Effects of dissolved ammonium addition and host feeding with Artemia salina on photoacclimation of the hermatypic coral Stylophora pistillata

Effects of nutrient treatments on photoacclimation of the hermatypic coral Stylophora pistillata (Esper) were studied. Studies on photoacclimation of colonies from different light regimes in the field were evaluated and used to design laboratory experiments. Coral colonies were collected in the Gulf of Eilat (Israel) from January to March 1993. Exterior branches of colonie s from different depths (1 to 40 m) displayed different trends in productio n characteristics at reduced and very low levels of illumination. From 24 /- 3% to 12 +/- 2% of incident surface photosynthetic active radiation (PAR (o)), zooxanthella population density and chlorophyll cl-fu per 10(6) zooxa nthellae increased, a trend seen in the range of light levels optimal for c oral growth (90 to 30% PAR(o)). The P-max of CO2 per 10(6) zooxanthellae de creased, while P-max of CO2 per 10(3) polyps increased, indicating an incre ase in zooxanthella population density at low light levels. Proliferous zoo xanthella frequency (PZF, a measure of zooxanthella division) declined sign ificantly at light levels < 18 +/- 3% PAR(o). At the lowest levels of illum ination (< 5% PAR(o)), zooxanthella population density decreased, as did th e PZF; chi a + c per 10(6) zooxanthellae was unchanged. In 28-d experiments , exterior coral branches from the upper surfaces of colonies from 3 m dept h (65 +/- 4% PAR(o)) were incubated in aquaria under bright (80 to 90% PAR( o)), reduced (20 to 30% PAR(o)), and extremely low (2 to 4% PAR(o)) light i ntensities. At each light intensity, the corals were maintained in three fe eding treatments: sea water (SW); ammonium enriched SW (SW + N); SW with Ar temia salina nauplii (SW + A). An increase in P-max of CO2 per 10(3) polyps was found in corals acclimated to reduced light (20 to 30% PAR,) in nutrie nt-enriched SW, while in SW, where the increase in zooxanthella population density was smaller, it did not occur. Nutrient enrichments (SW + N at 2 to 4% PAR(o) and SW + A at 20 to 30% PAR(o)) increased zooxanthella populatio n density, but had no effect on chi a + c per 10(6) zooxanthellae. Acclimat ion for 14 d to reduced (10 to 20% PAR(o)) and extremely low (1 to 3% PAR(o )) light intensities shifted C-14 photoassimilation into glycerol and other compounds (probably glycerides), rather than sugars. Both ammonium additio n and feeding with Artemia salina nauplii resulted in an increase in photos ynthetic assimilation of C-14 into amino acids. We conclude that acclimatio n to reduced light consists of two processes: an increase in photosynthetic pigments and in zooxanthella population density. Both processes require ni trogen, the increase in zooxanthella population density needing more; this adaptation is therefore limited in nitrogen-poor sea water.