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Protozoa were thought to be the primary predators of marine bacteria, but phytoplankton now look set to rival them for poll position. These photosynthetic marine microorganisms dominate the oceans and form the basis of many oceanic food webs, and their sheer numbers drive global cycling of carbon and nutrients. On page 224, Glen Tarran from the UK-based Plymouth Marine Laboratory, and his colleague Mikhail Zubkov of the National Oceanography Centre in Southampton, UK, quantify the surprising degree to which phytoplankton consume ocean bacteria — a phenomenon that could have implications for climate change.

Did you stumble across phytoplankton's predatory behaviour?

Somewhat, yes. We've known for more than 50 years that some marine microorganisms are mixotrophs — that is, can derive metabolic energy either by photosynthesis or by grazing on other organisms — but the phenomenon wasn't thought to be widespread. However, that wasn't our initial focus. We set out to quantify how open-ocean protozoa consume bacteria. During our experiments, we developed a method to sort protozoa and phytoplankton and, to our surprise, found phytoplankton grazing on bacteria as much as the protozoa were.

How many bacteria do phytoplankton consume?

Our work shows that phytoplankton are responsible for between 40% and 95% of the bacteria consumed in near-surface ocean waters in the North Atlantic near Iceland. We found similar numbers near the Cape Verde Islands off Africa's west coast, suggesting that this phenomenon is widespread.

What are the next steps?

Next month we will embark on a voyage across the Atlantic, from the United Kingdom to the Falkland Islands, looking for mixotrophy among phytoplankton as we pass through areas of varying temperature, nutrients and ocean dynamics. Our goal is to quantify how important mixotrophy is to global carbon cycling. Eventually we'd like to plug our data into global climate-change models.

Have we misunderstood marine food webs?

No, we've simply been limited in our ability to quantify their dynamics. Mike Zubkov and I have spent 10 years developing techniques to determine microbial dynamics. Mike is the big-picture guy forging new radioactive methods; I bring things down to Earth with the nuts and bolts implementation of the techniques. The open ocean is still a frontier, and we plan to further explore the role of phytoplankton in ocean nutrient dynamics.