Deep Chlorophyll Maximum Research Articles

Parasitic taxa are key to the vertical stratification and community variation of pelagic ciliates from the surface to the abyssopelagic zone.

An increase in upper-ocean thermal stratification is being observed worldwide due to global warming. However, how ocean stratification affects the vertical profile of plankton communities remains unclear. Understanding this is crucial for assessing the broader implications of ocean stratification. Pelagic ciliates cover multiple functional groups, and thus can serve as a model for studying the vertical distribution and functional strategies of plankton in stratified oceans. We hypothesize that pelagic ciliate communities exhibit vertical stratification caused by shifts in functional strategies, from free-living groups in the photic zone to parasitic groups in deeper waters. 306 samples from the surface to the abyssopelagic zone were collected from 31 stations in the western Pacific and analyzed with environmental DNA (the V4 region of 18S rDNA) metabarcoding of pelagic ciliates. We found a distinct vertical stratification of the entire ciliate communities, with a boundary at a depth of 200m. Significant distance-decay patterns were found in the photic layers of 5m to the deep chlorophyll maximum and in the 2,000m, 3000m and bottom layers, while no significant pattern occurred in the mesopelagic layers of 200m - 1,000m. Below 200m, parasitic Oligohymenophorea and Colpodea became more prevalent. A linear model showed that parasitic taxa were the main groups causing community variation along the water column. With increasing depth below 200m, the ASV and sequence proportions of parasitic taxa increased. Statistical analyses indicated that water temperature shaped the photic communities, while parasitic taxa had a significant influence on the aphotic communities below 200m. This study provides new insights into oceanic vertical distribution, connectivity and stratification from a biological perspective. The observed shift of functional strategies from free-living to parasitic groups at a 200m transition layer improves our understanding of ocean ecosystems in the context of global warming.

Temporal insights into deep chlorophyll maxima dynamics in the Indian sector of the Southern Ocean: a Bio-Argo float study

Temporal insights into deep chlorophyll maxima dynamics in the Indian sector of the Southern Ocean: a Bio-Argo float study

Periodically asymmetric responses of deep chlorophyll maximum to light and thermocline in a clear monomictic lake: Insights from monthly and diel scale observations

Deep chlorophyll maximum (DCM), a chlorophyll peak in the water column, has important implications for biogeochemical cycles, energy flow and water surface algal blooms in deep lakes. However, how an observed periodically asymmetric DCM response to environmental variables remains unclear, limiting our in-depth understanding and effective eco-environmental management of deep lakes. Based on both monthly field investigations in 2021 and diel continuous observations in 2021–2023 in clear, monomictic Lake Fuxian, Southwest China, the temporal dynamics and drivers of DCM were examined and periodic features of DCM were found, with a formation period (FP, February–July) and a weakening period (WP, August–December). On the monthly scale, although DCM dynamics were partly attributed to thermocline structures, the role of light penetration depths varied with period. In the FP, the influence of light on DCM was direct, i.e., increased depth and thickness but decreased magnitude. Differently, the influence of light mainly occurred by affecting thermocline structures in the WP, where water quality was another important driver. On the diel scale, light was a major reason for a thicker and lower (magnitude) DCM during day than at night, and the response of DCM to environmental factors between the FP and WP differed also more during day. This periodically asymmetric response of daytime DCM not only being caused by light but possibly also related to other physical factors such as lake surface water temperature, wind speed and precipitation. Bayesian network modelling suggested that water darkening and stratification intensification may promote a shallower, thinner and larger (magnitude) DCM in both FP and WP, but achieving such changes in DCM requires different light and thermocline thresholds. Our findings provide new information valuable for modelling DCM and for predicting the related surface algal blooms in deep lakes under climate change and eutrophication.

Novel isolates expand the physiological diversity of Prochlorococcus and illuminate its macroevolution.

