DNA viruses have a major influence on the ecology and evolution of cellular organisms1,2,3,4, but their overall diversity and evolutionary trajectories remain elusive5. Here we carried out a phylogeny-guided genome-resolved metagenomic survey of the sunlit oceans and discovered plankton-infecting relatives of herpesviruses that form a putative new phylum dubbed Mirusviricota. The virion morphogenesis module of this large monophyletic clade is typical of viruses from the realm Duplodnaviria6, with multiple components strongly indicating a common ancestry with animal-infecting Herpesvirales. Yet, a substantial fraction of mirusvirus genes, including hallmark transcription machinery genes missing in herpesviruses, are closely related homologues of giant eukaryotic DNA viruses from another viral realm, Varidnaviria. These remarkable chimaeric attributes connecting Mirusviricota to herpesviruses and giant eukaryotic viruses are supported by more than 100 environmental mirusvirus genomes, including a near-complete contiguous genome of 432 kilobases. Moreover, mirusviruses are among the most abundant and active eukaryotic viruses characterized in the sunlit oceans, encoding a diverse array of functions used during the infection of microbial eukaryotes from pole to pole. The prevalence, functional activity, diversification and atypical chimaeric attributes of mirusviruses point to a lasting role of Mirusviricota in the ecology of marine ecosystems and in the evolution of eukaryotic DNA viruses.

The United Nations high seas treaty has been a long time coming. Secured earlier this month after almost 20 years of effort, it will be the first international law to offer some protection to the nearly two-thirds of the ocean that is beyond national control. These parts of the ocean currently have few, if any, meaningful safeguards against pollution, overfishing and habitat destruction. The treaty is without doubt a major achievement.

Agreed under the UN Convention on the Law of the Sea, it represents several wins. Among them is the capacity to create marine protected areas through decisions of a conference of the parties to the treaty. It also recognizes that genetic resources of the high seas must benefit all of humanity. Moreover, companies planning commercial activities and organizations considering other large projects (such as potential climate interventions involving the ocean) will need to carry out environmental impact assessments.

The vast swathes of kelp forest growing along the world’s coastlines are estimated to generate US$500 billion a year on average, making them considerably more valuable than previous studies have suggested, according to an analysis that assessed economic contributions made by six types of the seaweed.

The study, published on 18 April in Nature Communications1, estimates that kelp forests provide services worth between $465 billion and $562 billion a year worldwide, mainly by providing a habitat for valuable fish and seafood species, and by removing nitrogen from contaminated seawater. The results suggest that each type of kelp forest (see ‘Seaweed services’) generates up to $147,100 per hectare annually, a figure that’s more than three times higher than previous estimates.

“Until now, most kelp-forest evaluations were regional,” says Cristina Piñeiro-Corbeira, a marine ecologist at the University of A Coruña in Spain who was not involved in the project. “This study is a step forward in understanding kelp forests and their importance for human well-being on a global scale.”

The Northern Lights dance across the horizon in emerald and lavender ribbons as the R/V Helmer Hanssen cruises through one of the darkest regions of the planet in the heart of winter. For more than a dozen researchers on this two-week voyage, the mission is simple but profound: to disappear.

Gliding through inky waters, our captain suddenly shuts off every exterior light on the ship and we become invisible, a maritime phantom. It is the ideal way to study marine organisms that exploit darkness and cold as few other forms of life can.

This is a new frontier for Arctic researchers. Until 2007, it wouldn’t have seemed profitable to stage an oceanographic biology expedition in midwinter. Scientists thought that most of the region’s marine ecosystem shut down throughout the months-long darkness of polar night. Without sunlight to power the growth of photosynthetic plankton, there would be nothing to eat for the larger zooplankton, which are the primary source of food for seabirds and fish. That was the conventional wisdom, at least.

