Nitrogen flux in the symbiotic sea anemone Anemonia viridis (Forskal)
The aim of this study was to investigate factors affecting the uptake of dissolved inorganic nitrogen (DIN) by the temperate symbiotic sea anemone, Anemonia viridis. Laboratory experiments were used to test DIN uptake rates of symbiotic and aposymbiotic (alga-free) individuals, to enable the prediction of the magnitude of DIN flux rates in anemones in their natural environment, and to allow the construction of a model of the DIN fluxes in the association. It was also hoped to assess the potential for fully autotrophic growth in this species. Uptake rates were measured by following depletion of DIN from small-volume incubation chambers that had been enriched to produce DIN concentrations across the reported range for temporate coastal waters. At light levels of 190 E.m^-2, ammonia was taken up by symbiotic anemones from all concentrations tested, whilst aposymbionts lost ammonia to the surrounding sea water. Nitrate was not taken up by any of the anemones. The relationship between the weight-specific ammonia flux rate and ambient ammonia concentration was linear for both symbionts and aposymbionts. In all cases the slope of the relationship was positive, symbionts showing increasing rates of uptake and apsoymbionts showing decreasing rates of efflux with increasing ammonia concentration. There was no evidence of Michaelis-Menten uptake kinetics across the range of concentrations used, despite extending this to 30 g-at NH3 - N.1-1 in an attempt to show saturation of uptake. In darkness, the elevation of the rate/concentration relationship was depressed for symbionts, but not for aposymbionts, so that symbionts showed efflux of ammonia at concentrations below 4 g-at NH_3-N.1^-1. The elevation increased with light intensity up to 50 E.m-2.s-1, whilst the slope of the relationship remained constant. Above 50 E.m^-2 the elevation could not increase, but the slope became greater. Feeding the anemones with squid mantle tissue three days prior to experiments had variable effects on ammonia flux rates, but there was a tendency for the elevation of the rate/concentration relationship to drop in recently fed anemones, compared to starved anemones at the same light intensity. Effects are likely to be more marked over the first 24 hours after feeding, and ammonia flux during this period may have significant consequences for the nitrogen balance of the association. Freshly isolated zooxanthellae took up ammonia, and showed Michaelis-Menten type uptake kinetics, with k_s between 3 and 7 g-at NH3 -N.1-1, and max between 0.025 and 0.043 g-at NH_3-N.(g chla)-1.h-1. Uptake rates were not affected by light level, and were an order of magnitude higher than those of the intact association over the range of experimental concentrations used. Incubation of the intact association with 15 NH3-enriched sea water resulted in the 15 N being incorporated into the zooxanthellae, and not the host tissue, over 30 minutes exposure to the labelled ammonia. This showed directly that the algae are responsible for the ammonia uptake of the association. The results obtained are consistent with a two-component uptake system, diffusion across the animal cell membrane being driven by a concentration gradient produced by algal ammonia assimilation. The observed kinetics are the result of two antagonistic processes; the catabolic production of ammonia by the animal cell, and the active uptake of ammonia by the algae. It appears that algal ammonia uptake is saturated at irradiances of 50 E.m^-2.s^-1 and below, and unsaturated at higher irradiances. Light levels and ammonia concentrations were measured in the vicinity of a natural population of Anemonia viridis in Loch Sween, Argyll. The laboratory experiments were used to predict the net metabolic gain or loss of DIN experienced by anemones under the measured field conditions. Ambient ammonia concentrations were negligible around the anemones in Loch Sween throughout the year, and under such conditions a net loss of nitrogen would have occurred, making a heterotrophic input of nitrogen necessary for growth. Net nitrogen gain may, however, occur at ambient ammonia concentrations that could exist in rockpools further south in Britain, where A. viridis is frequently the dominant animal species. A laboratory experiment was conducted to test for the possibility of autotrophic growth in the presence of adequate levels of DIN. Unfortunately, experimental conditions did not consistently provide high enough concentrations of ammonia for net nitrogen gain to be predicted, but anemones exposed to elevated levels of DIN for 8 weeks lost significantly less weight than anemones kept in sea water with negligible ammonia concentrations. The weight loss of the DIN-'fed' anemones corresponded closely to the weight of lost nitrogen predicted from the flux experiments, after conversion to equivalent weight of protein. It remains to be seen if Anemonia viridis can survive and grow autotrophically, but even in the nitrogen-poor environment of a Scottish sea loch, the algae can potentially supply 86-90% of the associations basal nitrogen requirements. The capacity for DIN uptake is clearly an important factor in the success of this species in temperate waters.