Prochlorococcus is a diverse picocyanobacterial genus and the most abundant phototroph on Earth. Its photosynthetic diversity divides it into high-light (HL)- or low-light (LL)-adapted groups representing broad phylogenetic grades-each composed of several monophyletic clades. Here, we physiologically characterize four new Prochlorococcus strains isolated from below the deep chlorophyll maximum in the North Pacific Ocean. We combine these physiological properties with genomic analyses to explore the evolution of photosynthetic antennae and discuss potential macroevolutionary implications. The isolates belong to deeply branching low-light-adapted clades that have no other cultivated representatives and display some unusual characteristics. For example, despite its otherwise low-light-adapted physiological characteristics, strain MIT1223 has low chl b2 content similar to high-light-adapted strains. Isolate genomes revealed that each strain contains a unique arsenal of pigment biosynthesis and binding alleles that have been horizontally acquired, contributing to the observed physiological diversity. Comparative genomic analysis of all picocyanobacteria reveals that Pcb, the major pigment carrying protein in Prochlorococcus, greatly increased in copy number and diversity per genome along a branch that coincides with the loss of facultative particle attachment. Collectively, these observations support a recently developed macroevolutionary model, in which niche-constructing radiations allowed ancestral lineages of picocyanobacteria to transition from a particle-attached to planktonic lifestyle and broadly colonize the euphotic zone.IMPORTANCEThe marine cyanobacterium, Prochlorococcus, is among the Earth's most abundant organisms, and much of its genetic and physiological diversity remains uncharacterized. Although field studies help reveal the scope of diversity, cultured isolates allow us to link genomic potential to physiological processes, illuminate eco-evolutionary feedbacks, and test theories arising from comparative genomics of wild cells. Here, we report the isolation and characterization of novel low-light (LL)-adapted Prochlorococcus strains that fill in multiple evolutionary gaps. These new strains are the first cultivated representatives of the LLVII and LLVIII paraphyletic grades of Prochlorococcus, which are broadly distributed in the lower regions of the ocean euphotic zone. Each of these grades is a unique, highly diverse section of the Prochlorococcus tree that separates distinct ecological groups: the LLVII grade branches between monophyletic clades that have facultatively particle-associated and constitutively planktonic lifestyles, whereas the LLVIII grade lies along the branch that leads to all high-light (HL)-adapted clades. Characterizing strains and genomes from these grades yields insights into the large-scale evolution of Prochlorococcus. The new LLVII and LLVIII strains are adapted to growth at very low irradiance levels and possess unique light-harvesting gene signatures and pigmentation. The LLVII strains represent the most basal Prochlorococcus group with a major expansion in photosynthetic antenna genes. Furthermore, a strain from the LLVIII grade challenges the paradigm that all LL-adapted Prochlorococcus exhibit high ratios of chl b:a2. These findings provide insights into the photophysiological evolution of Prochlorococcus and redefine what it means to be a low- vs high-light-adapted Prochlorococcus cell.

Diverse Responses of Upper Ocean Temperatures to Chlorophyll‐Induced Solar Absorption Across Different Coastal Upwelling Regions

AbstractChlorophyll in phytoplankton absorbs solar radiation (SR) and affects the thermal structure and dynamics within upwelling regions. However, research on this process across global‐scale coastal upwelling systems is still lacking. Here, we use a coupled ocean‐biogeochemical model to investigate differing responses to chlorophyll‐induced solar absorption between Pacific and Atlantic coastal upwelling regions. Chlorophyll‐induced solar absorption leads to colder Pacific coastal upwelling but warmer Atlantic coastal upwelling. In the Pacific, the shading effect of the surface chlorophyll maximum leads to colder subsurface water, which is then upwelled, contributing to cooling. The more stratified upper ocean leads to shallower mixed layer depth, intensifying offshore transport and upwelling. In the Atlantic, the absorption of SR by the subsurface chlorophyll maximum causes warmer and weaker upwelling. The processes described, in turn, trigger positive feedback to ocean biogeochemistry and potentially interact with climate dynamics, underscoring the necessity to incorporate them into Earth system models.

