IMAGE COURTESY OF ANDIA CHAVES-FONNEGRA
EDITOR'S CHOICE IN MARINE BIOLOGY
F. Zhang et al., “Phosphorus sequestration in the form of polyphosphate by microbial symbionts in marine sponges,” PNAS, 112:4381-86, 2015.
Fan Zhang, a graduate student in Russell Hill’s lab at the University of Maryland Center for Environmental Science, was using microscopy to study how Caribbean coral reef sponges process nitrogen. But the sponges autofluoresced so brightly that their nitrogen-fixing bacterial symbionts were difficult to see. To detect the bacteria, Zhang applied a blue fluorescent stain called DAPI, but to his surprise, he saw something else: bright yellow dots.
An Internet search suggested that polyphosphate—chains of phosphate molecules—could be the cause, and indeed, with specific extraction methods and scanning electron microscopy, Zhang’s team observed polyphosphate granules that accounted for up to 40 percent of the phosphorus in three sponge species. To find the source, the researchers cultured the symbiotic cyanobacteria, finding that they contained not only polyphosphate granules but the genes necessary to make them.
Sponges were already known to provide carbon and nitrogen to the reef community. The symbiont-synthesized polyphosphate granules sequestered in the sponges now made it clear “that sponges are right at the center of cycling of phosphorus in coral reef ecosystems,” Hill says.
Sponges may serve as sinks that remove phosphorus from the ecosystem, says Fleur van Duyl of the Royal Netherlands Institute for Sea Research. This could explain why phosphorus is considered the limiting nutrient on some reefs, she adds. Filling in the remaining details of the sponge phosphorus cycle could help researchers predict what might happen to the nutrient balance on reefs as the climate changes and sponges become more prevalent there, Hill says.