Today, the Southern Ocean (SO) is the primary sink of anthropogenic heat and carbon, yet anthropogenic warming and ozone depletion have altered its circulation and stratification, creating uncertainty about the persistence of this sink. Evidence from the last glacial period and deglaciation shows that the SO played a central role in past climate transitions by releasing heat and carbon to the atmosphere through sea-ice retreat and enhanced ventilation of deep and abyssal waters. Antarctic warming during weak Atlantic Meridional Overturning Circulation (AMOC) phases was likely amplified by SO feedbacks, such as increased deep-ocean convection, and a strengthening/poleward shift of Southern Hemisphere westerlies. However, changes in the abyssal circulation, its interaction with North Atlantic Deep Water, and its impact on oceanic carbon during the deglaciation remain debated. Finally, although sediment and modeling evidence suggests that Antarctic Ice Sheet (AIS) discharge can significantly affect climate and biogeochemistry, clear signals remain scarce in existing proxy records-either masked by dominant AMOC-driven variability or unresolved due to limited temporal resolution. Improved model representation and proxy records are needed to clarify the role of abyssal circulation and the interaction between the ocean and AIS.
Viruses dominate the deep-ocean biosphere (>200-m depth) yet remain profoundly understudied. As the most abundant biological entities on Earth, they govern the geochemistry, evolution, and ecology of deep-sea prokaryotic and eukaryotic communities across a vast array of deep-sea biomes. The last two decades have witnessed revolutionary advances in deep-sea sampling, sequencing, and computational biology, illuminating viral diversity and function across Earth's largest active biome. Here, we synthesize emerging patterns in deep-sea viral ecology, biogeography, and evolutionary dynamics while identifying critical knowledge gaps and opportunities. This review is pertinent in the face of deep-sea mining and climate change, which threaten to fundamentally alter these ecosystems and cascade through virus-host networks in ways we cannot yet predict. Understanding viral control of deep-sea biogeochemistry and resistance to environmental perturbation is essential for predicting ecosystem resilience in the face of anthropogenic pressures and informing policy on one of Earth's final frontiers.
Dissolved organic N (DON) and dissolved organic P (DOP) represent the largest pools of bioavailable N and P in oligotrophic surface waters. While both serve as assimilative nutrient sources for autotrophs when inorganic forms of N and P are scarce, the location and quantitative significance of their utilization remain unclear. Here, we review new geochemical metrics and modeling tools that link surface-ocean DON and DOP to basin-scale biogeochemical processes and identify their source and sink regions. We find that surface-ocean dissolved organic nutrients are influenced by both phosphate and iron stress, as well as denitrification in the Pacific and atmospheric dust deposition in the Atlantic, and support up to 66% and 80% of annual net community production in the western North Pacific and North Atlantic gyres, respectively. Collectively, this work refines our understanding of the role of organic nutrients in supporting the biological pump and the controls on upper-ocean nutrient cycling.
The Earth's climate has been kept under Goldilocks conditions because a variety of feedback systems maintain the atmospheric p CO 2 within a narrow range. The ocean, as a large reservoir of carbon compared with the atmosphere, plays a key role in the climate system, and studying ocean process can help us better understand this system. Cosmogenic nuclides produced in the atmosphere and their ratio to a terrestrial counterpart can provide detailed depictions of Earth surface process, and they have therefore been utilized widely since it became possible to measure them with accelerator mass spectrometry. Beryllium isotopes ( 10 Be/ 9 Be) are one of the most useful isotope systems for this purpose. In this article, we summarize recent developments in beryllium isotope chemistry and the isotopes’ relation to ocean current and ice sheet dynamics as well as weathering in relation to long-term climate.