Nutrient depletion and phytoplankton shifts driven by the Pearl River plume in the Taiwan Strait

The intrusion of the Pearl River plume into the Taiwan Strait provides a unique case study that challenges traditional assumptions about the impacts of nutrient-rich river plumes on coastal phytoplankton communities. In this study, we conducted a detailed analysis of nutrient dynamics and phytoplankton composition within the Taiwan Strait, focusing on the effects of the Pearl River plume. Our findings reveal significant nutrient depletion, particularly of nitrogen, in the surface waters as the plume extends seaward, resulting in nitrogen limitation and a marked reduction in phytoplankton biomass. Vertical stratification within the Taiwan Strait creates distinct ecological niches, with the mid-layer supporting a deep chlorophyll maximum and the surface layer becoming dominated by the picophytoplankton Synechococcus. This shift from diatom-dominated communities to Synechococcus dominance has far-reaching implications for carbon cycling and food web dynamics in the region. Our results suggest that the Pearl River plume’s influence on the Taiwan Strait represents a departure from the typical nutrient enrichment associated with river plumes, highlighting the complexity of coastal biogeochemical processes.

Quantifying the Role of Silicate and Dissolved Nitrogen in Co‐Limiting the Primary and Secondary Productivity of the Bay of Bengal Euphotic Zone

AbstractA single‐column coupled physical and biological model based on the North Pacific Ecosystem Model for Understanding Regional Oceanography (NEMURO) with nitrogen and silicon cycles is adapted for the Bay of Bengal (BoB) environment. The model simulated plankton biomass and nutrients along the track of Bio‐Argos over East and West BoB (from 2016 to 2017) are validated with the observations. The model reasonably simulates the perennial structure of subsurface chlorophyll maximum (SCM). Further, three experiments are carried out to know the limitations in primary and secondary production in terms of nitrogen (NO3 + NH4) and silicate (Si(OH)4) in the open ocean BoB. In a “no‐NO3”experiment, the nitrate limiting term [that is, NO3/(NO3 + KNO3)] is set to zero so that the difference from the control case gives the role of “regenerated production” in the total primary and secondary production. Similarly, a no‐NH4 experiment was conducted to infer the role of “new production.” The new (regenerated) production fuels 85 ± 1% (28 ± 6%) of the living biomass in the East part of open ocean BoB (East BoB). The corresponding values for the west part (West BoB) are 86 ± 1% (42 ± 2%). Among the primary producers, the new (regenerated) production contributed 72 ± 1% (24 ± 6%) in the East BoB and 74 ± 1% (37 ± 2%) in the West BoB. The silicate limits the diatom production by 46% ± 22% (45% ± 27%) of the actual amount of diatom in the East BoB (West BoB) diatom. Surface to 60 m depth, we observed severe nitrate limitation over East BoB and equal limitation of nitrate and silicate over West BoB from June to September; the remaining months suffered from moderate nitrate limitation in both regions. This study shows that nitrate appears to be a limiting nutrient compared to silicate at the surface of both East and West BoB. Since silicicline is deeper than nitracline, silicate is the potential limiting nutrient in BoB from 65 to 105 m depth in all seasons.

Small-Scale Biophysical Interactions and Dinophysis Blooms: Case Study in a Strongly Stratified Chilean Fjord

