. These results suggest that the carbon source for bamboo corals organic nodes may not always reside in the mixed layer, especially in oligotrophic regions, which has implications for 14C-based age model development in bamboo corals.

Response of microbial community of surface and deep chlorophyll maximum to nutrients and light in South China Sea

In oligotrophic seas such as South China Sea, the subsurface or deep chlorophyll maximum (DCM) is always present. The surface planktonic community receives sufficient light, but is short of nutrients. The DCM layer is under light limitation, but frequent supply of nutrients. Therefore, vertical mixing becomes critical in determining their community composition and drives their changes by responding to light and nutrients. In this study, we conducted an onboard experiment by collecting seawater samples at surface and the DCM layer and adding nutrients, and incubated them under full sunlight and 10% light, and examined the diversity of bacterial and eukaryotic communities and their cell abundance using 16/18S high throughput sequencing and FCM approaches. Our study found large differences in bacterial and eukaryotic community structure and cell abundance between the surface and DCM. After 72 hours of culture, taxonomically the incubated surface water was dominated by pico-eukaryotic phytoplankton, while the incubated DCM layer water is dominated by diatoms, which suggests diatoms are the main functional group of phytoplankton bloom after a vertical mixing event. These findings indicate that phytoplankton at the DCM respond to enhanced light and frequent supplied nutrients due to vertical mixing and thus maintain primary productivity in the otherwise oligotrophic oceans.

A Deep Seamount Effect Enhanced the Vertical Connectivity of the Planktonic Community Across 1,000 m Above Summit

AbstractSeamount effects, which are generally defined as hydrographic disturbances caused by topography and nutrient enrichment and biological aggregations around seamounts, are normally observed in shallow seamounts due to limited sampling efforts in deep seamounts. However, it remains unclear how and to what extent do deep seamounts leave their imprint on planktonic communities. Herein ciliates, a representative protist group, were chosen to explore the effect of deep seamount on planktonic community. By investigating the vertical and horizontal distribution of ciliate communities around the Kocebu Guyot (summit at −1,198 m) and in nonseamount area, we revealed an obvious deep seamount effect, which enhanced the vertical mixing of ciliate communities to an extent of over 1,000 m above the summit. The vertical mixing was manifested by a strong uplift of bottom dwellers from waters deeper than 500 m and a weak uplift from the 300 m layer to the deep chlorophyll maximum (about 150 m) layer. Network analysis showed that the ciliate cooccurrence relationship around the seamount was much more complex than that in nonseamount area. Statistical analysis indicated that seamount significantly weakened the limitation that water depth posed on vertical ciliate distribution. Overall, the ciliate communities presented a much higher‐resolution record of deep seamount effects than physico‐chemical data. Deep seamount could enhance the vertical mixing of waters and cooccurrence complexity of planktonic community to the euphotic layer. Considering the wide existence of deep seamounts, such an effect may have ecological significance and enhance the cycles of matter and energy of global oceans.

Zavarzinia marina sp. nov., a novel hydrocarbon-degrading bacterium isolated from deep chlorophyll maximum layer seawater of the West Pacific Ocean and emended description of the genus Zavarzinia.

A Gram-stain-negative, motile, non-spore-forming, strictly aerobic and rod-shaped bacterial strain, Adcm-6AT, was isolated from a seawater sample collected from the deep chlorophyll maximum layer in the West Pacific Ocean. Strain Adcm-6AT grew at 20-37 °C (optimum, 28-32 °C), at pH 6-11 (pH 7) and in the presence of 0-6 % (1-2 %) NaCl (w/v). Phylogenetic analysis based on 16S rRNA gene sequences indicated that it belonged to the genus Zavarzinia and had 97.7 and 96.9 % sequence similarity to Zavarzinia compransoris DSM 1231T and Zavarzinia aquatilis JCM 32263T, respectively. Digital DNA-DNA hybridization and average nucleotide identity values between strain Adcm-6AT and the two type strains were 22.2-22.9 % and 79.7-80.4 %, respectively. The principal fatty acids were C19:0 cyclo ω8c, summed feature 8 (C18:1 ω6c and/or C18:1 ω7c) and C16:0. The predominant respiratory quinone was Q-10. The polar lipids were diphosphatidylglycerol, two phosphatidylethanolamines, two phosphatidyglycerols and an unidentified lipid. The genomic DNA G+C content of strain Adcm-6AT was 67.7 %. Based on phylogenetic analysis and genomic-based relatedness indices, as well as phenotypic and genotypic characteristics, strain Adcm-6AT represents a novel species within the genus Zavarzinia, for which the name Zavarzinia marina sp. nov. is proposed. The type strain is Adcm-6AT (=MCCC M24951T=KCTC 82849T).

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.

