Surge ammonium uptake of the cultured seaweed, <i>Kappaphycus alvarezii</i> (Doty) Doty (Rhodophyta: Gigartinales)

Abstract

The possibility that the cultured seaweed Kappaphycus alvarezii may demonstrate surge ammonium uptake was studied using the perturbation method. Cultivars of the green morphotype of K. alvarezii were incubated at three different initial NH4+ concentrations (i.e., 10, 20, 30 μM) at the same light intensity. Additional incubations were conducted at 30 μM initial NH4+ concentration in the dark. K. alvarezii were also placed in a 30 μM NH4+-enriched seawater medium for 1 h before incubation at 30 μM initial NH4+ concentration. Levels of enrichment and time had significant effects on ammonium uptake rates. Surge ammonium uptake tended to be higher with higher initial ammonium concentration. Mean uptake rates at 10 μM NH4+ initial concentration were significantly lower compared to other treatments. Higher variability in uptake was noted during the first hour of incubation. Ammonium uptake in the dark was not significantly different from that in light (at 30 μM NH4+ initial concentration). Surge uptake was higher for incubations without additional enrichment than when the alga was additionally exposed to enriched medium before the experiment. On average, surge uptake (VS) occurred within the first 30 min and ranged from 15 to 35 μmol NH4+ g−1 DW h−1. Internally controlled uptake (VI) ranged from 5 to 15 μmol NH4+ g−1 DW h−1 and occurred from 30 min to 6 h after exposure to the nutrient. Externally controlled uptake (VE) was less than 5 μmol NH4+ g−1 DW h−1 and occurred from 6 to 8 h after exposure when the medium was significantly depleted of ammonium (as in 10 and 20 μM NH4+ initial concentrations). K. alvarezii from Danajon Reef, central Philippines thus exhibits surge ammonium uptake and some form of nitrogen limitation. Such a strategy is needed for surviving in environments with low or erratic nutrient supplies.