Revisiting Carbon Flux Through the Ocean's Twilight Zone

Photosynthetic light requirement near the theoretical minimum detected in Arctic microalgae

Photosynthesis is one of the most important biological processes on Earth, providing the main source of bioavailable energy, carbon, and oxygen via the use of sunlight. Despite this importance, the minimum light level sustaining photosynthesis and net growth of primary producers in the global ocean is still unknown. Here, we present measurements from the MOSAiC field campaign in the central Arctic Ocean that reveal the resumption of photosynthetic growth and algal biomass buildup under the ice pack at a daily average irradiance of not more than 0.04 ± 0.02 µmol photons m-2 s-1 in late March. This is at least one order of magnitude lower than previous estimates (0.3-5 µmol photons m-2 s-1) and near the theoretical minimum light requirement of photosynthesis (0.01 µmol photons m-2 s-1). Our findings are based on measurements of the temporal development of the under-ice light field and concurrent measurements of both chlorophyll a concentrations and potential net primary production underneath the sea ice at 86 °N. Such low light requirements suggest that euphotic zones where photosynthesis can occur in the world's oceans may extend further in depth and time, with major implications for global productivity estimates.

Microbial iron limitation in the ocean's twilight zone

Primary production in the sunlit surface ocean is regulated by the supply of key nutrients, primarily nitrate, phosphate and iron (Fe), required by phytoplankton to fix carbon dioxide into biomass1-3. Below the surface ocean, remineralization of sinking organic matter rapidly regenerates nutrients, and microbial metabolism in the upper mesopelagic 'twilight zone' (200-500 m) is thought to be limited by the delivery of labile organic carbon4,5. However, few studies have examined the role of nutrients in shaping microbial production in the mesopelagic6-8. Here we report the distribution and uptake of siderophores, biomarkers for microbial Fe deficiency9 across a meridional section of the eastern Pacific Ocean. Siderophore concentrations are high not only in chronically Fe-limited surface waters but also in the twilight zone underlying the North and South Pacific subtropical gyres, two key ecosystems for the marine carbon cycle. Our findings suggest that bacterial Fe deficiency owing to low Fe availability is probably characteristic of the twilight zone in several large ocean basins, greatly expanding the region of the marine water column in which nutrients limit microbial metabolism, with potential implications for ocean carbon storage.

RotoBOD─Quantifying Oxygen Consumption by Suspended Particles and Organisms

Sinking or floating is the natural state of planktonic organisms and particles in the ocean. Simulating these conditions is critical when making measurements, such as respirometry, because they allow the natural exchange of substrates and products between sinking particles and water flowing around them and prevent organisms that are accustomed to motion from changing their metabolism. We developed a rotating incubator, the RotoBOD (named after its capability to rotate and determine biological oxygen demand, BOD), that uniquely enables automated oxygen measurements in small volumes while keeping the samples in their natural state of suspension. This allows highly sensitive rate measurements of oxygen utilization and subsequent characterization of single particles or small planktonic organisms, such as copepods, jellyfish, or protists. As this approach is nondestructive, it can be combined with several further measurements during and after the incubation, such as stable isotope additions and molecular analyses. This makes the instrument useful for ecologists, biogeochemists, and potentially other user groups such as aquaculture facilities. Here, we present the technical background of our newly developed apparatus and provide examples of how it can be utilized to determine oxygen production and consumption in small organisms and particles.

It's time to broaden what we consider a 'blue carbon ecosystem'

Photoautotrophic marine ecosystems can lock up organic carbon in their biomass and the associated organic sediments they trap over millennia and are thus regarded as blue carbon ecosystems. Because of the ability of marine ecosystems to lock up organic carbon for millennia, blue carbon is receiving much attention within the United Nations' 2030 Agenda for Sustainable Development as a nature-based solution (NBS) to climate change, but classically still focuses on seagrass meadows, mangrove forests, and tidal marshes. However, other coastal ecosystems could also be important for blue carbon storage, but remain largely neglected in both carbon cycling budgets and NBS strategic planning. Using a meta-analysis of 253 research publications, we identify other coastal ecosystems-including mud flats, fjords, coralline algal (rhodolith) beds, and some components or coral reef systems-with a strong capacity to act as blue carbon sinks in certain situations. Features that promote blue carbon burial within these 'non-classical' blue carbon ecosystems included: (1) balancing of carbon release by calcification via carbon uptake at the individual and ecosystem levels; (2) high rates of allochthonous organic carbon supply because of high particle trapping capacity; (3) high rates of carbon preservation and low remineralization rates; and (4) location in depositional environments. Some of these features are context-dependent, meaning that these ecosystems were blue carbon sinks in some locations, but not others. Therefore, we provide a universal framework that can evaluate the likelihood of a given ecosystem to behave as a blue carbon sink for a given context. Overall, this paper seeks to encourage consideration of non-classical blue carbon ecosystems within NBS strategies, allowing more complete blue carbon accounting.

One-third of Southern Ocean productivity is supported by dust deposition

Natural iron fertilization of the Southern Ocean by windblown dust has been suggested to enhance biological productivity and modulate the climate1-3. Yet, this process has never been quantified across the Southern Ocean and at annual timescales4,5. Here we combined 11 years of nitrate observations from autonomous biogeochemical ocean profiling floats with a Southern Hemisphere dust simulation to empirically derive the relationship between dust-iron deposition and annual net community production (ANCP) in the iron-limited Southern Ocean. Using this relationship, we determined the biological response to dust-iron in the pelagic perennially ice-free Southern Ocean at present and during the last glacial maximum (LGM). We estimate that dust-iron now supports 33% ± 15% of Southern Ocean ANCP. During the LGM, when dust deposition was 5-40-fold higher than today, the contribution of dust to Southern Ocean ANCP was much greater, estimated at 64% ± 13%. We provide quantitative evidence of basin-wide dust-iron fertilization of the Southern Ocean and the potential magnitude of its impact on glacial-interglacial timescales, supporting the idea of the important role of dust in the global carbon cycle and climate6-8.

The Vertical Metabolic Activity and Community Structure of Prokaryotes along Different Water Depths in the Kermadec and Diamantina Trenches

Prokaryotes play a key role in particulate organic matter's decomposition and remineralization processes in the vertical scale of seawater, and prokaryotes contribute to more than 70% of the estimated remineralization. However, little is known about the microbial community and metabolic activity of the vertical distribution in the trenches. The composition and distribution of prokaryotes in the water columns and benthic boundary layers of the Kermadec Trench and the Diamantina Trench were investigated using high-throughput sequencing and quantitative PCR, together with the Biolog EcoplateTM microplates culture to analyze the microbial metabolic activity. Microbial communities in both trenches were dominated by Nitrososphaera and Halobacteria in archaea, and by Alphaproteobacteria and Gammaproteobacteria in bacteria, and the microbial community structure was significantly different between the water column and the benthic boundary layer. At the surface water, amino acids and polymers were used preferentially; at the benthic boundary layers, amino acids and amines were used preferentially. Cooperative relationships among different microbial groups and their carbon utilization capabilities could help to make better use of various carbon sources along the water depths, reflected by the predominantly positive relationships based on the co-occurrence network analysis. In addition, the distinct microbial metabolic activity detected at 800 m, which was the lower boundary of the twilight zone, had the lowest salinity and might have had higher proportions of refractory carbon sources than the shallower water depths and benthic boundary layers. This study reflected the initial preference of the carbon source by the natural microbes in the vertical scale of different trenches and should be complemented with stable isotopic tracing experiments in future studies to enhance the understanding of the complex carbon utilization pathways along the vertical scale by prokaryotes among different trenches.

