Photosynthesis is the hallmark of the do-it-yourself crowd. Organisms that rely on it need only light, carbon dioxide, and some inorganic nutrients to grow. There are exceptions, of course, such as carnivorous plants that live in low-nutrient habitats.

And here's a new one: microscopic algae that eat free-floating bacteria in the open ocean.

The smallest of the marine phytoplankton are unicellular algae less than one-tenth the width of a hair. They grow almost exclusively by photosynthesis, or so most scientists thought.

But working aboard a research vessel in the North Atlantic, and using isotopes to track the fate of nutrients in samples of seawater, Mikhail V. Zubkov of the National Oceanography Centre in Southampton and Glen A. Tarran of the Plymouth Marine Laboratory, both in England, have determined that the tiny algae obtain about a quarter of their biomass from bacteria.

So abundant are the little algae that they alone devour between 40 percent and 95 percent of all the bacteria eaten in the top, sunlit layer of the oceanthe rest succumb to other kinds of unicellular beings.

That algae should depend to such an extent on bacterivory came as a surprise. Perhaps it's more efficient to assimilate nutrients concentrated in bacteria than diffused in seawater, Zubkov and Tarran suggest. Whatever the reason, ecologists will have to revise their models of marine food chains to account for
algal appetites.

The findings were detailed in the journal Nature.