A few years ago, a team of scientists took an expensive robot, attached it to a buoy floating off the coast of Hawaii, and left it there. From the outside, it would have looked like an elaborate garbage can. Inside, it was busy. As it bobbed and flowed with the currents, it sucked in some of the surrounding water and passed it through a small circular filter. It added preservative to the filter, moved it to one side, and put a new one in its place. It did this every two hours.
After three days, the scientists came back for it. That was how they took the ocean’s pulse.
The open ocean is full of life. Even when you can’t see any fish or crabs or whales, there are microbes galore. Individually, they are too small to see. Together, they account for up to 90 percent of all the life in the ocean by weight. The most common of these—a bacterium called Prochlorococcus—was only discovered three decades ago, but it’s so abundant that there can be hundreds and thousands in a single millilitre of seawater. Like plants, it uses the sun’s energy to make nutrients, and releases oxygen in the process. Take five breaths; the oxygen in at least one of them came from Prochlorococcus. These microbes are the planet’s lungs.
The MIT team, led by Elizabeth Ottesen and Ed DeLong, used their robot to sample these microbes at regular intervals, and found that they have daily rhythms of genetic activity. As the sun rises and falls and the tides flood and ebb, these microbes switch their genes on and off in predictable 24-hour cycles. Prochlorococcus in particular is so regular that you could set your watch by its genes.
These microbes rely on the sun (they’re called phototrophs), so it was always fairly predictable that they should have some kind of daily rhythm. As Ottesen puts it, “They’re getting up in the morning for some photosynthesising, before powering down in the evening and switching to growth-related genes.”
But to the team’s surprise, they found similar rhythms in other marine bacteria that were further up the food chain. These ‘heterotrophs’ sustain themselves by feeding on organic matter or grazing on the phototrophs. And yet, they too have daily cycles in genes that bring in food, break it down, make DNA, and more.
Each species has its own particular peaks of activity, but many of them were cycling in sync, and some were tightly linked to whatever Prochlorococcus was doing. “The food webs are coupled down to the hour,” says Ottesen. “That was surprising. You don’t expect cow populations to respond to how grass changes over the course of the day.”
Think of these microbes as a city full of people, of day-workers and night-workers, larks and owls. Each of them gets up, goes to work, and falls asleep at different times of the day. And yet, they’re all connected. Some get up earlier than others, so they can drive the trains that ferry commuters to work. Some prepare food early in the morning so that others can take that food to shops and yet others can buy their lunch. Some start their radio shows in the evening so that others can have something to listen to when they drive home. Waves of choreographed activity coruscate through the city; so it is in the oceans.
Scientists have seen hints of these cycles in the lab, but Ottesen wanted to see how they play out in the actual ocean. “It’s important to work out not just how they behave in a zoo but in the wild,” she says. That task has always been difficult. Even if you park a ship in one spot, water will be constantly move around it, making it hard to study the same patch of ocean over time. The team’s robot, however, can go with the flow and collect samples all day long.
The team found some differences between the wild and captive bacteria. For example, in a lab, Prochlorococcus switches on the vast majority of its genes at dawn or dusk; in the wild, there’s another big peak at noon. “The study reveals the power of sophisticated sampling devices for studying ocean features that were heretofore inaccessible,” says MIT’s Sallie Chisholm. “It is heartening to see that the patterns observed in this study correspond quite well with those observed in cultures.”
“It’s an important demonstration that the ability of heterotrophic bacteria to respond, directly or otherwise, to the daily periodicity in light is more common than perhaps thought,” says Erik Zinser from the University of Tennessee in Knoxville. The challenge now, he adds, is to work out how these cycles arise, and what they mean for the oceans.
Or, indeed, the world. The phototrophs convert carbon dioxide into organic substances, and the heterotrophs effectively reverse that process as they eat and breathe. “The balance between them determines if the ocean is acting as a source or a sink of carbon dioxide,” says Ottesen. And that has implications not just for the oceans, but for the whole planet.
Reference: Ottesen, Young, Gifford, Eppley, Marin, Schuster, Scholin & DeLong. 2014. Multispecies diel transcriptional oscillations in open ocean heterotrophic bacterial assemblages. http://dx.doi.org/10.1126/science.1252476