Biological export production controls upper ocean calcium carbonate dissolution and CO2 buffer capacity

Marine biogenic calcium carbonate (CaCO3) cycles play a key role in ecosystems and in regulating the ocean's ability to absorb atmospheric carbon dioxide (CO2). However, the drivers and magnitude of CaCO3 cycling are not well understood, especially for the upper ocean. Here, we provide global-scale evidence that heterotrophic respiration in settling marine aggregates may produce localized undersaturated microenvironments in which CaCO3 particles rapidly dissolve, producing excess alkalinity in the upper ocean. In the deep ocean, dissolution of CaCO3 is primarily driven by conventional thermodynamics of CaCO3 solubility with reduced fluxes of CaCO3 burial to marine sediments beneath more corrosive North Pacific deep waters. Upper ocean dissolution, shown to be sensitive to ocean export production, can increase the neutralizing capacity for respired CO2 by up to 6% in low-latitude thermocline waters. Without upper ocean dissolution, the ocean might lose 20% more CO2 to the atmosphere through the low-latitude upwelling regions.

Diel metabolic patterns revealed by in situ transcriptome and proteome in a vertically migratory copepod

Zooplankton undergo a diel vertical migration (DVM) which exposes them to gradients of light, temperature, oxygen, and food availability on a predictable daily schedule. Disentangling the co-varying and potentially synergistic interactions on metabolic rates has proven difficult, despite the importance of this migration for the delivery of metabolic waste products to the distinctly different daytime (deep) and nighttime (surface) habitats. This study examines the transcriptomic and proteomic profiles of the circumglobal migratory copepod, Pleuromamma xiphias, over the diel cycle. The transcriptome showed that 96% of differentially expressed genes were upregulated during the middle of the day - the period often considered to be of lowest zooplankton activity. The changes in protein abundance were more spread out over time, peaking (42% of comparisons) in the early evening. Between 9:00 and 15:00, both the transcriptome and proteome datasets showed increased expression related to chitin synthesis and degradation. Additionally, at 09:00 and 22:00, there were increases in myosin and vitellogenin proteins, potentially linked to the stress of migration and/or reproductive investment. Based on protein abundances detected, there is an inferred switch in broad metabolic processes, shifting from electron transport system in the day to glycolysis and glycogen mobilization in the afternoon/evening. These observations provide evidence of the diel impact of DVM on transcriptomic and proteomic pathways that likely influence metabolic processes and subsequent excretion products, and clarify how this behaviour results in the direct rapid transport of waste metabolites from the surface to the deep ocean.

Latitudinal gradient and influencing factors of deep-sea particle export along the Kyushu-Palau Ridge in the Philippine Sea

The export of particulate organic matter (POM) to deep-sea is crucial for deep-sea ecosystems. However, in situ measurements of large-scale POM export flux are scarce in the tropical and subtropical western Pacific, leading to reliance on biogeochemical models or sediment trap data from a few stations. To address this gap, the underwater vision profiler was used to measure particulate density and to calculate particulate organic carbon (POC) fluxes along the Kyushu-Palau Ridge (KPR) in the Philippine Sea. The results revealed a significant latitudinal gradient of POC fluxes: 37 % of the POC output from 200 m depth was preserved to 2000 m in the Western Pacific Warm Pool and up to 51 % was preserved in the North Pacific Subtropical Gyre. The near-bottom POC fluxes north of 25°N (1.64 ± 0.80 mg m-2 d-1) were significantly higher than the average near-bottom value of the entire transect (0.60 ± 0.43 mg m-2 d-1). Multiple linear regression analysis showed that the chlorophyll concentration had a significant positive effect on the POC fluxes at all depths, except near the bottom, while local factors such as mesoscale eddies and the interaction effect between the topography and current velocity only had significant effects on the POC fluxes at depths of >2000 m. Particle size spectrum analysis revealed that particles ranging from 64 to 323 μm in size exerted a dominant influence on the increase in the POC fluxes in the near-bottom layers situated north of 25°N. These findings indicated that the spatial heterogeneity of POC fluxes in the western Pacific was governed not only by upper ocean primary productivity but also by mesoscale processes, current velocity, and topography. These results provided crucial fundamental information for cartography of the distribution and simulation of the dynamics of deep-sea organisms along the KPR in the Philippine Sea.

Planktonic microbial signatures of sinking particle export in the open ocean's interior

A considerable amount of particulate carbon produced by oceanic photosynthesis is exported to the deep-sea by the "gravitational pump" (~6.8 to 7.7 Pg C/year), sequestering it from the atmosphere for centuries. How particulate organic carbon (POC) is transformed during export to the deep sea however is not well understood. Here, we report that dominant suspended prokaryotes also found in sinking particles serve as informative tracers of particle export processes. In a three-year time series from oceanographic campaigns in the Pacific Ocean, upper water column relative abundances of suspended prokaryotes entrained in sinking particles decreased exponentially from depths of 75 to 250 m, conforming to known depth-attenuation patterns of carbon, energy, and mass fluxes in the epipelagic zone. Below ~250 m however, the relative abundance of suspended prokaryotes entrained in sinking particles increased with depth. These results indicate that microbial entrainment, colonization, and sinking particle formation are elevated at mesopelagic and bathypelagic depths. Comparison of suspended and sinking particle-associated microbes provides information about the depth-variability of POC export and biotic processes, that is not evident from biogeochemical data alone.

Diel, seasonal and vertical changes in the abundance, biomass and community structure of pelagic polychaetes at the subtropical station S1 in the western North Pacific: comparison with the results from the subarctic station K2

Information on pelagic polychaete community structure in the western North Pacific is available for the subarctic region (Station K2) but not for the subtropical region. Hence, we analyzed day-night vertically stratified samples collected in eight layers within the first 1000 m of the water column during four seasons in 1 year, using the same sampling method as St. K2, at the subtropical region (Station S1). At St. S1, 27 species of pelagic polychaetes belonging to 13 genera and six families were identified. The annual mean abundance was 35.0 ind. 1000 m-3 and the biomass was 17.3 mg WW 1000 m-3. At St. S1, the numbers of genera and species were higher and the annual mean abundance and biomasses were much lower than St. K2. The pelagic polychaetes often peaked in the mesopelagic layer at St. K2, with the carnivores and particle feeders peaking in the epipelagic and mesopelagic layers, respectively. At St.S1, the carnivorous species predominated throughout the entire water column, and were most abundant in the epipelagic layer. Thus, In the western Pacific Ocean, the subarctic pelagic polychaete community structure changed vertically with feeding ecology. On the other hand, the subtropical community may be adapted to conditions of high irradiance and light transmission.

Integrated FT-ICR MS and metabolome reveals diatom-derived organic matter by bacterial transformation under warming and acidification

Bacterial transformation and processing of diatom-derived organic matter (OM) is extremely important for the cycling of production and energy in marine ecosystems; this process contributes to the production of microbial food webs. In this study, a cultivable bacterium (Roseobacter sp. SD-R1) from the marine diatom Skeletonema dohrnii were isolated and identified. A combined Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS)/untargeted metabolomics approach was used to synthesize the results of bacterial transformation with dissolved OM (DOM) and lysate OM (LOM) under warming and acidification through laboratory experiments. Roseobacter sp. SD-R1 had different preferences for the conversion of molecules in S. dohrnii-derived DOM and LOM treatments. The effects of warming and acidification contribute to the increased number and complexity of molecules of carbon, hydrogen, oxygen, nitrogen, and sulfur after the bacterial transformation of OM. The chemical complexity generated by bacterial metabolism provides new insights into the mechanisms that shape OM complexity.

