Safety factors and nutrient uptake by seaweeds

Abstract

The differences in the kinetic characteristics of nutrient (nitrate, ammonium and phosphate) uptake by seaweeds are evaluated using published half-saturation constants (K m ), maximum rates of uptake (V max ) and calculated safety factors for nitrate, ammonium and phosphate uptake. The safety factor is the ratio of maximum nutrient uptake rate to uptake rate at maximum ambient concentration of nutrient and provides a simple estimate of the amount of surplus capacity of a nutrient uptake system. Frequency distributions of K m values for nitrate, ammonium and phosphate uptake show similar patterns, though values tend to be lower for phosphate uptake and greater for ammonium uptake. There is no relationship between the K m values for nitrate or ammonium uptake and seaweed surface area:volume ratio (SA:V). Frequency distributions of V max :K m values for nitrate, ammonium and phosphate uptake show similar patterns, but V max :K m values tend to be lower for phosphate uptake and greater for ammonium uptake. Moreover, rates of nitrate uptake (at 5 μM nitrate) are comparable to rates of ammonium uptake (at 1.5 μM ammonium) at any value of seaweed SA:V ratio, i.e. the efficiency of ammonium uptake is greater. For nitrate and phosphate uptake by seaweeds and ammonium uptake by phytoplankton values for safety factors are low. In contrast, values for ammonium uptake by seaweeds are high. It is suggested that the reason for the high surplus capacity for ammonium uptake in seaweeds is a combination of the size of the plant and the spatially and temporally variable concentration of ammonium in the seawater that surrounds these plants.