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An analysis of 24 coral reef viromes challenges the view that lytic phage are believed to predominate when the density of their hosts increase and shows instead that lysogeny is more important at high host densities; the authors also show that this model is consistent with predator–prey dynamics in a range of other ecosystems, such as animal-associated, sediment and soil systems. Microbial viruses can control host abundances via density-dependent lytic predator–prey dynamics. Less clear is how temperate viruses, which coexist and replicate with their host, influence microbial communities. Here we show that virus-like particles are relatively less abundant at high host densities. This suggests suppressed lysis where established models predict lytic dynamics are favoured. Meta-analysis of published viral and microbial densities showed that this trend was widespread in diverse ecosystems ranging from soil to freshwater to human lungs. Experimental manipulations showed viral densities more consistent with temperate than lytic life cycles at increasing microbial abundance. An analysis of 24 coral reef viromes showed a relative increase in the abundance of hallmark genes encoded by temperate viruses with increased microbial abundance. Based on these four lines of evidence, we propose the Piggyback-the-Winner model wherein temperate dynamics become increasingly important in ecosystems with high microbial densities; thus ‘more microbes, fewer viruses’. Live-and-let-live marine phage Lytic phage can control the abundance of their microbial hosts in a density-dependent manner with 'kill-the-winner' predation dynamics. It was widely assumed that lytic phages would dominate in nutrient-rich conditions favouring high host density, and that lysogenic phage, which integrate into their hosts instead of lysing them, tend to dominate when host numbers are low. This meta-analysis of 24 coral reef viromes challenges that view. Ben Knowles et al . find that lysogeny is more important than lysis at high, rather than low host densities. The authors term this the 'Piggyback-the-Winner' model, and show that it is consistent with predator–prey dynamics in a range of other ecosystems, including animal-associated, sediment, and soil systems.

The leakage of cold, methane-rich fluids from subsurface reservoirs to the sea floor sustains some of the richest ecosystems on the sea bed. These cold-seep communities consume around two orders of magnitude more oxygen than the surrounding sea floor as a result of the microbial consumption of seep methane. The leakage of cold, methane-rich fluids from subsurface reservoirs to the sea floor at specific sites on continental slopes, termed cold seeps, sustains some of the richest ecosystems on the sea bed. These seep-fuelled communities utilize around two orders of magnitude more oxygen per unit area than non-seep seafloor communities. Much of the oxygen is consumed by microbes and animal–microbe symbioses that use methane as an energy source. The proportion of methane consumed varies with fluid flow rate, ranging from 80% in seeps with slow fluid flow to less than 20% in seeps where fluid flow is high. Assuming the presence of a few tens of thousands of active cold seep systems on continental slopes worldwide, we estimate that the total efflux of methane to the overlying ocean could reach 0.02 Gt of carbon annually. As much more methane is lost from continental slopes, be it through emission to the hydrosphere or consumption by microbes, than can be produced, we suggest that a substantial fraction of the methane that fuels seep ecosystems is sourced from deep carbon buried kilometres under the sea floor.

Phytoplankton production drives marine ecosystem trophic-structure and global fisheries yields. Phytoplankton biomass is particularly influential near coral reef islands and atolls that span the oligotrophic tropical oceans. The paradoxical enhancement in phytoplankton near an island-reef ecosystem—Island Mass Effect (IME)—was first documented 60 years ago, yet much remains unknown about the prevalence and drivers of this ecologically important phenomenon. Here we provide the first basin-scale investigation of IME. We show that IME is a near-ubiquitous feature among a majority (91%) of coral reef ecosystems surveyed, creating near-island ‘hotspots’ of phytoplankton biomass throughout the upper water column. Variations in IME strength are governed by geomorphic type (atoll vs island), bathymetric slope, reef area and local human impacts (for example, human-derived nutrient input). These ocean oases increase nearshore phytoplankton biomass by up to 86% over oceanic conditions, providing basal energetic resources to higher trophic levels that support subsistence-based human populations. The Island Mass Effect (IME), where island proximity enhances phytoplankton biomass, remains a poorly understood phenomenon. Here, the authors show the IME is a common feature among Pacific reefs, driving increased production that creates biological hotspots in otherwise barren ocean basins.