Air-sea exchanges of momentum and kinetic energy involve interactions between wind and ocean surface currents, mediated by the effects of surface waves. The wind stress that transfers momentum into the ocean depends on the velocity difference between winds and currents. Wind stress is also hypothesized to depend on the height and steepness of wind-generated waves. Waves are advected by ocean currents and also refracted as they pass through spatially varying currents. In turn, the impacts of waves and currents feed back on the wind, altering the structure of the atmospheric boundary layer. Together, wind-wave-current interactions shape ocean circulation, weather, and climate. However, gaps remain in our understanding, particularly in quantifying feedbacks between the components. Advances in our understanding will be facilitated by simultaneous measurements of key variables, via in situ observation or future satellite systems capable of obtaining global-scale observations.
Organic sulfur (OS) in the ocean is produced in vast quantities by primary producers that fix inorganic sulfate into proteins, metabolites, and other ubiquitous biomolecules. As biogenic OS is transported and transformed through the marine environment, it is joined by OS from two additional sources: abiogenic OS from sulfurization under anoxic conditions, and geological OS from the weathering of sediments and rocks. Important differences in the properties of the OS from these sources affect its fate in the environment and underlie the formation of recalcitrant dissolved organic matter and sedimentary kerogen. This review builds connections between the rapid OS cycle in the surface ocean and these longer-lived reservoirs, applying our growing knowledge of particle fluxes and organic matter dynamics at the sediment-water interface. Future studies on marine OS are poised to help us better understand the implications of these fluxes for the carbon cycle and climate across human and geological timescales.
Marine dissolved organic matter (DOM) represents one of Earth's most complex exometabolomes, playing a central role in marine carbon cycling and long-term sequestration. Despite its biogeochemical importance, the molecular complexity of DOM has long challenged its analytical characterization. Here, we review recent advancements in structure-resolved analytical techniques for DOM. In addition to spectroscopic methods, we focus on liquid chromatography-tandem mass spectrometry and ion mobility spectrometry, as these technologies can provide unprecedented molecular-level insights into DOM composition. By integrating high-resolution analytical techniques with computational pipelines, researchers are now able to resolve previously obscured molecular structures, which has the potential to refine models of DOM cycling and its interactions with microbial communities. Continued innovation in structure-resolved methodologies will be essential for unraveling the molecular complexity of marine DOM and understanding its implications for global biogeochemical processes.
Eutrophication of the Baltic Sea was recognized more than half a century ago, but it remains a major threat to the sea's ecosystem. Requirements developed by the Baltic Marine Environment Protection Commission (formed in 1974) and subsequently implemented in national and European Union law have led to reductions of phosphorus river load by approximately 50% and nitrogen river load by approximately 30% since the 1980s, but so far, the measures have failed to significantly improve the surface water quality. A decades-long accumulation of phosphate and oxygen-sapping substances appeared to reduce the efficiency of the lateral supply of oxygen from intrusions and major Baltic inflows via the narrow Baltic Straits. The dynamic change of, in particular, phosphate cycling in deep waters during these inflows contrasts with the sluggish response to river load reduction measures. Seasonal phosphate recycling in surface water results mainly from exchange with the large deep-water phosphate pool, and this key exchange can be better interpreted based on an improved understanding of its physical drivers.
The molecular revolution of the 1990s brought insights into the tremendous breadth of ecological and evolutionary diversity harbored within the bacterial and archaeal domains of life, enabling scientists to peer into the proverbial microbial black box. Many of these early molecular efforts focused on microbes in marine surface waters, given their global relevance and ease of extraction from seawater via filtration. From molecular surveys of marine microbial communities, there emerged a limited number of taxa with marked numerical dominance and distribution across ocean realms. One of these lineages is the now well-studied Roseobacteraceae family. Three decades of studying roseobacter members, many of which are amenable to both laboratory culture and genetic manipulation, have led to discoveries in how microbial heterotrophs process diverse marine organic matter, drive biogeochemical cycles, and interact with primary producers.