Diarrhetic shellfish poisoning (DSP) toxins and pectenotoxins (PTXs) produced by endemic species of Dinophysis, mainly D. acuta and D. acuminata, threaten public health and negatively impact the shellfish industry worldwide. Despite their socioeconomic impact, research on the environmental drivers of DSP outbreaks in the Chilean fjords is scanty. From 22 to 24 March 2017, high spatial–temporal resolution measurements taken in Puyuhuapi Fjord (Northern Patagonia) illustrated the short-term (hours, days) response of the main phytoplankton functional groups (diatoms and dinoflagellates including toxic Dinophysis species) to changes in water column structure. Results presented here highlight the almost instantaneous coupling between time–depth variation in density gradients, vertical shifts of the subsurface chlorophyll maximum, and its evolution to a buoyancy-driven thin layer (TL) of diatoms just below the pycnocline the first day. A second shallower TL of dinoflagellates, including Dinophysis acuta, was formed on the second day in a low-turbulence lens in the upper part of the pycnocline, co-occurring with the TL of diatoms. Estimates of in situ division rates of Dinophysis showed a moderate growth maximum, which did not coincide with the cell density max. This suggests that increased cell numbers resulted from cell entrainment of off-fjord populations combined with in situ growth. Toxin profiles of the net tow analyses mirrored the dominance of D. acuminata/D. acuta at the beginning/end of the sampling period. This paper provides information about biophysical interactions of phytoplankton, with a focus on Dinophysis species in a strongly stratified Patagonian fjord. Understanding these interactions is crucial to improv predictive models and early warning systems for toxic HABs in stratified systems.

Dynamics of subsurface chlorophyll maxima in the northern Indian Ocean

Subsurface chlorophyll maxima (SCM) significantly contributes to oceanic primary productivity, emphasizing the need to study its dynamics and governing mechanisms. We used datasets from various platforms to investigate relationships between the SCM characteristics (SCM depth (ZSCM), SCM magnitude (Chlmax), SCM thickness (TSCM)) and environmental variables modulated by various physical processes in the Northern Indian Ocean (NIO). In the Arabian Sea (western NIO), seasonal processes like convective mixing and upwelling, primarily regulated the SCM characteristics. In the Bay of Bengal (eastern NIO), SCM characteristics were jointly influenced by fresh water influx, barrier layer formation, presence of eddies, and the propagation of Kelvin and Rossby waves. Any changes in these oceanic processes, potentially driven by climate change, could therefore impact oceanic primary production. Additionally, a positive association obtained between Chlmax and downward CO2 flux, while a shallower ZSCM, associated with higher concentrations of DMS, indicated SCM's role in regulating atmospheric gases.

Submesoscale ocean dynamic process contributions to diurnal subsurface chlorophyll variation along Lagrangian recirculation inside mesoscale eddies: A case study in the Southern Ocean

Studies regarding oceanic mesoscale and submesoscale processes and their impact on chlorophyll are mainly confined to weeks to decadal time scales. Based on biogeochemical-Argo float observations and altimeter data in the Southern Ocean in summer of 2016, we show the day-night chlorophyll difference inside a cyclonic eddy (ΔChlTCE) and an anticyclonic eddy (ΔChlTACE) associated with submesoscale processes. A diurnal cycle of chlorophyll is observed in the upper 50 m, with ΔChlTCE (1.5 mg m−3) as much as ten times that of ΔChlTACE (0.15 mg m−3). However, there are similar ratios of day-night chlorophyll difference to the maximum chlorophyll concentration in a day for the cyclonic and anticyclonic eddies (∼67%). Submesoscale processes present different impacts on the subsurface chlorophyll between the cyclonic and anticyclonic eddies on the diurnal scale. More significant submesoscale processes in the cyclonic eddy dominate the subsurface negative ΔChlTCE. It causes the phytoplankton to penetrate the bottom of the mixed layer and extend ∼50 m below the mixed layer. In contrast, submesoscale processes and their associated with vertical buoyancy flux only generate weak negative subsurface ΔChlTACE. The strong vertical gradient of ΔChlTACE is mainly dominated by the vertical displacement of the deep chlorophyll maximum.

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.