Nutrient availability limits phytoplankton growth throughout much of the global ocean. Here we synthesize available experimental data to identify three dominant nutrient limitation regimes: nitrogen is limiting in the stratified subtropical gyres and in the summertime Arctic Ocean, iron is most commonly limiting in upwelling regions, and both nutrients are frequently co-limiting in regions in between the nitrogen and iron limited systems. Manganese can be co-limiting with iron in parts of the Southern Ocean, whilst phosphate and cobalt can be co-/serially limiting in some settings. Overall, an analysis of experimental responses showed that phytoplankton net growth can be significantly enhanced through increasing the number of different nutrients supplied, regardless of latitude, temperature, or trophic status, implying surface seawaters are often approaching nutrient co-limitation. Assessments of nutrient deficiency based on seawater nutrient concentrations and nutrient stress diagnosed via molecular biomarkers showed good agreement with experimentally-assessed nutrient limitation, validating conceptual and theoretical links between nutrient stoichiometry and microbial ecophysiology. Nutrient limitation is a key constraint on ocean productivity. Here, by analysing a compilation of field experiments spanning the global ocean, this study shows that increasing the number of different nutrients supplied significantly increases net phytoplankton growth, suggesting multiple nutrients are often approaching co-limiting levels.

Marine macroalgae are dominant primary producers in coastal zones. A review of the published literature suggests that macroalgae may play an important role in carbon sequestration. Vegetated coastal habitats have been identified as important carbon sinks. In contrast to angiosperm-based habitats such as seagrass meadows, salt marshes and mangroves, marine macroalgae have largely been excluded from discussions of marine carbon sinks. Macroalgae are the dominant primary producers in the coastal zone, but they typically do not grow in habitats that are considered to accumulate large stocks of organic carbon. However, the presence of macroalgal carbon in the deep sea and sediments, where it is effectively sequestered from the atmosphere, has been reported. A synthesis of these data suggests that macroalgae could represent an important source of the carbon sequestered in marine sediments and the deep ocean. We propose two main modes for the transport of macroalgae to the deep ocean and sediments: macroalgal material drifting through submarine canyons, and the sinking of negatively buoyant macroalgal detritus. A rough estimate suggests that macroalgae could sequester about 173 TgC yr −1 (with a range of 61–268 TgC yr −1 ) globally. About 90% of this sequestration occurs through export to the deep sea, and the rest through burial in coastal sediments. This estimate exceeds that for carbon sequestered in angiosperm-based coastal habitats.

Human pressures on the ocean are thought to be increasing globally, yet we know little about their patterns of cumulative change, which pressures are most responsible for change, and which places are experiencing the greatest increases. Managers and policymakers require such information to make strategic decisions and monitor progress towards management objectives. Here we calculate and map recent change over 5 years in cumulative impacts to marine ecosystems globally from fishing, climate change, and ocean- and land-based stressors. Nearly 66% of the ocean and 77% of national jurisdictions show increased human impact, driven mostly by climate change pressures. Five percent of the ocean is heavily impacted with increasing pressures, requiring management attention. Ten percent has very low impact with decreasing pressures. Our results provide large-scale guidance about where to prioritize management efforts and affirm the importance of addressing climate change to maintain and improve the condition of marine ecosystems. Human pressure on the ocean is thought to be increasing globally, yet the magnitude and patterns of these changes are largely unknown. Here, the authors produce a global map of change in cumulative human pressures over the past 5 years, and show that ∼66% of the ocean has experienced elevated human impact.

