Ocean microbes use 'substitute' nutrients when phosphorus is low.
They might seem minuscule and insignificant, but marine microorganisms have a massive effect on global nutrient cycles. Phytoplankton form the basis of marine food chains and, from them, nutrients are transported through the system. Phosphorus is an essential nutrient for all organisms — it is a component of both DNA and cell membranes — so shortages can limit microbial growth. But recent studies have found that some species can thrive even when phosphorus is scarce. The mystery was how.
Benjamin Van Mooy, a marine chemist at Woods Hole Oceanographic Institution in Massachusetts, became interested in phosphorus cycling as a graduate student, when he discovered that 15–20% of marine phosphorus taken up by microbes in the South Pacific winds up in phospholipids, the fatty molecules that are a major component of cell membranes. He wondered whether the same was true in regions where phosphorus availability is much lower.
Unlike those for nitrogen and iron, the oceanic phosphorus cycle is a 'closed' system, meaning that there is never any addition of phosphorus from outside the system. Previous work suggested that phosphorus cycling in the Atlantic Ocean might provide an interesting contrast to that in the Pacific, so Van Mooy and his group headed for the Sargasso Sea. This elliptical stretch of water in the middle of the North Atlantic is Earth's only shoreless sea. It is bounded by ocean currents and known for its floating masses of seaweed. It was once considered a 'dead zone' owing to its lack of larger sea life, but the discovery of rich microbial diversity has prompted numerous studies, including a metagenomic survey by genomics pioneer Craig Venter. “The Sargasso Sea has become Mecca for many scientists,” says Van Mooy.
By incubating sea-water samples with a radioactive phosphorus isotope, the team established that only about 1–2% of the phosphorus taken up in the Sargasso is used to make lipids. “At first, I thought we'd made a mistake,” Van Mooy says. When the findings were confirmed, he knew they needed to culture these plankton in the lab to determine how they were able to thrive on so little phosphorus.
Unfortunately, Van Mooy has a brown thumb. “I'm a chemist. Everything I've ever tried to grow in culture has died within a week,” he says. So he turned to friends in the related field of marine lipid research. Together, they sought to test a hypothesis that had been lingering in the literature — that some plankton species can use non-phosphorus 'substitute' molecules in their membranes.
On page 69, Van Mooy and his colleagues show that sulphur- and nitrogen-containing 'substitute' lipids, which in microbial membranes function similarly to phospholipids, are more abundant than phospholipids in some Sargasso Sea species. In fact, a survey of individual species in culture found that some species of algae, and photosynthetic bacteria called cyanobacteria, can use substitute lipids to reduce their phosphorus requirements by 10–30%.
The biggest challenge, Van Mooy says, was substantiating that theirs were the first descriptions of nitrogen-containing substitute lipids in the ocean. “Our work provides an example of how much remains to be discovered about the biochemicals that are present in marine organisms — and puts a biochemical face on the idea that all cells are not created equal.”