Controls of Cross‐Shore Planktonic Ecosystem Structure in an Idealized Eastern Boundary Upwelling System

AbstractEastern boundary upwelling systems (EBUSs) are among the most productive regions in the ocean because deep, nutrient‐rich waters are brought up to the surface. Previous studies have identified winds, mesoscale eddies and offshore nutrient distributions as key influences on the net primary production in EBUSs. However uncertainties remain regarding their roles in setting cross‐shore primary productivity and ecosystem diversity. Here, we use a quasi‐two‐dimensional (2D) model that combines ocean circulation with a spectrum of planktonic sizes to investigate the impact of winds, eddies, and offshore nutrient distributions in shaping EBUS ecosystems. A key finding is that variations in the strength of the wind stress and the nutrient concentration in the upwelled waters control the distribution and characteristics of the planktonic ecosystem. Specifically, a strengthening of the wind stress maximum, driving upwelling, increases the average planktonic size in the coastal upwelling zone, whereas the planktonic ecosystem is relatively insensitive to variations in the wind stress curl. Likewise, a deepening nutricline shifts the location of phytoplankton blooms shore‐ward, shoals the deep chlorophyll maximum offshore, and supports larger phytoplankton across the entire domain. Additionally, increased eddy stirring of nutrients suppresses coastal primary productivity via “eddy quenching,” whereas increased eddy restratification has relatively little impact on the coastal nutrient supply. These findings identify the wind stress maximum, isopycnal eddy diffusion, and nutricline depth as particularly influential on the coastal ecosystem, suggesting that variations in these quantities could help explain the observed differences between EBUSs, and influence the responses of EBUS ecosystems to climate shifts.

Particulate mercury export in the Central Pacific Ocean using 234Th[sbnd]238U disequilibria

Mercury (Hg) is a potent neurotoxin that enters the food web and may contaminate commercial, recreational, subsistence, and ceremonial fish stocks. Understanding the pathways by which this contamination occurs in marine systems is thus an essential component of minimizing consumer health risk. Our knowledge of the biogeochemical cycling of mercury, however, is relatively limited. Temporal changes in sinking particulate mercury (PHg) fluxes throughout the upper 400 m were examined at Station ALOHA (22°N, 158°W) in the North Pacific Subtropical Gyre (NPSG) and spatially along a north-south transect to the Equator (17.5°N to 5°N x 155°W) using a combination of in situ pumps and Uranium-238/Thorium-234 disequilibria as a tracer of particle export. Our results indicate that Station ALOHA is characterized by seasonally variable export fluxes of PHg, with highest fluxes occurring in May (175 m, 346 pmol m−2 day−1), with the advent of summer zooplankton growth, and in September (400 m, 356 pmol m−2 day−1), coinciding with a diazotroph mediated summer export pulse. PHg fluxes in May and September were higher than those previously measured in the equatorial Pacific at 150 m and continued to be high (> 100 pmol Hg m−2 d−1) down to 400 m, thereby providing a significant source of Hg to the mesopelagic food web. In contrast to Station ALOHA, at 8 and 5°N, PHg fluxes attenuated rapidly with depth, and fluxes were generally lower, with a maximum flux of 86 pmol m−2 d−1 (5°N). Depth profiles at 8 and 5°N were significantly different from one another, with PHg fluxes higher throughout the water column at 5°N and characterized by a subsurface peak in Hg flux 3 times higher than at 8°N (86 vs. 29 pmol Hg m−2 d−1). Monomethylmercury (MeHg) fluxes (max = 1.09 ± 0.57 pmol m−2 d−1) and concentrations (max = 0.14 fmol L−1) comprised only a small percentage of the total PHg pool. These results suggest that PHg cycling significantly differed between the NPSG and near the equator at least during an El Niño year. At Station ALOHA, microbial reworking of small particles below the deep chlorophyll maximum coupled with changes in zooplankton grazing drive seasonal export variability. In contrast near the equator, low fluxes associated with low biological productivity result in significantly lower PHg transport to depth during an El Niño year.

