Nutrition of algal-invertebrate symbiosis. II. Effects of exogenous nitrogen sources on growth, photosynthesis and the rate of excretion by algal symbionts in vivo and in vitro

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Abstract
The effect of increasing concentrations of ammonia, nitrate and urea was tested on (1) axenic cultures of the symbiont Gymnodinium microadriaticum , (2) symbionts freshly isolated from the tissues of the staghorn coral Acropora cervicornis , and (3) symbionts in vivo in this same host. Rates of symbiont photosynthesis and excretion were measured by using H 14 CO 3 –. Rates of skeletogenesis in A. cervicornis were measured using 45 Ca 2+ . Growth was determined by cell count. Analysis of growth rates in established cultures and freshly isolated symbionts, shows no apparent stimulation by these nitrogen additions at the concentrations used. Growth is comparable to controls. Ammonia, in concentrations ranging between 20 and 200μM, increases the photosynthetic rate of symbionts in vivo and in vitro . The rate of excretion of labelled photosynthate is also stimulated. Calcification by the host is increased proportionally in these same experiments. Ammonia addition has no measurable effect in the dark or in the presence of 10 -5 M 3, 3:4-dichlorophenyl-1 :1-dimethyl urea (DCMU). Similar results are obtained with urea additions in vivo . Increasing concentrations of nitrate in vivo and in vitro and urea in vitro do not stimulate photosynthesis, excretion or calcification (nitrate only) in these experiments.