Nitrate uptake in the scleractinian coral Stylophora pistillata

We assessed the uptake rates of nitrate by the scleractinian coral Stylophora pistillata by following 15N from seawater into the coral tissue. Two sets of corals were first prepared, with ‘‘nitrate-enriched’’ corals grown in 5 mmol L21 NO and control corals grown in #1 mmol L21 NO . Uptake rates at 0.3 and 3 mmol L21 [15N]NO 2 2 2 3 3 3 were then measured. Most of the %15N enrichment occurred in the zooxanthellae fraction. Uptake rates were not significantly different between nitrate-enriched and control corals, suggesting that they were not dependent on a nitrate acclimation. These rates increased with the in situ nitrate concentration and varied from 1.2 6 0.2 ng h21 cm22 N to 6.1 6 1.1 ng h21 cm22 N in the algal fraction at 0.3 and 3 mmol L21 [15N]NO , respectively. In a second 3 experiment, two sets of corals were prepared, with ‘‘ammonium-enriched’’ corals grown in 5 mmol L21 NH and 4 control corals grown in ,1 mmol L21 NH . Uptake rates at 3 mmol L21 [15N]NO were measured. These rates were 1 2 4 3 significantly lower with high NH concentrations in seawater. In the algal fraction, they ranged from 0.1 to 0.6 ng 4 h21 cm22 N in NH -enriched corals and from 2.2 to 4.5 ng h21 cm22 N in control corals. Nitrate can therefore be 4 considered as an important source of nitrogen for corals, at least when ammonium concentrations are low in seawater. Nitrate and ammonium are the major sources of nitrogen in the marine environment for primary production (Codispoti 1989). Reef waters usually contain low levels of inorganic nutrients. The ranges of nitrogen concentrations are typically of 0.3 to 1 mmol L21 nitrate and 0 to 0.4 mmol L21 ammonium (Bythell 1990, D’Elia and Wiebe 1990, Furnas 1991). Scleractinian corals thriving in these nutrient-poor waters have developed adaptations for conserving nitrogen. They live in symbiosis with dinoflagellates called zooxanthellae that can take up and retain dissolved inorganic nitrogen from the surrounding seawater (Muscatine 1980; Wilkerson and Trench 1986; Falkowski et al. 1993; Wang and Douglas 1998) or recycled from the host (Rahav et al. 1989). Many studies have investigated the uptake of ammonium by cultured and freshly isolated zooxanthellae (D’Elia et al. 1983; Domotor and D’Elia 1984; Yellowlees et al. 1994) as well as by the entire association (Muscatine and D’Elia 1978; Burris 1983; Wilkerson and Trench 1986; Bythell 1990; Hoegh-Guldberg and Williamson 1999; Grover et al. 2002). Ammonium is assimilated into glutamine through the action of glutamine synthetase. This enzyme is present in both the host and algal fractions. Corals are able to efficiently take up and retain ammonium from seawater, even when concentrations are as low as those measured in reef waters 1 Corresponding author ([email protected]).