Niche differentiation in microorganisms capable of using alternative reduced nitrogen sources studied across depth and between oxic and anoxic ocean regions

IntroductionAssimilation of reduced nitrogen is less energetically costly than assimilation of oxidized forms. In the open ocean, ammonium is generally absent from the water column, including in oxygen-deficient zones (ODZs). Some microorganisms can use alternative organic reduced nitrogen forms like urea and cyanate, as indicated by the presence of cyanase (cynS) and urease (ureC) genes.MethodsHere we examine the Hawaii Ocean Time series, two stations in the Eastern Tropical South Pacific ODZ and one in the Eastern Tropical North Pacific ODZ, using phylogenetic read placement of metagenomic reads to define the proportion of each taxon capable of using cyanate and/or urea in oxic and anoxic environments.ResultsAn improved phylogenetic tree found that Thioglobaceae and Verrucomicrobia had the capability to use urea. Our detailed examination of all the microbial groups able to use cyanate and urea illuminated that niche differentiation, an adaptation to minimize competition, determines chosen nitrogen sources, partitioning by depth and oxygen. Urease genes were found in Picocyanobacteria and SAR11 in surface waters, Thaumarchaeota and Nitrospina in deep waters, Thioglobaceae and Cand. Scalindua in ODZs, and Verrucomicrobia in the deep oxycline. In the ODZs, the percentage of Anammox bacteria that contained cynS was double that of those containing ureC, and their cynS transcripts were abundant, indicating a preference for cyanate over urea.DiscussionWhile Prochlorococcus could utilize cyanate in the deep chlorophyll maximum, in the ODZs, Prochlorococcus uses nitrite rather than compete with Cand. Scalindua for cyanate, even though cyanate is present. SAR11 and Prochlorococcus may compete for urea in surface waters, but for SAR11, the presence of ureC was negatively correlated with nitrate concentration (p = 10−17), with ~ 40% of SAR11 genomes containing the ureC gene in oxic surface waters but none at depth, indicating that SAR11 bacteria switched to using nitrate when available. In the oxycline above the ODZ, where Thaumarchaeota and Nitrospina both could use urea, 50% of Nitrospina were also able to use cyanate, and their cyanase transcripts were present. This use of dissolved organic N should allow a higher biomass of N-cycling microbes and higher N-transformation rates than in a system competing for ammonia only.

Short-term time-series observations of phytoplankton light-absorption and productivity in Prydz Bay, coastal Antarctica

The optical characteristics of coastal Antarctic waters exhibit complexity due to the dynamic hydrography influenced by meltwater intrusion, which alters nutrient levels, thermohaline structure, and optically active substances (OAS) regimes. Studies on bio-optical variability and its implications on phytoplankton productivity (PP) are scanty in coastal polar regions. On this backdrop, time-series measurements (72 h at 6 h intervals) of bio-optical properties such as phytoplankton biomass (chlorophyll-a), absorption (aph), and total suspended matter (TSM) concurrently with PP were measured to understand their interplay and variability in relation to the ambient physicochemical settings in the under-sampled Prydz Bay, coastal Antarctica. Our findings revealed thermohaline stratification within the bay, likely attributed to the inflow of less saline meltwater from nearby glaciers and minimal wind activity. The consistent presence of sub-surface chlorophyll maximum (SCM) beneath the stratified layer underscored the light-acclimatization response of shade-adapted phytoplankton. Surface waters exhibited higher TSM compared to deeper layers, indicating glacial melt influence, while the depth of the sunlit layer remained relatively stable, suggesting limited water mass movement and/or variability in OAS at the study site. An inverse relation between chlorophyll-a and chlorophyll-a-specific phytoplankton light absorption (a*ph(λ)) manifested ‘pigment package effect’ within the prevailing phytoplankton community, implying reduced light-absorption efficiency and consequent lower PP. Compared to chlorophyll-a, the phytoplankton light absorption (aph(λ)) emerged as a better proxy for explaining PP variability. Nutrient availability was not limiting, which was conducive to micro (large) phytoplankton growth. Classification of phytoplankton size classes (micro, nano, and pico) based on the B/R ratio (aph at Blue (443 nm)/Red (676 nm) region) confirmed the dominance of larger (micro) phytoplankton that are more susceptible to package effect, thus have implications on reduced PP potential of this polar marine ecosystem.

