Nitrogen fixation in coral reef sponges with symbiotic cyanobacteria

Abstract

NITROGEN FIXATION by endosymbiotic cyanobacteria (blue-green algae) results in an important input of nitrogen into terrestrial ecosystems¹; however, there is as yet little information on its significance in the marine environment^2,3. Symbiosis of cyanobacteria with marine animals, although rare, is known to occur in an echiuroid worm⁴ and in sponges from coral reefs⁵ and the Mediterranean⁶. In the present study of sponge–cyanobacterial symbioses, several sponges from a coral reef in the Red Sea were tested, using the acetylene reduction technique⁷, immediately after their collection on reef-based platforms for their ability to fix nitrogen. Nitrogenase activity was detected in two sponges with cyanobacteria but not in a third with no cyanobacteria. This is the first demonstration of nitrogenase activity in an animal with symbiotic cyanobacteria. In two previous reports of nitrogen fixation in marine animals, the activity was attributed to bacteria in the gut^8,9. Although these preliminary experiments are not sufficient to assess the significance of cyanobacterial nitrogenase activity in sponges, it is possible that any additional fixed nitrogen would be beneficial to sponges in tropical waters which are low in available nitrogen¹⁰ and in particulate nutrients^11,12.