Sedimentary Nitrogen Uptake & Assimilation in the Temperate Zooxanthellate Anemone Anthopleura aureoradiata

This study investigated the potential for, and efficiency of, particulate nitrogen uptake from the sediment and subsequent consequences of this on the nutrient status of endosymbiotic dinoflagellates (zooxanthellae) in the temperate zooxanthellate anemone Anthopleura aureoradiata. Sediment was collected from a mudflat and labelled with (15NH4)2SO4 before being provided to A. aureoradiata at low (5 g dry weight) and high sediment (20 g dry weight) loads for 6 hours. While no discernible change in the isotopic content of the sediment could be detected, analysis of the host and algal symbionts revealed that 15N had been taken up. Uptake by the host was similar at both high and low sediment loads, but the algal symbionts acquired more nitrogen at the lower load (1.13 versus 0.93 atom % 15N in the low and high loads, respectively). Evaluation of this particulate nitrogen uptake from the sediment was further examined by measuring the nitrogen status of the zooxanthellae. This was determined by measuring the extent to which ammonium (40 muM NH+4) enhanced the rate of zooxanthellar dark carbon fixation above that seen in filtered seawater (FSW) alone; the enhancement ratio was expressed as [dark NH+4 rate/dark FSW rate]. VD'/VL, a further index of nitrogen status, was also calculated where VD' = [dark NH+4 rate - dark FSW rate] and VL = rate of carbon fixation in the light. When anemones were starved for 2-8 weeks, zooxanthellar nitrogen deficiency became apparent at greater than or equal to 4 weeks, with NH+4/FSW and VD'/VL averaging up to 2.90 and 0.11, respectively. In comparison, when anemones were fed 5 times per week for 8 weeks the addition of ammonium had little effect, indicating nitrogen sufficiency; NH+4/FSW and VD'/VL values were 1.03 and -1.0 x 10-3, respectively. The nitrogen status of zooxanthellae from anemones starved and incubated with and without sediment was examined with no apparent difference between sediment and no sediment treatments; zooxanthellar nitrogen deficiency became apparent at greater than or equal to 4 weeks in both treatments, with NH+4/FSW and VD'/VL averaging up to 3.73 and 0.17 for the sediment treatment and 2.74 and 0.15 for the no sediment treatment, respectively. The nitrogen status of zooxanthellae from anemones found on a mudflat (Pauatahanui Inlet) and a rocky intertidal site (Kau Bay) was different. Zooxanthellae from mudflat anemones were nitrogen sufficient with NH+4/FSW and VD'/VL values averaging up to 1.26 and -6.0 x 10-3, respectively. Nitrogen deficient zooxanthellae were present in anemones from the rocky intertidal. Anemones from tide pools in the upper littoral zone had NH+4/FSW and VD'/VL values of 2.99 and 0.11, respectively, while anemones from the mid littoral zone had NH+4/FSW and VD'/VL of 2.90 and 0.13, respectively; there was no significant difference in nitrogen status between zooxanthellae from high shore tide pool anemones and aerially exposed mid-littoral anemones. These results suggest that while particulate nitrogen can be taken up from the sediment by this species, dissolved inorganic nitrogen such as ammonium in the seawater, and especially the interstitial water surrounding infaunal anemones on mudflats, may be a more important source of nitrogen in the field.