Stable microbial community diversity across large-scale Antarctic water masses

The distinctive environmental attributes of the Southern Ocean underscore the indispensability of microorganisms in this region. We analyzed 208 samples obtained from four separate layers (Surface, Deep Chlorophyll Maximum, Middle, and Bottom) in the neighboring seas of the Antarctic Peninsula and the Cosmonaut Sea to explore variations in microbial composition, interactions and community assembly processes. The results demonstrated noteworthy distinctions in alpha and beta diversity across diverse communities, with the increase in water depth, a gradual rise in community diversity was observed. In particular, the co-occurrence network analysis exposed pronounced microbial interactions within the same water mass, which are notably stronger than those observed between different water masses. Co-occurrence network complexity was higher in the surface water mass than in the bottom water mass. Yet, the surface water mass exhibited greater network stability. Moreover, in the phylogenetic-based β-nearest taxon distance analyses, deterministic processes were identified as the primary factors influencing community assembly in Antarctic microorganisms. This study contributes to exploring diversity and assembly processes under the complex hydrological conditions of Antarctica.

Evaluation of Vertical Patterns in Chlorophyll‐A Derived From a Data Assimilating Model of Satellite‐Based Ocean Color

AbstractSatellite‐based sensors of ocean color have become the primary tool to infer changes in surface chlorophyll, while BGC‐Argo floats are now filling the information gap at depth. Here we use BGC‐Argo data to assess depth‐resolved information on chlorophyll‐a derived from an ocean biogeochemical model constrained by the assimilation of surface ocean color remote sensing. The data‐assimilating model replicates well the general seasonality and meridional gradients in surface and depth‐resolved chlorophyll‐a inferred from the float array in the Southern Ocean. On average, the model tends to overestimate float‐based chlorophyll, particularly at times and locations of high productivity such as the beginning of the spring bloom, subtropical deep chlorophyll maxima, and non‐iron limited regions of the Southern Ocean. The highest model RMSE in the upper 50 m with respect to the float array is of 0.6 mg Chl m−3, which should allow the detection of seasonal changes in float‐based biomass (varying between 0.01 and >1 mg Chl m−3) but might hinder the identification of subtle changes in chlorophyll at narrow local scales. Both model and float profiling data show good agreement with in situ data from station ALOHA, with model estimates showing a slight accuracy edge in inferring depth‐resolved observations. Uncertainties in float bio‐optical estimates impede their use as a reliable benchmark for validation, but the general qualitative agreement between model and float data provides confidence in the ability of model to replicate biogeochemical features below the surface, where data is not directly constrained by the assimilation of satellite ocean color.

Mesoscale variability of phosphorus stocks, hydrological and biological processes in the mixed layer in the Eastern Mediterranean Sea in autumn and during an unusually dense winter phytoplankton bloom

We investigated spatiotemporal variations of nutrients, dissolved organic pools (C, N, P), phosphomonoesterase (PME) and phosphodiesterase (PDE) activities, heterotrophic prokaryotic production and planktonic microorganisms within the mixed layer (ML) in the Eastern Mediterranean Sea. We characterized two contrasted situations: autumn 2018 (highly stratified period, deep chlorophyll maximum within 100 m depth) and winter 2019 (including a bloom period). We compared the distribution of biogeochemical variables within the mixed layer and hydrological vertical structure between the different stations using a principal component analysis. Six groups of stations were identified (one group in autumn, 5 in winter), based on variable physical descriptors but also environmental biogeochemical conditions related to i) the seasonal aspect (for instance, all stations sampled within the Ierapetra anticyclone in autumn clustered in one, single group); ii) transitions between cyclonic and anticyclonic structures with a large range of ML depths (18–269 m) and indications of intense, preceding winter convection events: iii) progression of a high phytoplankton bloom during the winter cruise inferred from a series of observations: a strong nitrate drawdown, important growth of Synechococcus, pico and nano eukaryotes, accumulation of chlorophyll a (>60 mg m−2), primary production rates up to 509 mg C m−2 d−1, changes in the pigments’ diversity, increase in biomass-specific ectoenzymatic activities and of heterotrophic prokaryotic production, all in conjunction with the vicinity of the Rhodes gyre. Here, we studied the distribution of biological and biogeochemical properties within the mixed layer, in particular by employing sensitive methods for the detection of low phosphate concentrations and of the labile dissolved organic phosphorus pool. From this data set, we demonstrate that the surface mixed layer classically considered as a P-depleted and uniform layer in the Eastern Mediterranean Sea was highly biologically dynamic, and prone to rapid spatio-temporal changes in phosphatase activities and phytoplankton dynamics. Altogether, these data reveal a strong short-term population dynamics. The results highlight the role of mixing episodes in winter, which provide pulsed supplies of phosphate and/or nitrate from the deeper layer to the euphotic zone, triggering transient blooms that often go undetected by satellites.