Phytoplankton Distributions in the Kuroshio-Oyashio Region of the Northwest Pacific Ocean: Implications for Marine Ecology and Carbon Cycle

The Northwest Pacific Ocean (NWPO) is a significant sink for atmospheric CO2 but a paucity of large-scale phytoplankton surveys in the upper and lower euphotic zone results in uncertainties in estimates of the efficiency of the biological carbon pump there. Here, we report the spatial distribution of lipid biomarkers from diatoms (brassicasterol/epi-brassicasterol), dinoflagellates (dinosterol), and haptophytes (C37 alkenones) as proxies of phytoplankton biomass and community structure in suspended particles from the surface and deep chlorophyll maximum (DCM) layers across low- and mid-latitude regions of the NWPO. Our observations suggest that these lipid biomarkers can be used as indicators of the vertical distributions of phytoplankton biomass, which was comparable between the surface and DCM layers. Water masses with different nutrient concentrations strongly controlled the variations of lipid biomarkers, showing high biomass and the dominance of diatoms in the eutrophic Oyashio region, whereas low biomass and high proportions of dinoflagellates and haptophytes occurred in the oligotrophic Kuroshio region. Diatoms predominated in the DCM, likely enhancing carbon sequestration in the deep ocean and in sediments. Our results quantitatively demonstrate the horizontal and vertical variations of phytoplankton biomass and community structure, leading to an improved understanding of ecosystem function and biogeochemical cycles in this important region of the NWPO.

Coccolithophore Abundance, Degree of Calcification, and Their Contribution to Particulate Inorganic Carbon in the South China Sea

AbstractParticulate inorganic carbon (PIC) production and export to deep ocean are important processes in water column carbonate cycling. Biochemistry investigations have shown that the seawater PIC in eutrophic high‐latitude waters is contributed by marine calcifying algae coccolithophores which are constituted by the nearly monospecies Emiliania huxleyi. This allows the seawater PIC to be predicted in these high‐fertility regions based on satellite remote sensing. However, in waters with low surface PIC productivity, which still account for 70% of global ocean areas, the subsurface PIC maximum may not be well disclosed by satellite. Here, with a biochemistry cruise, we investigated the seawater coccolithophore cell and coccolith abundances, degree of E. huxleyi coccolith calcification, and PIC concentration in the deep chlorophyll maximum layers in the South China Sea, with the aim to improve our understanding of PIC production in these low productivity regions. Our results demonstrate the control of nutrient levels in the water column on the geographical (horizontal) distribution of Noelaerhabdaceae coccolithophores (E. huxleyi and Gephyrocapsa oceanica) in the investigated area, as well as an insensitive response of the calcification degree of E. huxleyi coccolith to carbonate chemistry. Although Noelaerhabdaceae coccoliths were the major components of deep‐sea sediment carbonate, they on average contributed to <18% of suspended PIC for the investigated seawater, indicating multiple sources of highly variable suspended PIC in the subsurface oligotrophic water. The production of Noelaerhabdaceae coccoliths greatly depends on local environments, which highlights the importance of field investigations in low‐fertility areas for global PIC productivity evaluation.

Picophytoplankton in the West Pacific Ocean: A Snapshot.

Marine picophytoplankton have crucial ecological value and make an important contribution to marine primary productivity. While biomass of phytoplankton in general is projected to decline as a result of global warming, picophytoplankton will likely dominate in the future oceans due to their growth advantages in an oligotrophic environment. To better understand the biography of picophytoplankton, we undertook a comprehensive study of the distribution patterns of picophytoplankton, carbon biomass, and Chl a concentrations, etc. based on large-scale sampling in the tropical Western Pacific Ocean. In terms of cellular abundance, Prochlorococcus was the most abundant group (averaging [1.03 ± 0.40] × 104 cells/mL), followed by Synechococcus (averaging [1.31 ± 1.22] × 103 cells/mL) and then picoeucaryote (averaging [4.83 ± 2.84] × 102 cells/mL). The picophytoplankton size-fractionated chlorophyll a (Pico-Chl a) accounted for about 30% of the total Chl a, with Prochlorococcus and picoeukaryotes contributing 41 and 35%, respectively, of the Pico-Chl a-normalized carbon biomass, indicating the ecological importance of picophytoplankton as the primary producers. In terms of biogeographic distribution, the picophytoplankton communities exhibited contrasting patterns. The surface distribution of Prochlorococcus and Synechococcus was concentrated in the low latitude of the 142°E section, while picoeucaryote was more abundant near the 130°E and equator sections. Synechococcus was higher in the shallow layer at 25 m, and it was extremely tolerant of high-light irradiation, while Prochlorococcus and picoeucaryote were distributed in the deep Chlorophyll maximum layer (DCM) (about 100 m). From the carbon-to-Chlorophyll a ratios, which was derived from Prochlorococcus and picoeucaryote population groups, we found that the ratio varied widely, from 0.19 to 75.56, and was highest at the depth of 200 m. Of these, Prochlorococcus had an important contribution. The correlation analysis of environmental factors showed that Prochlorococcus, Synechococcus, and picoeucaryote were negatively correlated with nutrient concentration. We concluded that Prochlorococcus group was dominant in the WPO, both in abundance and biomass, and the various abiotic factors such as temperature, salinity, and nutrient concentrations were closely correlated with the spatial variation in the picophytoplankton community. These findings aid our understanding of how contrasting environmental conditions influence picophytoplankton community and the importance of picophytoplankton in contributing the carbon pool in the oligotrophic ocean.