Nitrogen fixation in symbiotic marine sponges: Ecological significance and difficulties in detection

There has been considerable speculation, and some evidence, that coral reef sponges can fix atmospheric nitrogen through some of their microbial symbionts, particularly symbiotic cyanobacteria. Many Indo-Pacific coral reef sponges can satisfy much of their requirement for carbon energy compounds via translocation from photosynthetic symbionts, and a similar mechanism has been invoked to explain how some sponges could supplement the low amount of available nitrogen in clear tropical waters. Attempts to measure nitrogen fixation using the acetylene reduction test have proven technically difficult and given ambiguous results. However, fixation was demonstrated unambiguously with incorporation of the stable isotope ¹⁵N₂ into the amino acids glutamine, glutamate and aspartate of Callyspongia muricina, although at relatively low rates. The variability in measuring acetylene reduction in 23 sponge species is attributed to several factors: the number of cellular and matrix barriers that must be passed by acetylene and ethylene; the difficulty of maintaining sponges alive under experimental conditions; possible metabolism of ethylene by symbiotic bacteria; and possible toxiciry of the reagents.