Marine viral ecology emerged as a distinct discipline approximately 25 years ago. Despite significant progress, direct assessments of viral impacts on carbon flux remain scarce. Here, we integrate recent advances and knowledge gaps in marine viral ecology and a comprehensive conceptual viral-engine framework, highlighting the various ways in which viruses play a fundamental role in shaping marine ecosystem dynamics. Moreover, we present a meta-analysis of virus-mediated microbial mortality rates to examine the role of viruses in driving seasonal and global patterns in microbial biomass. We illustrate how viruses fundamentally shape marine ecosystem dynamics and serve as key drivers of microbial turnover, nutrient recycling, and global carbon cycling, positioning them as an engine driving oceanic biogeochemical processes.
Ammonia oxidation is a fundamental step in the marine nitrogen cycle, catalyzing the conversion of ammonia to nitrite or nitric oxide and generating reductive power for the autotrophic growth of microorganisms. The ecology, diversity, and properties of ammonia-oxidizing microbes in the ocean's plankton have been extensively studied, but these microbes can also live in association or symbiosis with marine hosts such as sponges, corals, jellyfish, bivalves, and crustaceans. Sequencing-based studies have revealed that ammonia-oxidizing archaea of the family Nitrosopumilaceae are prevalent in various marine hosts, although other ammonia-oxidizing taxa are also found and coexist within the same host. Ammonia oxidation rates are highly variable between host species, even between closely related taxa. Limited knowledge is available on the metabolic interactions that ammonia-oxidizing microbes have, but theoretical considerations indicate that they could make significant contributions to carbon fixation for their hosts. Additionally, ammonia-oxidizing microbes appear to also have undergone specific genomic adaptations to their host environment, and the hosts may also enable ammonia oxidation to occur in habitats where planktonic counterparts might be limited. This review identifies key knowledge gaps and highlights the need for further research to fully understand the ecological significance of symbiotic ammonia oxidation in marine ecosystems.
The Chesapeake Bay is a large estuarine system spanning multiple jurisdictions and serves as a model for estuarine health worldwide. Historically, nutrient loading degraded water quality, prompting the need for regulation. Water clarity, one component of water quality, is vital for benthic communities and serves as a key indicator of overall ecosystem health. Here, physical resuspension and salinity gradients, nutrient and sediment inputs, production of organic detritus by phytoplankton, and benthic communities all interact to drive clarity patterns, with high spatial variability. Trends over the last 40 years show improvement, though with a temporary increase in organic detritus in response to reduced sediment inputs and algal release from light limitation. Continued reductions in nutrient and sediment inputs have led to improved clarity across all metrics and a re-expansion of submerged aquatic vegetation. Future management should continue reductions in nutrient and sediment inputs while addressing climate-related shifts in estuarine dynamics.
It is increasingly obvious that, even when reaching net-zero emissions, removal of anthropogenic CO2 from the atmosphere will be required. Some ocean-based removal technologies, while not proven for routine operation at scale, show promise. All of these rely on inducing a flux of CO2 from the atmosphere into the ocean that is directly attributable to the removal intervention. Crucial for the economic viability of these technologies is the quantification of the cumulative net air-sea flux of CO2 that an intervention can verifiably deliver. Because this flux is the difference between a realistic case with and a hypothetical case without intervention, it cannot be determined by observation alone-one must rely on a combination of informative observations and skillful models. Major uncertainties in the quantification of net CO2 uptake include the removal of seawater with a dissolved inorganic carbon deficit from direct contact with the atmosphere and the inevitable rebalancing of carbon among Earth's mobile carbon pools.
Built infrastructure, such as seawalls and levees, has long been used to reduce shoreline erosion and protect coastal properties from flood impacts. In contrast, natural and nature-based features (NNBF), including marshes, mangroves, oyster reefs, coral reefs, and seagrasses, offer not only coastal protection but also a range of valuable ecosystem services. There is no clear understanding of the capacity of either natural habitats or NNBF integrated with traditional engineered infrastructure to withstand extreme events, nor are there well-defined breakpoints at which these habitats fail to provide coastal protection. Evaluating existing NNBF strategies using a standardized set of metrics can help to assess their effectiveness to better inform design criteria. This review identifies a selection of NNBF projects with long-term monitoring programs and synthesizes the monitoring data to provide a literature-based performance assessment. It also explores the integration of NNBF with existing gray infrastructure to enhance overall effectiveness.
