Methylamine uptake by zooxanthellae‐invertebrate symbioses: Insights into host ammonium environment and nutrition

Abstract

Cnidarians with endosymbiotic algae (=zooxanthellae) take up dissolved inorganic nutrients from seawater, but neither the physiological mechanisms nor the effect of host nutrition on transport kinetics is known. We used the NH4+ analogue [14C]methylamine ([14C]MA) to examine these aspects of NH4+ uptake by a sea anemone (Aiptasia pallida) and a coral (Madracis decactis).Both intact symbioses and isolated zooxanthellae took up [14C]MA. In anemones, uptake rates per algal cell increased with time after feeding. Uptake rates for isolates from hosts unfed for 7– 10 d were linear for at least 200 min, slightly light‐dependent, and conformed to Michaelis‐Menten kinetics (Ks = 68 µM; V = 3.8 mol 10−18 cell−1 s−1). Isolates from well‐fed hosts took up [14C]MA much less rapidly at all concentrations tested and did not exhibit saturable uptake kinetics. NH4+ competitively inhibited [14C]MA uptake by isolated algae (inhibition constant = 4.0 µM) and reduced [14C]MA uptake by intact symbiotic anemones. We hypothesize that [14C]MA (and by analogy NH4+) uptake occurs by a “depletion‐diffusion” mechanism in intact symbiotic anemones with zooxanthellae maintaining very low intracellular [14C]MA and NH4+ concentrations in host tissue and that [14C]MA uptake kinetics will be useful in evaluating the nutritional status of corals and similar symbiotic associations under field conditions.