Viral Dynamics in the Tropical Pacific Ocean: A Comparison between Within and Outside a Warm Eddy.

In mesoscale eddies, the chemical properties and biological composition are different from those in the surrounding water due to their unique physical processes. The mechanism of physical-biological coupling in warm-core eddies is unclear, especially because no studies have examined the effects of environmental factors on bacteria and viruses. The purpose of the present study was to examine the influence of an anticyclonic warm eddy on the relationship between bacterial and viral abundances, as well as viral activity (viral production), at different depths. At the core of the warm eddy, the bacterial abundance (0.48 to 2.82 × 105 cells mL-1) fluctuated less than that outside the eddy (1.12 to 7.03 × 105 cells mL-1). In particular, there was a four-fold higher viral-bacterial abundance ratio (VBR) estimated within the eddy, below the layer of the deep chlorophyll maximum, than outside the eddy. An anticyclonic warm eddy with downwelling at its center may contribute to viruses being transmitted directly into the deep ocean through adsorption on particulate organic matter while sinking. Overall, our findings provide valuable insights into the interaction between bacterial and viral abundances and their ecological mechanisms within a warm eddy.

The Messinian paleoenvironment in the Mediterranean (Monte dei Corvi, Ancona): A comparison with the modern oceanographic conditions

The Messinian was characterized by peculiar biogeochemical dynamics and climate in the Mediterranean region, testified by widespread deoxygenation events (sapropel) and evaporite deposition. To constrain the Mediterranean response to past perturbation it is crucial to understand the current environmental crisis related to climate change. In this regard, benthic and planktic calcareous fossils provide valuable insights into surface and bottom water conditions during deoxygenation events.Here we studied in high resolution 4 sapropel-bearing cycles of the Monte dei Corvi (Ancona, Italy), which recorded the behavior of the Adriatic Deep-Water formation, a major controlling factor for the oxygenation in the modern Eastern Mediterranean. Our analysis unveils fluctuations in planktic and benthic assemblages driven by variations in insolation parameters. Sapropel interbeds deposited during insolation maxima exhibit warm-oligotrophic and Deep Chlorophyll Maximum taxa, suggesting warming and freshening of surface water, leading to weakened Adriatic Deep-Water formation and reduced oxygen delivery to the bottom. Marly limestone/marlstone interbeds exhibit the dominance of cold-eutrophic taxa and an abundance of fecal pellets with monospecific/oligospecific calcareous nannofossils taxa (Umbilicosphaera jafari and Reticulofenestra perplexa), suggesting moderately high salinity and sustained productivity during phases of strong mixing.Comparisons between Messinian and the present-day setting reveal significant differences in the abundance and distribution of calcareous planktic assemblage, mostly due to heightened productivity during the Messinian, a response to restricted conditions that increased the basin's susceptibility to nutrient-delivering runoff. This circumstance played a significant role in the widespread deoxygenation and accumulation of organic carbon during the Messinian.The uncertain trajectory of primary production in the Mediterranean complicates precise predictions of the future oxygen balance. Insights from the Messinian underscore the crucial role of primary productivity in shaping bottom oxygen conditions, emphasizing the necessity for ongoing investigations.