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 by approximately 50% and nitrogen 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.

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 50 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.

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 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.