Zooplankton diel vertical migration (DVM) is a globally ubiquitous phenomenon and a critical component of the ocean's biological pump. During DVM, zooplankton metabolism leads to carbon and nutrient export to mesopelagic depths, where carbon can be sequestered for decades to millennia, while also introducing labile, energy-rich food sources to midwater ecosystems. Three pervasive metabolic pathways allow zooplankton to sequester carbon: fecal pellet egestion, dissolved organic matter excretion, and respiration. Additionally, there are several less well-parameterized sources of DVM transport associated with growth, feeding, reproduction, and mortality. These processes are challenging to measure in situ and difficult to extrapolate from laboratory experiments, making them some of the most poorly constrained factors in assessments and models of the biological pump. In this review, we evaluate and compare observational and modeling approaches to estimate zooplankton DVM and the resulting active carbon flux, highlighting major discrepancies and proposing directions for future research.
Present seasonally or year-round in polar and subpolar seas, sea ice is one of the most complex and biologically rich ecosystems on Earth. Throughout the history of our planet, sea ice has periodically covered vast proportions of the world's oceans, and it may also serve as a plausible habitat on other ocean worlds. In this review, we provide a comprehensive overview of current knowledge on sea ice as a habitat, both on Earth and in extraterrestrial environments. We focus on bacteria, microalgae, and their associated viruses, describing the key physicochemical characteristics that shape this unique ecosystem. Additionally, we explore hypotheses on how microorganisms colonize sea ice, survive by protecting themselves and altering their environment, and ultimately proliferate and evolve. Finally, we consider the potential role of the sea-ice microbiome in the evolution of life on Earth and its possible existence beyond our planet.
The Atlantic Ocean circulation, in particular its zonally averaged north-south volume transport indicated by the Atlantic Meridional Overturning Circulation (AMOC), is sensitive to surface buoyancy anomalies. It may undergo a transition to a climate-disrupting state within a century under continuing greenhouse gas emissions. The potential climate and societal impacts are expected to be large, and therefore reliable estimates of the probability of the onset of such a collapse before the year 2100 are crucial for policymakers. This article addresses whether current Earth system models are fit for purpose to capture present-day AMOC stability and presents the current status of estimates of collapse onset probabilities.
Organic detrital particles drift and sink through all ocean waters. This marine snow mediates the global carbon cycle by sequestering carbon in the deep sea and fuels ocean ecosystems by feeding deep-sea organisms. These global processes are ultimately controlled by the collection of events that occur at the scale of individual marine snowflakes. These particles are incredibly diverse, with physical characteristics and compositions determined by the myriad processes that lead to their formation and transformation over time. When that diversity is classified, we can calculate the quantity of carbon that particles transport to the deep sea. Each marine snowflake is a microcosm, with distinct organisms and metabolisms concentrated within the organic matter of a particle. Resolving the biology of individual marine snowflakes is possible through innovations in physical collection and the development of autonomous imaging platforms. Accounting for particle-specific biology generates major advancements in ocean biogeochemistry and ecology.
For over 50 years I have studied corals, coral reefs, and reef-associated animals. Although much of my work was underpinned by genetics and I have dabbled in theory, I am a natural historian at heart. The many topics I pursued reflect in part what sparked my fancy but were also greatly shaped by a series of chance events, unexpected data, and unplanned opportunities. Many of the findings and ideas for which I am now known were initially met with skepticism and rejection—success required stubborn faith in my intuitions and convictions and the support of many assistants, collaborators, mentors, and leaders. Watching the sudden loss of the reefs that dazzled me as a graduate student and the subsequent steady decline of ocean life around the world has driven my interests in conservation and communication, and in the end, perhaps surprisingly, made me focus on the positive.