Carbon Export in the Ocean: A Biologist's Perspective

Understanding the nature of organic matter flux in the ocean remains a major goal of oceanography because it impacts some of the most important processes in the ocean. Sinking particles are important for carbon dioxide removal from the atmosphere and its movement to the deep ocean. They also feed life below the ocean's productive surface and sustain life in the deep sea, in addition to depositing organic matter on the seafloor. However, the magnitude of all of these processes is dependent on the transformation of sinking particles during their journey through the water column. This review focuses on the movement of organic matter from the surface ocean to the deep sea via the biological carbon pump and examines the processes that prevent this downward movement-namely, attenuation via microbial colonization and zooplankton feeding. It also discusses how the depth-specific interactions among microbes, zooplankton, and aggregates determine carbon export as well as nutrient recycling, which in turn impact ocean production and Earth's climate.

Quantifying the Ocean's Biological Pump and Its Carbon Cycle Impacts on Global Scales

The biological pump transports organic matter, created by phytoplankton productivity in the well-lit surface ocean, to the ocean's dark interior, where it is consumed by animals and heterotrophic microbes and remineralized back to inorganic forms. This downward transport of organic matter sequesters carbon dioxide from exchange with the atmosphere on timescales of months to millennia, depending on where in the water column the respiration occurs. There are three primary export pathways that link the upper ocean to the interior: the gravitational, migrant, and mixing pumps. These pathways are regulated by vastly different mechanisms, making it challenging to quantify the impacts of the biological pump on the global carbon cycle. In this review, we assess progress toward creating a global accounting of carbon export and sequestration via the biological pump and suggest a path toward achieving this goal.

Prokaryotic Life in the Deep Ocean's Water Column

The oceanic waters below a depth of 200 m represent, in terms of volume, the largest habitat of the biosphere, harboring approximately 70% of the prokaryotic biomass in the oceanic water column. These waters are characterized by low temperature, increasing hydrostatic pressure, and decreasing organic matter supply with depth. Recent methodological advances in microbial oceanography have refined our view of the ecology of prokaryotes in the dark ocean. Here, we review the ecology of prokaryotes of the dark ocean, present data on the biomass distribution and heterotrophic and chemolithoautotrophic prokaryotic production in the major oceanic basins, and highlight the phylogenetic and functional diversity of this part of the ocean. We describe the connectivity of surface and deep-water prokaryotes and the molecular adaptations of piezophilic prokaryotes to high hydrostatic pressure. We also highlight knowledge gaps in the ecology of the dark ocean's prokaryotes and their role in the biogeochemical cycles in the largest habitat of the biosphere.

Metabolic activity and community structure of prokaryotes associated with particles in the twilight zone of the South China Sea

The twilight zone is an important depth of the ocean where particulate organic matter (POM) remineralization takes place, and prokaryotes contribute to more than 70% of the estimated remineralization. However, little is known about the microbial community and metabolic activity associated with different particles in the twilight zone. The composition and distribution of particle-attached prokaryotes in the twilight zone of the South China Sea (SCS) were investigated using high-throughput sequencing and quantitative PCR, together with the Biolog Ecoplate™ microplates culture to analyze the microbial metabolic activity. We found that α- and γ-Proteobacteria dominating at the lower and upper boundary of the twilight zone, respectively; Methanosarcinales and Halobacteriales of the Euyarchaeota occupied in the larger particles at the upper boundary. Similar microbial community existed between euphotic layer and the upper boundary. Higher amount of shared Operational Taxonomic Units (OTUs) in the larger particles along the water depths, might be due to the fast sinking and major contribution of carbon flux of the larger particles from the euphotic layer. In addition to polymers as the major carbon source, carbohydrates and amino acids were preferentially used by microbial community at the upper and lower boundary, respectively. This could potentially be attributed to the metabolic capabilities of attached microbial groups in different particles, and reflected the initial preference of the carbon source by the natural microbes in the twilight zone as well. The microbial structure and carbon metabolic profiles could be complemented with metatranscriptomic analysis in future studies to augment the understanding of the complex carbon cycling pathways in the twilight zone.

Intra- and inter-spatial variability of meiofauna in hadal trenches is linked to microbial activity and food availability

Hadal trenches are depocenters for organic material, and host intensified benthic microbial activity. The enhanced deposition is presumed to be reflected in elevated meiofaunal standing-stock, but available studies are ambiguous. Here, we investigate the distribution of meiofauna along the Atacama Trench axis and adjacent abyssal and bathyal settings in order to relate the meiofauna densities to proxies for food availability. Meiofauna densities peaked at the sediment surface and attenuated steeply with increasing sediment depth. The distribution mirrored the vertical profile of the microbial-driven oxygen consumption rate demonstrating a close linkage between microbial activity and meiofauna density. Meiofaunal standing-stock along the trench axis varied by a factor of two, but were markedly higher than values from the abyssal site at the oceanic plate. Overall, meiofaunal densities poorly correlated with common proxies for food availability such as total organic carbon and phytopigments, but strongly correlated with the microbial benthic O₂ consumption rate. We argue that microbial biomass likely represents an important meiofaunal food source for hadal meiofauna. Observations from three trench systems underlying surface water of highly different productivity confirmed elevated meiofaunal densities at the trench axis as compared to abyssal sites on oceanic plates. Food availability appear to drive elevated abundance and variations in meiofauna densities in hadal sediments.

Influence of Seasonal Variability in Flux Attenuation on Global Organic Carbon Fluxes and Nutrient Distributions

The biological carbon pump is a key component of the marine carbon cycle. This surface-to-deep flux of carbon is usually assumed to follow a simple power law function, which imposes that the surface export flux is attenuated throughout subsurface waters at a rate dictated by the parameterization exponent. This flux attenuation exponent is widely assumed as constant. However, there is increasing evidence that the flux attenuation varies both spatially and seasonally. While the former has received some attention, the consequences of the latter have not been explored. Here we aim to fill the gap with a theoretical study of how seasonal changes in both flux attenuation and sinking speed affect nutrient distributions and carbon fluxes. Using a global ocean-biogeochemical model that represents detritus explicitly, we look at different scenarios for how these varies seasonally, particularly the relative "phase" with respect to solar radiation and the "strength" of seasonality. We show that the sole presence of seasonality in the model-imposed flux attenuation and sinking speed leads to a greater transfer efficiency compared to the non-seasonal flux attenuation scenario, resulting in an increase of over 140% in some cases when the amplitude of the seasonality imposed is 60% of the non-seasonal base value. This work highlights the importance of the feedback taking place between the seasonally varying flux attenuation, sinking speed and other processes, suggesting that the assumption of constant-in-time flux attenuation and sinking speed might underestimate how much carbon is sequestered by the biological carbon pump.

