Mike Zubkov of the National Oceanography Centre in Southampton and Glen Tarran from Plymouth Marine Laboratory obtained their result by studying the feeding habits of algae less than five microns in size in the North Atlantic Ocean. The work was carried out during a cruise last August aboard the Royal Research Ship Discovery.
Zubkov and Tarran were investigating how many bacteria protozoa eat and decided to find out whether algae feed on bacteria, too. The researchers observed that algae eat about one bacterium per hour whereas protozoa consume about three to five bacteria per hour. But as there are more algae than protozoa in the ocean, the algae collectively eat at least as many bacteria as the protozoa do.
The result means that planktonic algae have two feeding mechanisms: photosynthesis – whereby they convert sunlight into energy like plants do – and eating bacteria. Indeed the latter mechanism provides as much as 25% of the algae's overall energy needs.
The fact that algae are as important as protozoa at feeding on bacteria adds a fundamental piece to the marine web jigsaw puzzle, says Tarran. In particular, it is important for our understanding of the cycling of major elements, like carbon and nitrogen, from the atmosphere into the ocean. For example, phytoplankton growth depends less on inorganic nitrogen input (from the atmosphere) when there are bacteria present.
Zubkov and Tarran obtained their results by allowing samples of algae and protozoa to feed on native oceanic bacteria that had been radioactively labelled with hydrogen-3 and sulphur-35 tracers. By separating the two types of organism and measuring the amount of radioactivity that had built up in their bodies, the researchers were able to calculate the amounts of bacteria ingested by each.
The work was reported in Nature.