A measure of marine life

    The extraordinary emerging images of ocean microbiology need the fourth dimension of time.

    The globe floating in the void might almost be the first, haunting glimpse of an alien world, blue-green and dense with life. But this is the view through a microscope, not a telescope, and the globe is a crucial inhabitant of this planet, not a token of another. Just a micrometre in circumference, Prochlorococcus makes up for in number what it lacks in size. This tiny bacterium is the most common photosynthetic organism on Earth, providing a substantial fraction of the planet's carbon fixation.

    Until just over two decades ago, moreover, Prochlorococcus was unknown. Its ubiquitous presence in non-coastal, non-polar waters is one of many recent discoveries by which ocean microbiology has re-emphasized the primacy of microbes in Earth's biosphere. That primacy, analysed in this week's Insight starting on page 179, holds everywhere, notably in soils. But in soils, every pore and grain provides its own microenvironment. The seas, transparent and well mixed, are where this microbe-centric view of life is most clearly visible.

    “New discoveries have revealed a previously unimagined profusion of microbial life.”

    The new discoveries have revealed a previously unimagined profusion of microbial life, with perhaps 1,000 times more organisms per unit volume than scientists thought in the 1980s. Not only are there new players, such as Prochlorococcus, but whole new classes of player, such as the Archaea now known to exist far more widely than suggested by their early reputation as niche extremophiles. There are metabolisms that were previously unknown, such as those of bacteria using sunlight and proteorhodopsin to power their lives. And, perhaps most exciting of all, there is an extraordinary amount of genetic diversity and gene transfer — the latter often mediated by unexpectedly abundant viruses.

    These findings are largely based on improved technologies, such as satellite imaging that can read out chlorophyll abundances, flow cytometry that can distinguish the tiniest cells, and gene sequencing that can make sense of raw genomes from the water. Between them they have provided a picture of life in the oceans covering every scale from the pigment molecule to the planet as a whole. Yet for all their data-gathering power, these technologies are still largely blind to the temporal dimension — a problem that urgently needs addressing.

    The oceans, after all, are patterned in time as well as space. The Hawaii Ocean Time-Series, running now for two decades, has seen intriguing signs of long-term oscillations between nitrogen and phosphate-limited microbial assemblages. But such thorough, regularly assessed measurements of the physical, chemical and biological environment in the oceans are almost nonexistent.

    To reproduce such time series in dozens of ecologically and oceanographically distinct provinces around the world would be costly, and hard to justify on the basis of traditional hypothesis-driven science. But that is not the correct yardstick for this work. At a time when humankind's carbon emissions are producing rapid changes in Earth's climate, recording those changes as they reverberate through the seas is a necessity if they are to be understood, and their future course predicted.

    More generally, there is a growing number of areas in which scientists' ability to gather information currently exceeds their ability to understand it. Although it may go against the grain, it is worth considering that gathering those data regardless of comprehension is worth some effort, even if there are opportunity costs to current science. The scientists of the future, with greater knowledge, craft and insight, will be grateful for them.

    This argument applies particularly strongly to attempts to monitor Earth and its oceans as a whole. Unlike the photosynthetic galaxies strewn across the seas, this planet is the only available example of its type for humanity to understand. The light-rich, life-rich seas are a key to that understanding.

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    A measure of marine life. Nature 459, 140 (2009). https://doi.org/10.1038/459140a

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