In situ experimental evidences for responses of abyssal benthic biota to shifts in phytodetritus compositions linked to global climate change

Abyssal plains cover more than half of Earth's surface, and the main food source in these ecosystems is phytodetritus, mainly originating from primary producers in the euphotic zone of the ocean. Global climate change is influencing phytoplankton abundance, productivity, and distribution. Increasing importance of picoplankton over diatom as primary producers in surface oceans (especially projected for higher latitudes) is projected and hence altering the quantity of organic carbon supplied to the abyssal seafloor as phytodetritus, consequences of which remain largely unknown. Here, we investigated the in situ responses of abyssal biota from viruses to megafauna to different types of phytoplankton input (diatoms or cyanobacteria which were labeled with stable isotopes) at equatorial (oligotrophic) and temperate (eutrophic) benthic sites in the Pacific Ocean (1°N at 4277 m water depth and 39°N at 5260 m water depth, respectively). Our results show that meiofauna and macrofauna generally preferred diatoms as a food source and played a relatively larger role in the consumption of phytodetritus at higher latitudes (39°N). Contrarily, prokaryotes and viruses showed similar or even stronger responses to cyanobacterial than to diatom supply. Moreover, the response of prokaryotes and viruses was very rapid (within 1-2 days) at both 1°N and 39°N, with quickest responses reported in the case of cyanobacterial supply at higher latitudes. Overall, our results suggest that benthic deep-sea eukaryotes will be negatively affected by the predicted decrease in diatoms in surface oceans, especially at higher latitudes, where benthic prokaryotes and viruses will otherwise likely increase their quantitative role and organic carbon cycling rates. In turn, such changes can contribute to decrease carbon transfer from phytodetritus to higher trophic levels, with strong potential to affect oceanic food webs, their biodiversity and consequently carbon sequestration capacity at the global scale.

Nitrifiers drive successions of particulate organic matter and microbial community composition in a starved macrocosm

Organic carbon produced by nitrifiers plays an important role in maintaining the microbial metabolism in the aphotic ocean layer with carbon and energy scarcity. However, the contribution of nitrifiers to organic carbon processing remains unclear. To explore how nitrification impacts the material cycle in the starved ecosystem, we set up an ultra-large volume, long-term incubation experiment. Seawater collected from the Halifax coastal ocean was pumped into the Aquatron Tower Tank located at Dalhousie University, Canada, and was incubated under dark conditions for 73 days. The results indicated that the relative abundance of nitrifiers increased and nitrification was strengthened in the dark system where energy and organic carbon were scarce. The importance of nitrogenous compounds in particulate materials increased over the course of the incubation. Correlation analysis showed that the relative abundances of nitrifiers and particulate organic compounds containing nitrogen were significantly and positively correlated. Furthermore, network analysis suggested that metabolic processes related to nitrogenous and aromatic compounds are most important to particle associated bacteria. This study suggests that the nitrifiers could produce a series of organic compounds that result in the alteration of organic matter composition by promoting the degradation of recalcitrant aromatic compounds, which has important implications for organic matter processing in the starved dark ecosystem.

Seasonal Variability of Photosynthetic Microbial Eukaryotes (<3 µm) in the Kara Sea Revealed by 18S rDNA Metabarcoding of Sediment Trap Fluxes

This survey is the first to explore the seasonal cycle of microbial eukaryote diversity (<3 µm) using the NGS method and a 10-month sediment trap (2018–2019). The long-term trap was deployed from September to June in the northwestern part of the Kara Sea. A water sample collected before the sediment trap was deployed and also analyzed. The taxonomic composition of microbial eukaryotes in the water sample significantly differed from sediment trap samples, characterized by a high abundance of Ciliophora reads and low abundance of Fungi while trap samples contained an order of magnitude less Ciliophora sequences and high contribution of Fungi. Photosynthetic eukaryotes (PEs) accounting for about 34% of total protists reads were assigned to five major divisions: Chlorophyta, Cryptophyta, Dinoflagellata, Haptophyta, and Ochrophyta. The domination of phototrophic algae was revealed in late autumn. Mamiellophyceae and Trebouxiophyceae were the predominant PEs in mostly all of the studied seasons. Micromonas polaris was constantly present throughout the September–June period in the PE community. The obtained results determine the seasonal dynamics of picoplankton in order to improve our understanding of their role in polar ecosystems.

Microbes mediating the sulfur cycle in the Atlantic Ocean and their link to chemolithoautotrophy

Only about 10%-30% of the organic matter produced in the epipelagic layers reaches the dark ocean. Under these limiting conditions, reduced inorganic substrates might be used as an energy source to fuel prokaryotic chemoautotrophic and/or mixotrophic activity. The aprA gene encodes the alpha subunit of the adenosine-5'-phosphosulfate (APS) reductase, present in sulfate-reducing (SRP) and sulfur-oxidizing prokaryotes (SOP). The sulfur-oxidizing pathway can be coupled to inorganic carbon fixation via the Calvin-Benson-Bassham cycle. The abundances of aprA and cbbM, encoding RuBisCO form II (the key CO2 fixing enzyme), were determined over the entire water column along a latitudinal transect in the Atlantic from 64°N to 50°S covering six oceanic provinces. The abundance of aprA and cbbM genes significantly increased with depth reaching the highest abundances in meso- and upper bathypelagic layers. The contribution of cells containing these genes also increased from mesotrophic towards oligotrophic provinces, suggesting that under nutrient limiting conditions alternative energy sources are advantageous. However, the aprA/cbbM ratios indicated that only a fraction of the SOP is associated with inorganic carbon fixation. The aprA harbouring prokaryotic community was dominated by Pelagibacterales in surface and mesopelagic waters, while Candidatus Thioglobus, Chromatiales and the Deltaproteobacterium_SCGC dominated the bathypelagic realm. Noticeably, the contribution of the SRP to the prokaryotic community harbouring aprA gene was low, suggesting a major utilization of inorganic sulfur compounds either as an energy source (occasionally coupled with inorganic carbon fixation) or in biosynthesis pathways.

A Visual Tour of Carbon Export by Sinking Particles

To better quantify the ocean's biological carbon pump, we resolved the diversity of sinking particles that transport carbon into the ocean's interior, their contribution to carbon export, and their attenuation with depth. Sinking particles collected in sediment trap gel layers from four distinct ocean ecosystems were imaged, measured, and classified. The size and identity of particles was used to model their contribution to particulate organic carbon (POC) flux. Measured POC fluxes were reasonably predicted by particle images. Nine particle types were identified, and most of the compositional variability was driven by the relative contribution of aggregates, long cylindrical fecal pellets, and salp fecal pellets. While particle composition varied across locations and seasons, the entire range of compositions was measured at a single well-observed location in the subarctic North Pacific over one month, across 500 m of depth. The magnitude of POC flux was not consistently associated with a dominant particle class, but particle classes did influence flux attenuation. Long fecal pellets attenuated most rapidly with depth whereas certain other classes attenuated little or not at all with depth. Small particles (<100 μm) consistently contributed ∼5% to total POC flux in samples with higher magnitude fluxes. The relative importance of these small particle classes (spherical mini pellets, short oval fecal pellets, and dense detritus) increased in low flux environments (up to 46% of total POC flux). Imaging approaches that resolve large variations in particle composition across ocean basins, depth, and time will help to better parameterize biological carbon pump models.

