Light, balanced and unbalanced growth and ammonium assimilation kinetics of Ulva

Abstract

Macroalgae are important primary producers in coastal and estuarine waters and they depend on two essential resources: light and nutrients, but there are occasions when one or both can limit growth rate. Therefore, an improved understanding of how they acquire nutrients and how light and/or nutrient limitation impacts their growth and physiology is essential to understanding the ecology and aquaculture of macroalgae, particularly fast-growing, and potentially nuisance, species of Ulva. The kinetics of ammonium assimilation in Ulva was investigated using three techniques. The unifying component of all three methods is that the internally-controlled phase (Vi) of the perturbation method is numerically equal to the maximum rate of ammonium assimilation and therefore Vmax of the Michaelis-Menten formula is effectively already defined. The first method used two phases (Vi and Ve) of the perturbation method to determine kinetics, the second used the multiple-perturbation method and the third the CCCP method, which, unlike the other two methods, has independent evidence to support its validity. The relationships between rate of ammonium uptake and ammonium concentration for the first two methods had little correspondence to the relationship between rate of ammonium assimilation and ammonium concentration using the CCCP method, suggesting that the first two methods were not measuring the kinetics of ammonium assimilation. Following transfer of U. pertusa to decreased photon flux densities (10, 15, 30 and 50% ambient light) in outdoor cultures there was an initial phase (first 7 days) of unbalanced growth in which tissue at decreased photon flux density had similar, and not significantly different, growth rates measured as increases in total chlorophyll to that for the control (100% ambient light). Growth rates measured as increases in tissue fresh weight, dry weight, ash-free dry weight or surface area were significantly lower at decreased photon flux densities. After the period of unbalanced growth, a new steady-state was achieved and growth became balanced again, with no significant differences in the relationships between growth rate measured by any constituent and % ambient light. The qmin (photon flux density at which growth rate is zero) derived from the Droop relationship was 1.3 ± 0.1 mol photons m⁻² d⁻¹. There was a strong positive relationship between total chlorophyll and nitrogen (as %N) content, suggesting that growth rates measured as increases in tissue nitrogen would be similar to those calculated as increases in total chlorophyll. U. pertusa maintained at iv 10% ambient light in spring had significantly greater rates of ammonium assimilation when measured at 30 or 300 μmole m⁻² s⁻¹ compared with those from 100% light, but there was little difference in rates of assimilation for U. pertusa maintained at 10% and 100% ambient light in autumn and winter, possibly because the growth rate at 10% was close to qmin. Growth of U. pertusa in summer was N-limited and addition of nitrogen at low photon flux densities in indoor cultures led to high growth rates measured as increases in total chlorophyll. However, the relationship between total chlorophyll and nitrogen (as %N) content was poor, suggesting that increases in light harvesting pigment-protein complexes accounted for little of the increase in % N particularly in the 100% light treatment. Depriving U. pertusa of nutrients (nitrogen and phosphorus) resulted in even lower total chlorophyll content than with natural nitrogen deficiency. The increase in total chlorophyll content after addition of nitrate and phosphate at low photon flux density combined with the increase in biomass, yielded a growth rate measured as an increase in total chlorophyll of 0.67 d-1 that exceeds the highest growth rate ever measured in Ulva. Issues relating to the use of the Michaelis-Menten and Droop equations in uptake kinetics and light-limited growth rate, respectively, the advantages and disadvantages of using indoor and outdoor cultures, the relationship between total chlorophyll and %N in N-deficient U. pertusa and future experiments are discussed.