Our oceans are critical to the health of our planet and its inhabitants. Increasing pressures on our marine environment are triggering an urgent need for continuous and comprehensive monitoring of the oceans and stressors, including anthropogenic activity. Current ocean observational systems are expensive and have limited temporal and spatial coverage. However, there exists a dense network of fibre-optic (FO) telecommunication cables, covering both deep ocean and coastal areas around the globe. FO cables have an untapped potential for advanced acoustic sensing that, with recent technological break-throughs, can now fill many gaps in quantitative ocean monitoring. Here we show for the first time that an advanced distributed acoustic sensing (DAS) interrogator can be used to capture a broad range of acoustic phenomena with unprecedented signal-to-noise ratios and distances. We have detected, tracked, and identified whales, storms, ships, and earthquakes. We live-streamed 250 TB of DAS data from Svalbard to mid-Norway via Uninett’s research network over 44 days; a first step towards real-time processing and distribution. Our findings demonstrate the potential for a global Earth-Ocean-Atmosphere-Space DAS monitoring network with multiple applications, e.g. marine mammal forecasting combined with ship tracking, to avoid ship strikes. By including automated processing and fusion with other remote-sensing data (automated identification systems, satellites, etc.), a low-cost ubiquitous real-time monitoring network with vastly improved coverage and resolution is within reach. We anticipate that this is a game-changer in establishing a global observatory for Ocean-Earth sciences that will mitigate current spatial sampling gaps. Our pilot test confirms the viability of this ‘cloud-observatory’ concept.

The coastal ocean represents an important global carbon sink and is a focus for interventions to mitigate climate change and meet the Paris Agreement targets while supporting biodiversity and other ecosystem functions. However, the fate of the flux of carbon exported from seaweed forests—the world’s largest coastal vegetated ecosystem—is a key unknown in marine carbon budgets. Here we provide national and global estimates for seaweed-derived particulate carbon export below 200 m depth, which totalled 3–4% of the ocean carbon sink capacity. We characterized export using models of seaweed forest extent, production and decomposition, as well as shelf–open ocean water exchange. On average, 15% of seaweed production is estimated to be exported across the continental shelf, which equates to 56 TgC yr −1 (range: 10–170 TgC yr −1 ). Using modelled sequestration timescales below 200 m depth, we estimated that each year, 4–44 Tg seaweed-derived carbon could be sequestered for 100 years. Determining the full extent of seaweed carbon sequestration remains challenging, but critical to guide efforts to conserve seaweed forests, which are in decline globally. Our estimate does not include shelf burial and dissolved and refractory carbon pathways; still it highlights a relevant potential contribution of seaweed to natural carbon sinks. Coastal seaweed transported to the open ocean contributes up to 3–4% of the particulate organic carbon sinking into the deeper ocean, according to combined ecological and biogeochemical modelling.

Ecosystems are controlled by ‘bottom-up’ (resources) and ‘top-down’ (predation) forces. Viral infection is now recognized as a ubiquitous top-down control of microbial growth across ecosystems but, at the same time, cell death by viral predation influences, and is influenced by, resource availability. In this Review, we discuss recent advances in understanding the biogeochemical impact of viruses, focusing on how metabolic reprogramming of host cells during lytic viral infection alters the flow of energy and nutrients in aquatic ecosystems. Our synthesis revealed several emerging themes. First, viral infection transforms host metabolism, in part through virus-encoded metabolic genes; the functions performed by these genes appear to alleviate energetic and biosynthetic bottlenecks to viral production. Second, viral infection depends on the physiological state of the host cell and on environmental conditions, which are challenging to replicate in the laboratory. Last, metabolic reprogramming of infected cells and viral lysis alter nutrient cycling and carbon export in the oceans, although the net impacts remain uncertain. This Review highlights the need for understanding viral infection dynamics in realistic physiological and environmental contexts to better predict their biogeochemical consequences.

The Arctic is warming faster than any other region in the world. The resultant large-scale shift in sea ice cover could increase oceanic emissions of dimethylsulphide, a climate-relevant trace gas generated by ice algae and phytoplankton. The Arctic is warming faster than any other region in the world. Among the changes already witnessed, the loss of seasonal sea ice is by far the most striking. This large-scale shift in sea-ice cover could affect oceanic emissions of dimethylsulphide — a climate-relevant trace gas generated by ice algae and phytoplankton. During the spring melt period, conditions at the margin of Arctic sea ice favour the growth of these organisms. As a result, high levels of dimethylsulphide can accumulate at the bottom of the ice, in leads, and in the water column at the ice edge during the spring melt season. Production of dimethylsulphide is not limited to the sea-ice edge, however. Significant concentrations have also been detected in the seasonal ice-free zone in spring and summer. Preliminary observations, together with model results, suggest that the production and emission of dimethylsulphide will increase in the Arctic as seasonal sea-ice cover recedes. If it escapes to the atmosphere, this newly generated dimethylsulphide could potentially cool the Arctic climate.