Relating sinking and suspended microbial communities in the California Current Ecosystem: digestion resistance and the contributions of phytoplankton taxa to export

We used 16S, 18S, plastid and internal transcribed spacer (for Synechococcus strains) sequencing to quantify relative microbial abundances in water-column samples and on sediment-trap-collected particles across an environmental gradient in the California Current Ecosystem (CCE) spanning a > 60-fold range of surface chlorophyll. Most mixed-layer dominant eukaryotes and prokaryotes were consistently underrepresented on sinking particles. Diatoms were the only phototrophic taxa consistently overrepresented. Even within this class, however, one genus (Thalassiosira) was a particle-enriched dominant, while a similarly abundant species was poorly represented. Synechococcus was significantly enriched on sinking particles at only one of four sites, but clade I was disproportionately abundant on sinking particles throughout the region compared with clade IV, the euphotic-zone co-dominant. The most abundant microbes on particles across the CCE were organisms with distributional maxima close to the sediment-trap depth (rhizarians), microbes associated with metazoans or sinking particles as a nutritional habitat (certain alveolates, Gammaproteobacteria) and organisms that resist digestive degradation of their DNA (Thalassiosira, Synechococcus). For assessing taxon contributions of phytoplankton to carbon export, our results highlight the need for sequence-based quantitative approaches that can be used to integrate euphotic-zone abundances, compute rates and account for taxon differences in preservation of sequence markers through trophic processing.

From Nano-Gels to Marine Snow: A Synthesis of Gel Formation Processes and Modeling Efforts Involved with Particle Flux in the Ocean

Marine gels (nano-, micro-, macro-) and marine snow play important roles in regulating global and basin-scale ocean biogeochemical cycling. Exopolymeric substances (EPS) including transparent exopolymer particles (TEP) that form from nano-gel precursors are abundant materials in the ocean, accounting for an estimated 700 Gt of carbon in seawater. This supports local microbial communities that play a critical role in the cycling of carbon and other macro- and micro-elements in the ocean. Recent studies have furthered our understanding of the formation and properties of these materials, but the relationship between the microbial polymers released into the ocean and marine snow remains unclear. Recent studies suggest developing a (relatively) simple model that is tractable and related to the available data will enable us to step forward into new research by following marine snow formation under different conditions. In this review, we synthesize the chemical and physical processes. We emphasize where these connections may lead to a predictive, mechanistic understanding of the role of gels in marine snow formation and the biogeochemical functioning of the ocean.

Illuminating key microbial players and metabolic processes involved in the remineralization of particulate organic carbon in the ocean's twilight zone by metaproteomics

The twilight zone (from the base of the euphotic zone to the depth of 1000 m) is the major area of particulate organic carbon (POC) remineralization in the ocean, and heterotrophic microbes contribute to more than 70% of the estimated remineralization. However, little is known about the microbial community and metabolic activity directly associated with POC remineralization in this chronically understudied realm. Here, we characterized the microbial community proteomes of POCs collected from the twilight zone of three contrasting sites in the Northwest Pacific Ocean using a metaproteomic approach. The particle-attached bacteria from Alteromonadales, Rhodobacterales, and Enterobacteriales were the primary POC remineralizers. Hydrolytic enzymes, including proteases and hydrolases, that degrade proteinaceous components and polysaccharides, the main constituents of POC, were abundant and taxonomically associated with these bacterial groups. Furthermore, identification of diverse species-specific transporters and metabolic enzymes implied niche specialization for nutrient acquisition among these bacterial groups. Temperature was the main environmental factor driven the active bacterial groups and metabolic processes, and Enterobacteriales replaced Alteromonadales as the predominant group under low temperature. This study provides insight into the key bacteria and metabolic processes involved in POC remineralization, and niche complementarity and species substitution among bacterial groups are critical for efficient POC remineralization in the twilight zone. IMPORTANCE The Ocean's twilight zone is a critical zone where more than 70% of the sinking particulate organic carbon (POC) are remineralized. Therefore, the twilight zone determines the size of biological carbon storage in the ocean, and regulates the global climate. Prokaryotes are major players that govern remineralization of POC in this region. However, knowledge of microbial community structure and metabolic activity is still lacking. This study unveiled microbial communities and metabolic activities of POCs collected from the twilight zone of three contrasting environments in the Northwest Pacific Ocean using a metaproteomic approach. Alteromonadales, Rhodobacterales and Enterobacteriales were the major remineralizers of POC. They excreted diverse species-specific hydrolytic enzymes to split POC to solubilized POC or dissolved organic carbon. Temperature played a crucial role in regulating the community composition and metabolism. Furthermore, niche complementarity or species substitution among bacterial groups guaranteed the efficient remineralization of POC in the twilight zone.

Respiration by "marine snow" at high hydrostatic pressure: Insights from continuous oxygen measurements in a rotating pressure tank

It is generally anticipated that particulate organic carbon (POC) for most part is degraded by attached microorganisms during the descent of "marine snow" aggregates toward the deep sea. There is, however, increasing evidence that fresh aggregates can reach great depth and sustain relatively high biological activity in the deep sea. Using a novel high-pressure setup, we tested the hypothesis that increasing levels of hydrostatic pressure inhibit POC degradation in aggregates rapidly sinking to the ocean interior. Respiration activity, a proxy for POC degradation, was measured directly and continuously at up to 100 MPa (corresponding to 10 km water depth) in a rotating pressure tank that keeps the aggregates in a sinking mode. Model diatom-bacteria aggregates, cultures of the aggregate-forming diatom Skeletonema marinoi, and seawater microbial communities devoid of diatoms showed incomplete and complete inhibition of respiration activity when exposed to pressure levels of 10-50 and 60-100 MPa, respectively. This implies reduced POC degradation and hence enhanced POC export to hadal trenches through fast-sinking, pressure-exposed aggregates. Notably, continuous respiration measurements at ≥50 MPa revealed curved instead of linear oxygen time series whenever S. marinoi was present, which was not captured by discrete respiration measurements. These curvatures correspond to alternating phases of high and low respiration activity likely connected to pressure effects on unidentified metabolic processes in S. marinoi.

A hybrid underwater robot for multidisciplinary investigation of the ocean twilight zone

Mesobot, an autonomous underwater vehicle, addresses specific unmet needs for observing and sampling a variety of phenomena in the ocean's midwaters. The midwater hosts a vast biomass, has a role in regulating climate, and may soon be exploited commercially, yet our scientific understanding of it is incomplete. Mesobot has the ability to survey and track slow-moving animals and to correlate the animals' movements with critical environmental measurements. Mesobot will complement existing oceanographic assets such as towed, remotely operated, and autonomous vehicles; shipboard acoustic sensors; and net tows. Its potential to perform behavioral studies unobtrusively over long periods with substantial autonomy provides a capability that is not presently available to midwater researchers. The 250-kilogram marine robot can be teleoperated through a lightweight fiber optic tether and can also operate untethered with full autonomy while minimizing environmental disturbance. We present recent results illustrating the vehicle's ability to automatically track free-swimming hydromedusae (Solmissus sp.) and larvaceans (Bathochordaeus stygius) at depths of 200 meters in Monterey Bay, USA. In addition to these tracking missions, the vehicle can execute preprogrammed missions collecting image and sensor data while also carrying substantial auxiliary payloads such as cameras, sonars, and samplers.

