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 NH 4 + analogue [ 14 C]methylamine ([ 14 C]MA) to examine these aspects of NH 4 + uptake by a sea anemone ( Aiptasia pallida ) and a coral ( Madracis decactis ). Both intact symbioses and isolated zooxanthellae took up [ 14 C]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 ( K s = 68 µ M; V = 3.8 mol 10 −1 8 cell −1 s −1 ). Isolates from well‐fed hosts took up [ 14 C]MA much less rapidly at all concentrations tested and did not exhibit saturable uptake kinetics. NH 4 + competitively inhibited [ 14 C]MA uptake by isolated algae (inhibition constant = 4.0 µ M) and reduced [ 14 C]MA uptake by intact symbiotic anemones. We hypothesize that [ 14 C]MA (and by analogy NH 4 +) uptake occurs by a “depletion‐diffusion” mechanism in intact symbiotic anemones with zooxanthellae maintaining very low intracellular [ 14 C]MA and NH 4 + concentrations in host tissue and that [ 14 C]MA uptake kinetics will be useful in evaluating the nutritional status of corals and similar symbiotic associations under field conditions.