COLIMITATION OF PHYTOPLANKTON GROWTH BY NICKEL AND NITROGEN

Axenic cultures of Thalassiosira weissflogii need nickel when urea is the source of nitrogen for growth, but have no measurable Ni requirement when growing on NH4+. This result is due to the Ni-containing enzyme, urease, which is necessary for urea catabolism in this diatom. In Ni-deficient cultures, where urea is the sole source of N, urease activity is low and growth rate is reduced. In Ni-replete cultures, urease activity is enhanced and the maximum growth rate achieved. Saturated uptake rates of Ni by urea-grown cells are faster than by NH4+-grown cells and are enhanced under Ni-limiting conditions. Ni-limited T. weissflogii cells are also N limited, even with high concentrations of urea in the medium, because they have insufficient urease activity to provide adequate N for growth. In these cultures, maximum growth rates are thus obtained by supplying either Ni or NH4+. These results demonstrate colimitation of T. weissflogii growth by Ni and N under laboratory conditions and provide an archetypal example of a trace metal and macronutrient interaction that may be applicable to other nutrients.