Rapid photoacclimation during episodic deep mixing augments the biological carbon pump

Episodic deep mixing events are one component of the biological carbon pump that physically transports organic carbon into the mesopelagic. Episodic deep mixing also disrupts summertime thermal stratification thereby changing the light field and nutrient concentrations available for phytoplankton growth. Phytoplankton survival and growth below the mixed layer following restratification depends on how rapidly cells can employ a variety of photoacclimation processes in response to the environmental changes. To compare the relative timescales of summertime episodic deep mixing events with the timescales of phytoplankton photoacclimation processes, we first analyzed autonomous float data to survey the frequency and magnitude of deep mixing events in the western North Atlantic Ocean. Next, we simulated a sustained deep mixing event in the laboratory and measured rates of acclimation processes ranging from light harvesting to growth in a model diatom and green alga. In both algae increases in chlorophyll (Chl) were coupled to growth, but growth of the green alga lagged the diatom by about a day. In float profiles, significant increases in Chl and phytoplankton carbon (C _phyto) were detected below the mixed layer following episodic deep mixing events. These events pose a previously unrecognized source of new production below the mixed layer that can significantly boost the amount of carbon available for export to the deep ocean.

Metaproteomics Reveals Similar Vertical Distribution of Microbial Transport Proteins in Particulate Organic Matter Throughout the Water Column in the Northwest Pacific Ocean

Solubilized particulate organic matter (POM) rather than dissolved organic matter (DOM) has been speculated to be the major carbon and energy sources for heterotrophic prokaryotes in the ocean. However, the direct evidence is still lack. Here we characterized microbial transport proteins of POM collected from both euphotic (75 m, deep chlorophyll maximum DCM, and 100 m) and upper-twilight (200 m and 500 m) zones in three contrasting environments in the northwest Pacific Ocean using a metaproteomic approach. The proportion of transport proteins was relatively high at the bottom of the euphotic zone (200 m), indicating that this layer was the most active area of microbe-driven POM remineralization in the water column. In the upper-twilight zone, the predicted substrates of the identified transporters indicated that amino acids, carbohydrates, taurine, inorganic nutrients, urea, biopolymers, and cobalamin were essential substrates for the microbial community. SAR11, Rhodobacterales, Alteromonadales, and Enterobacteriales were the key contributors with the highest expression of transporters. Interestingly, both the taxonomy and function of the microbial communities varied among water layers and sites with different environments; however, the distribution of transporter types and their relevant organic substrates were similar among samples, suggesting that microbial communities took up similar compounds and were functionally redundant in organic matter utilization throughout the water column. The similar vertical distribution of transport proteins from the euphotic zone to the upper twilight zone among the contrasting environments indicated that solubilized POM rather than DOM was the preferable carbon and energy sources for the microbial communities.

Introducing climate change into the biochemistry and molecular biology curriculum

Our climate is changing due to anthropogenic emissions of greenhouse gases from the production and use of fossil fuels. Present atmospheric levels of CO₂ were last seen 3 million years ago, when planetary temperature sustained high Arctic camels. As scientists and educators, we should feel a professional responsibility to discuss major scientific issues like climate change, and its profound consequences for humanity, with students who look up to us for knowledge and leadership, and who will be most affected in the future. We offer simple to complex backgrounds and examples to enable and encourage biochemistry educators to routinely incorporate this most important topic into their classrooms.

Reconciling atmospheric CO2, weathering, and calcite compensation depth across the Cenozoic

The Cenozoic era (66 to 0 million years) is marked by long-term aberrations in carbon cycling and large climatic shifts, some of which challenge the current understanding of carbon cycle dynamics. Here, we investigate possible mechanisms responsible for the observed long-term trends by using a novel approach that features a full-fledged ocean carbonate chemistry model. Using a compilation of pCO2, pH, and calcite compensation depth (CCD) observational evidence and a suite of simulations, we reconcile long-term Cenozoic climate and CCD trends. We show that the CCD response was decoupled from changes in silicate and carbonate weathering rates, challenging the continental uplift hypothesis. The two dominant mechanisms for decoupling are shelf-basin carbonate burial fractionation combined with proliferation of pelagic calcifiers. The temperature effect on remineralization rates of marine organic matter also plays a critical role in controlling the carbon cycle dynamics, especially during the warmer periods of the Cenozoic.

LifeWatch observatory data: phytoplankton observations in the Belgian Part of the North Sea

Background

This paper describes a phytoplankton data series generated through systematic observations in the Belgian Part of the North Sea (BPNS). Phytoplankton samples were collected during multidisciplinary sampling campaigns, visiting nine nearshore stations with monthly frequency and an additional eight offshore stations on a seasonal basis.

New information

The data series contain taxon-specific phytoplankton densities determined by analysis with the Flow Cytometer And Microscope (FlowCAM®) and associated image-based classification. The classification is performed by two separate semi-automated classification systems, followed by manual validation by taxonomic experts. To date, 637,819 biological particles have been collected and identified, yielding a large dataset of validated phytoplankton images. The collection and processing of the 2017–2018 dataset are described, along with its data curation, quality control and data storage. In addition, the classification of images using image classification algorithms, based on convolutional neural networks (CNN) from 2019 onwards, is also described. Data are published in a standardised format together with environmental parameters, accompanied by extensive metadata descriptions and finally labelled with digital identifiers for traceability. The data are published under a CC‐BY 4.0 licence, allowing the use of the data under the condition of providing the reference to the source.

Exploring Evolutionary Fitness in Biological Systems Using Machine Learning Methods

Here, we propose a computational approach to explore evolutionary fitness in complex biological systems based on empirical data using artificial neural networks. The essence of our approach is the following. We first introduce a ranking order of inherited elements (behavioral strategies or/and life history traits) in considered self-reproducing systems: we use available empirical information on selective advantages of such elements. Next, we introduce evolutionary fitness, which is formally described as a certain function reflecting the introduced ranking order. Then, we approximate fitness in the space of key parameters using a Taylor expansion. To estimate the coefficients in the Taylor expansion, we utilize artificial neural networks: we construct a surface to separate the domains of superior and interior ranking of pair inherited elements in the space of parameters. Finally, we use the obtained approximation of the fitness surface to find the evolutionarily stable (optimal) strategy which maximizes fitness. As an ecologically important study case, we apply our approach to explore the evolutionarily stable diel vertical migration of zooplankton in marine and freshwater ecosystems. Using machine learning we reconstruct the fitness function of herbivorous zooplankton from empirical data and predict the daily trajectory of a dominant species in the northeastern Black Sea.

The value of scientific research on the ocean's biological carbon pump

The ocean's biological carbon pump (BCP) sequesters carbon from the surface to the deep ocean and seabed, constituting one of Earth's most valuable ecosystem services. Significant uncertainty exists surrounding the amounts and rates of organic carbon sequestered in the oceans, however. With improved understanding of BCP sequestration, especially its scale, world policymakers would be positioned to make more informed decisions regarding the mitigation of carbon emissions. Here, an analytical model of the economic effects of global carbon emissions-including scientific uncertainty about BCP sequestration-was developed to estimate the value of marine scientific research concerning sequestration. The discounted net economic benefit of a putative 20-year scientific research program to narrow the range of uncertainty around the amount of carbon sequestered in the ocean is on the order of $0.5 trillion (USD), depending upon the accuracy of predictions, the convexities of climate damage and economic output functions, and the initial range of uncertainty.

Deep-sea meiofauna assemblages with special reference to marine nematodes in the Caiwei Guyot and a Polymetallic Nodule Field in the Pacific Ocean

Meiofauna particularly marine nematodes around the Caiwei Guyot in the northwest Pacific Ocean and a Polymetallic Nodule Field in the northeast Pacific Ocean were studied. Due to the geographic structure, the Caiwei Guyot and the Polymetallic Nodule Field had different environmental characteristics. Meiofaunal abundances around the Guyot area ranged from 9.18 to 25.59 ind./10 cm², which were much lower than those in the Polymetallic Nodule Field. Marine nematode was the most dominant group. A total of 123 species, belonging to 74 genera and 29 families were found. Xyalidae (21.43%), Cyatholaimidae (9.82%), Linhomoeidae (8.03%) were the dominant families. The values of species number, Margalef's species richness and Shannon-Wiener diversity index ranged from 15 to 62, 4.75 to 12.84 and 2.58 to 3.93, respectively. The combination of water depth, silt-clay content and chlorophyll-a concentration can best explain the differences of nematode community. This study provides a baseline for deep-sea meiofauna distribution.

