AMMONIUM ENHANCEMENT OF DARK CARBON FIXATION AND NITROGEN LIMITATION IN ZOOXANTHELLAE SYMBIOTIC WITH THE REEF CORALS MADRACIS-MIRABILIS AND MONTASTREA-ANNULARIS

The nutrient status (limitation vs sufficiency) of dinoflagellates (zooxanthellae) symbiotic with reef corals in Bermuda was assessed in 1989 and 1990 by measuring the enhancement of dark carbon fixation with 20 muM ammonium by isolated symbionts. A colony of Madracis mirabilis was kept in the laboratory and fed daily or starved for one month. Symbionts from fed portions of the colony had ammonium-enhancement ratios (NH4 dark+; SW(dark); SW = seawater without added ammoniUm) similar to those of the original field population (1.2 to 1.3). Ammonium-enhancement ratios increased with starvation of the host (xBAR greater-than-or-equal-to 1.7) as did values for V(D') : V(L) [(ammonium dark rate - seawater dark rate): light rate in seawater]. Both parameters indicated decreasing nitrogen sufficiency of the algae when the host was not fed, but starvation appeared to affect these algae less than symbionts of sea anemones. Field samples of zooxanthellae from M. mirabilis (Three Hill Shoals and Bailey's Bay Flats) yielded results similar to those for fed corals, but those taken from Bailey's Bay Flats in May 1990 yielded exceptionally high values for enhancement (>3) and V(D') : V(L) indicating pronounced nitrogen limitation at the time of sampling. We sampled zooxanthellae from populations of Montastrea annularis at 8 m (Three Hill Shoals) and 24 m (Soldier's Point) depths. Enhancement and V(D') : V(L) values for zooxanthellae from the 8 m corals were density-dependent: symbionts from corals with ''normal'' symbiont densities displayed the most nitrogen limitation (enhancement values = 1.4 to 2.0), while those from bleached corals with lower density exhibited enhancement and V(D') : V(L) values typical of nitrogen-sufficient algae. Symbionts isolated from the 25 m corals yielded the highest values, and appeared to exhibit the least nitrogen-sufficiency for this species.