When I started my PhD at the Roscoff Biological Station in 1989, I joined a laboratory where I began studying extremophiles — organisms that can cope with challenging conditions, such as high temperatures or extreme pH values, that no other life forms can survive. These harsh environments are characteristic of hydrothermal systems such as deep-sea or shallow-water vents.

At the time, there were no prior methods or even the proper lab equipment — the word ‘extremophile’ was not widespread. I had to set up my entire lab from scratch, and ended up identifying a species from a deep-sea thermal vent in Roscoff, France.

About a decade ago, I began collaborating with geologist Bernard Pelletier after he told me that he’d discovered a hydrothermal ‘chimney’ in Prony Bay in south New Caledonia, a French territory in the South Pacific Ocean.

A chimney is a porous tower formed from the build-up of minerals that percolate to the surface as heated fluid from the ocean’s depths meets cold seawater. And I am still involved with this research.

In this picture, I’m taking samples of that heated fluid. Our research group studies the microorganisms inside the chimney and in the fluid. We try to decipher how this microbial ecosystem works and identify the energy source that fuels the cells living there. We sequence any genetic material found in the water and in the microorganisms that dwell there. It’s a many-thousand-parts puzzle.

This research helps us to better understand what life is and what could be evidence for life. These environments are the best analogues for those that existed between 4.3 and 4.5 billion years ago, when prebiotic organic molecules gave rise to early forms of life.

So our work could help us to identify life-forming events here and on other planets.

Marine heatwaves devastate ecosystems and the coastal communities that rely on them. Weeks, months or years of unusually warm waters can bleach corals, spur harmful algal blooms and wipe out seaweeds. They might kill or strand marine animals and disrupt food webs and fisheries1. Billions of US dollars are lost to such events around the world each year2.

For example, in 2013, an area of water in the northeast Pacific Ocean more than three times the size of Texas, known as The Blob, warmed by nearly 3°C. Over 18 months, these warm waters spread across the entire west coast of North America, from the Gulf of Alaska to the tip of the Baja Peninsula in Mexico. Seabirds starved and stocks of Pacific cod collapsed. Tuna moved north, as far as Alaska. Humpback whales drawn towards the coast became entangled in fishing nets. Mysterious creatures, such as glowing tropical sea pickles, or pyrosomes, arrived in northern waters.

Ocean scientists are striving to better understand such phenomena, and whether climate change is making marine heatwaves more frequent and more intense. But right now the field has a problem: the definitions and communications describing what a marine heatwave is are confusing.

The abyssal ocean circulation is a key component of the global meridional overturning circulation, cycling heat, carbon, oxygen and nutrients throughout the world ocean1,2. The strongest historical trend observed in the abyssal ocean is warming at high southern latitudes2,3,4, yet it is unclear what processes have driven this warming, and whether this warming is linked to a slowdown in the ocean’s overturning circulation. Furthermore, attributing change to specific drivers is difficult owing to limited measurements, and because coupled climate models exhibit biases in the region5,6,7. In addition, future change remains uncertain, with the latest coordinated climate model projections not accounting for dynamic ice-sheet melt. Here we use a transient forced high-resolution coupled ocean–sea-ice model to show that under a high-emissions scenario, abyssal warming is set to accelerate over the next 30 years. We find that meltwater input around Antarctica drives a contraction of Antarctic Bottom Water (AABW), opening a pathway that allows warm Circumpolar Deep Water greater access to the continental shelf. The reduction in AABW formation results in warming and ageing of the abyssal ocean, consistent with recent measurements. In contrast, projected wind and thermal forcing has little impact on the properties, age and volume of AABW. These results highlight the critical importance of Antarctic meltwater in setting the abyssal ocean overturning, with implications for global ocean biogeochemistry and climate that could last for centuries.

Since 2008, population densities of shallow-reef fishes, invertebrates and seaweeds around Australia have generally decreased near the northern limits of species’ ranges, and increased near their southern limits. Endemic invertebrates and seaweeds that prefer cold waters showed the steepest declines, and are prevented by deep-ocean barriers from moving south as temperatures rise.