Inorganic and Organic Carbon Uptake Processes and Their Connection to Microbial Diversity in Meso- and Bathypelagic Arctic Waters (Eastern Fram Strait)

The deep Arctic Ocean is increasingly vulnerable to climate change effects, yet our understanding of its microbial processes is limited. We collected samples from shelf waters, mesopelagic Atlantic Waters (AW) and bathypelagic Norwegian Sea Deep Waters (NSDW) in the eastern Fram Strait, along coast-to-offshore transects off Svalbard during boreal summer. We measured community respiration, heterotrophic carbon production (HCP), and dissolved inorganic carbon utilization (DICu) together with prokaryotic abundance, diversity, and metagenomic predictions. In deep samples, HCP was significantly faster in AW than in NSDW, while we observed no differences in DICu rates. Organic carbon uptake was higher than its inorganic counterpart, suggesting a major reliance of deep microbial Arctic communities on heterotrophic metabolism. Community structure and spatial distribution followed the hydrography of water masses. Distinct from other oceans, the most abundant OTU in our deep samples was represented by the archaeal MG-II. To address the potential biogeochemical role of each water mass-specific microbial community, as well as their link with the measured rates, PICRUSt-based predicted metagenomes were built. The results showed that pathways of auto- and heterotrophic carbon utilization differed between the deep water masses, although this was not reflected in measured DICu rates. Our findings provide new insights to understand microbial processes and diversity in the dark Arctic Ocean and to progress toward a better comprehension of the biogeochemical cycles and their trends in light of climate changes.

Antarctic Futures: An Assessment of Climate-Driven Changes in Ecosystem Structure, Function, and Service Provisioning in the Southern Ocean

In this article, we analyze the impacts of climate change on Antarctic marine ecosystems. Observations demonstrate large-scale changes in the physical variables and circulation of the Southern Ocean driven by warming, stratospheric ozone depletion, and a positive Southern Annular Mode. Alterations in the physical environment are driving change through all levels of Antarctic marine food webs, which differ regionally. The distributions of key species, such as Antarctic krill, are also changing. Differential responses among predators reflect differences in species ecology. The impacts of climate change on Antarctic biodiversity will likely vary for different communities and depend on species range. Coastal communities and those of sub-Antarctic islands, especially range-restricted endemic communities, will likely suffer the greatest negative consequences of climate change. Simultaneously, ecosystem services in the Southern Ocean will likely increase. Such decoupling of ecosystem services and endemic species will require consideration in the management of human activities such as fishing in Antarctic marine ecosystems.

Role of suspension feeders in antarctic pelagic-benthic coupling: Trophic ecology and potential carbon sinks under climate change

Sea-ice and coastal glacier loss in the Western Antarctic Peninsula open new ice-free areas. They allowing primary production and providing new seabed for colonisation, both acting as a negative feedback of climate change. However, the injection of sediment-laden runoff from the melting of land-terminating glaciers may reduce this feedback. Changes in particulate matter will affect nutrition and excretion (faeces stoichiometry and properties) of suspension feeders, reshaping coastal carbon dynamics and pelagic-benthic coupling. Absorption efficiency and biodeposition of Euphausia superba and Cnemidocarpa verrucosa were quantified for different food treatments and varying sediment concentrations. Both species showed high overall absorption efficiency for free-sediment diets, but were negatively affected by sediment addition. High sediment conditions increased krill biodeposition, while it decreased in ascidians. Energy balance estimation indicated high carbon sink potential in ascidians, but it is modulated by food characteristics and negatively affected by sediment inputs in the water column.

Impacts of Physical and Biological Processes on Spatial and Temporal Variability of Particulate Organic Carbon in the North Pacific Ocean during 2003-2017

The North Pacific Ocean is a significant carbon sink region, but little is known about the dynamics of particulate organic carbon (POC) and the influences of physical and biological processes in this region at the basin scale. Here, we analysed high-resolution surface POC data derived from MODIS-Aqua during 2003-2017, together with satellite-derived sea surface chlorophyll and temperature (SST). There are large spatial and temporal variations in surface POC in the North Pacific. Surface POC is much lower in the subtropical region (<50 mg m-3) than in the subarctic region (>100 mg m-3), primarily resulting from the south-to-north variability in biological production. Our analyses show significant seasonal and interannual variability in surface POC. In particular, there is one peak in winter-spring in the western subtropical region and two peaks in late spring and fall in the western subarctic region. Surface POC is positively correlated with chlorophyll (r = ~1) and negatively correlated with SST (r = ~-0.45, P < 0.001) south of 45°N, indicating the strong influence of physically driven biological activity on the temporal variability of POC in the subtropical region. There is a significantly positive but relatively lower correlation coefficient (0.6-0.8) between POC and chlorophyll and an overall non-significantly positive correlation between POC and SST north of 45°N, reflecting the reduction in the POC standing stock due to the fast sinking of large particles. The climate modes of the Pacific Decadal Oscillation, El Niño-Southern Oscillation and North Pacific Gyre Oscillation have large impacts on POC in various seasons in the subtropical region and weak influences in the subarctic region. Surface POC was anomalously high after 2013 (increased by ~15%) across the basin, which might be the result of complex interactions of physical and biological processes associated with an anomalous warming event (the Blob).

Modelling optimal behavioural strategies in structured populations using a novel theoretical framework

Understanding complex behavioural patterns of organisms observed in nature can be facilitated using mathematical modelling. The conventional paradigm in animal behavior modelling consists of maximisation of some evolutionary fitness function. However, the definition of fitness of an organism or population is generally subjective, and using different criteria can lead us to contradictory model predictions regarding optimal behaviour. Moreover, structuring of natural populations in terms of individual size or developmental stage creates an extra challenge for theoretical modelling. Here we revisit and formalise the definition of evolutionary fitness to describe long-term selection of strategies in deterministic self-replicating systems for generic modelling settings which involve an arbitrary function space of inherited strategies. Then we show how optimal behavioural strategies can be obtained for different developmental stages in a generic von-Foerster stage-structured population model with an arbitrary mortality term. We implement our theoretical framework to explore patterns of optimal diel vertical migration (DVM) of two dominant zooplankton species in the north-eastern Black Sea. We parameterise the model using 7 years of empirical data from 2007-2014 and show that the observed DVM can be explained as the result of a trade-off between depth-dependent metabolic costs for grazers, anoxia zones, available food, and visual predation.

