Effects of host nutritional status and seasonality on the nitrogen status of zooxanthellae in the temperate coral Plesiastrea versipora (Lamarck)

The nitrogen status of endosymbiotic dinoflagellates (zooxanthellae) in the temperate coral Plesiastrea versipora (Lamarck) was determined by measuring the extent to which ammonium (40 μM NH4+) enhanced the rate of zooxanthellar dark carbon fixation above that seen in filtered seawater (FSW) alone; the enhancement ratio was expressed as [dark NH4+ rate/dark FSW rate]. VD′/VL, a further index of nitrogen status, was also calculated where VD′ = [dark NH4+ rate − dark FSW rate] and VL = rate of carbon fixation in the light. When corals were starved for 2–8 weeks, zooxanthellar nitrogen deficiency became apparent at ≥ 4 weeks, with NH4+/FSW and VD′/VL averaging up to 2.08 and 0.0061, respectively. A decrease in light-saturated photosynthesis per zooxanthella also occurred, with the photosynthetic rate after 4–6 weeks being just 81% of that seen prior to starvation. In comparison, when corals were fed 5 times per week for 8 weeks the addition of ammonium had little effect, indicating nitrogen sufficiency; NH4+/FSW and VD′/VL were 1.03 and 0.0003, respectively. Photosynthetic rates of zooxanthellae from well-fed corals were up to 1.7 times greater than those of zooxanthellae from starved corals. The nitrogen status of zooxanthellae from corals in the field exhibited seasonal differences. Autumn samples were nitrogen sufficient, with NH4+/FSW = 1.003 and VD′/VL = 0.0001. In contrast, a small degree of nitrogen deficiency was seen in winter and spring, when NH4+/FSW averaged 1.075 and 1.249, and VD′/VL averaged 0.0013 and 0.0014, respectively. The greatest degree of nitrogen deficiency was observed in summer, when NH4+/FSW averaged 1.318 and VD′/VL averaged 0.0036. Given the clear links between food supply and nitrogen status seen under experimental conditions, and the likelihood that the zooxanthellae are also able to take up nutrients directly from the seawater, the fluctuations in nitrogen status may reflect temporal fluctuations in seawater nutrient concentrations and plankton abundance. The nutrient status of these temperate zooxanthellae in the field is in contrast to the marked nitrogen deficiency seen in zooxanthellae from nutrient-poor coral reef waters, and raises the possibility that temperate zooxanthellae can store nitrogen for use when exogenous nutrients and food are less readily available. This, in turn, may contribute to the considerable stability of temperate zooxanthellar populations under highly variable environmental conditions.