Error Budget in the Validation of Radiometric Products Derived from OLCI around the China Sea from Open Ocean to Coastal Waters Compared with MODIS and VIIRS

The accuracy of remote-sensing reflectance (Rrs) estimated from ocean color imagery through the atmospheric correction step is essential in conducting quantitative estimates of the inherent optical properties and biogeochemical parameters of seawater. Therefore, finding the main source of error is the first step toward improving the accuracy of Rrs. However, the classic validation exercises provide only the total error of the retrieved Rrs. They do not reveal the error sources. Moreover, how to effectively improve this satellite algorithm remains unknown. To better understand and improve various aspects of the satellite atmospheric correction algorithm, the error budget in the validation is required. Here, to find the primary error source from the OLCI Rrs, we evaluated the OLCI Rrs product with in-situ data around the China Sea from open ocean to coastal waters and compared them with the MODIS-AQUA and VIIRS products. The results show that the performances of OLCI are comparable to those MODIS-AQUA. The average percentage difference (APD) in Rrs is lowest at 490 nm (18%), and highest at 754 nm (79%). A more detailed analysis reveals that open ocean and coastal waters show opposite results: compared to coastal waters the satellite Rrs in open seas are higher than the in-situ measured values. An error budget for the three satellite-derived Rrs products is presented, showing that the primary error source in the China Sea was the aerosol estimation and the error on the Rayleigh-corrected radiance for OLCI, as well as for MODIS and VIIRS. This work suggests that to improve the accuracy of Sentinel-3A in the coastal waters of China, the accuracy of aerosol estimation in atmospheric correction must be improved.

Revealing Evolutionarily Optimal Strategies in Self-Reproducing Systems via a New Computational Approach

Modelling the evolution of complex life history traits and behavioural patterns observed in the natural world is a challenging task. Here, we develop a novel computational method to obtain evolutionarily optimal life history traits/behavioural patterns in population models with a strong inheritance. The new method is based on the reconstruction of evolutionary fitness using underlying equations for population dynamics and it can be applied to self-reproducing systems (including complicated age-structured models), where fitness does not depend on initial conditions, however, it can be extended to some frequency-dependent cases. The technique provides us with a tool to efficiently explore both scalar-valued and function-valued traits with any required accuracy. Moreover, the method can be implemented even in the case where we ignore the underlying model equations and only have population dynamics time series. As a meaningful ecological case study, we explore optimal strategies of diel vertical migration (DVM) of herbivorous zooplankton in the vertical water column which is a widespread phenomenon in both oceans and lakes, generally considered to be the largest synchronised movement of biomass on Earth. We reveal optimal trajectories of daily vertical motion of zooplankton grazers in the water column depending on the presence of food and predators. Unlike previous studies, we explore both scenarios of DVM with static and dynamic predators. We find that the optimal pattern of DVM drastically changes in the presence of dynamic predation. Namely, with an increase in the amount of food available for zooplankton grazers, the amplitude of DVM progressively increases, whereas for static predators DVM would abruptly cease.

Picoplankton Distribution and Activity in the Deep Waters of the Southern Adriatic Sea

Southern Adriatic (Eastern Mediterranean Sea) is a region strongly dominated by large-scale oceanographic processes and local open-ocean dense water formation. In this study, picoplankton biomass, distribution, and activity were examined during two oceanographic cruises and analyzed in relation to environmental parameters and hydrographic conditions comparing pre and post-winter phases (December 2015, April 2016). Picoplankton density with the domination of autotrophic biomasses was higher in the pre-winter phase when significant amounts of picoaoutotrophs were also found in the meso-and bathy-pelagic layers, while Synechococcus dominated the picoautotrophic group. Higher values of bacterial production and domination of High Nucleic Acid content bacteria (HNA bacteria) were found in deep waters, especially during the post-winter phase, suggesting that bacteria can have an active role in the deep-sea environment. Aerobic anoxygenic phototrophic bacteria accounted for a small proportion of total heterotrophic bacteria but contributed up to 4% of bacterial carbon content. Changes in the picoplankton community were mainly driven by nutrient availability, heterotrophic nanoflagellates abundance, and water mass movements and mixing. Our results suggest that autotrophic and heterotrophic members of the picoplankton community are an important carbon source in the food web in the deep-sea, as well as in the epipelagic layer. Besides, viral lysis may affect the activity of the picoplankton community and enrich the water column with dissolved organic carbon.

Drivers of Carbon Export Efficiency in the Global Ocean

The export of organic carbon from the surface ocean forms the basis of the biological carbon pump, an important planetary carbon flux. Typically, only a small fraction of primary productivity (PP) is exported (quantified as the export efficiency: export/PP). Here we assemble a global data synthesis to reveal that very high export efficiency occasionally occurs. These events drive an apparent inverse relationship between PP and export efficiency, which is opposite to that typically used in empirical or mechanistic models. At the global scale, we find that low PP, high export efficiency regimes tend to occur when macrozooplankton and bacterial abundance are low. This implies that a decoupling between PP and upper ocean remineralization processes can result in a large fraction of PP being exported, likely as intact cells or phytoplankton-based aggregates. As the proportion of PP being exported declines, macrozooplankton and bacterial abundances rise. High export efficiency, high PP regimes also occur infrequently, possibly associated with nonbiologically mediated export of particles. A similar analysis at a biome scale reveals that the factors affecting export efficiency may be different at regional and global scales. Our results imply that the whole ecosystem structure, rather than just the phytoplankton community, is important in setting export efficiency. Further, the existence of low PP, high export efficiency regimes imply that biogeochemical models that parameterize export efficiency as increasing with PP may underestimate export flux during decoupled periods, such as at the start of the spring bloom.

Shallow particulate organic carbon regeneration in the South Pacific Ocean

Particulate organic carbon (POC) produced in the surface ocean sinks through the water column and is respired at depth, acting as a primary vector sequestering carbon in the abyssal ocean. Atmospheric carbon dioxide levels are sensitive to the length (depth) scale over which respiration converts POC back to inorganic carbon, because shallower waters exchange with the atmosphere more rapidly than deeper ones. However, estimates of this carbon regeneration length scale and its spatiotemporal variability are limited, hindering the ability to characterize its sensitivity to environmental conditions. Here, we present a zonal section of POC fluxes at high vertical and spatial resolution from the GEOTRACES GP16 transect in the eastern tropical South Pacific, based on normalization to the radiogenic thorium isotope ²³⁰Th. We find shallower carbon regeneration length scales than previous estimates for the oligotrophic South Pacific gyre, indicating less efficient carbon transfer to the deep ocean. Carbon regeneration is strongly inhibited within suboxic waters near the Peru coast. Canonical Martin curve power laws inadequately capture POC flux profiles at suboxic stations. We instead fit these profiles using an exponential function with flux preserved at depth, finding shallow regeneration but high POC sequestration below 1,000 m. Both regeneration length scales and POC flux at depth closely track the depths at which oxygen concentrations approach zero. Our findings imply that climate warming will result in reduced ocean carbon storage due to expanding oligotrophic gyres, but opposing effects on ocean carbon storage from expanding suboxic waters will require modeling and future work to disentangle.

Coupling carbon and energy fluxes in the North Pacific Subtropical Gyre

The major biogeochemical cycles of marine ecosystems are driven by solar energy. Energy that is initially captured through photosynthesis is transformed and transported to great ocean depths via complex, yet poorly understood, energy flow networks. Herein we show that the chemical composition and specific energy (Joules per unit mass or organic carbon) of sinking particulate matter collected in the North Pacific Subtropical Gyre reveal dramatic changes in the upper 500 m of the water column as particles sink and age. In contrast to these upper water column processes, particles reaching the deep sea (4000 m) are energy-replete with organic carbon-specific energy values similar to surface phytoplankton. These enigmatic results suggest that the particles collected in the abyssal zone must be transported by rapid sinking processes. These fast-sinking particles control the pace of deep-sea benthic communities that live a feast-or-famine existence in an otherwise energy-depleted habitat.