HOW CRITICAL IS THE CRITICAL N:P RATIO?¹

Abstract

The value of cellular N:P that corresponds to co‐limitation by N and P, the critical (Rcrit) or optimum ratio, has been used to infer the competitive advantage of phytoplankton growing in P‐impoverished systems. Using a revised quota model, with a normalized quota function and capable of simulating surge transport, the interactions between the minimum P‐quota (PCo), the shape of the P‐quota–cell growth relationship (affected by constant KQP), and transport kinetics in affecting the utility of Rcrit are considered. For a low PCo to endow an organism with a high Rcrit over a wide range of growth rates, the P‐quota curve must be more hyperbolic (KQP low) rather than linear (KQP high). PCo and KQP also affect the half saturation constant for growth, KgP ; this and the capacity to transport nutrients at rates above those required to sustain steady‐state growth endows a competitive advantage. However, the kinetics of transport into the organism have a greater potential for affecting KgP than changing the kinetics of internal P usage. Thus, the value of Rcrit is not a critical factor affecting competition except in extreme oligotrophic conditions. For competition between species, nutrient transport, accumulation, and resource utilization are all important. However, the efficiency of internal resource utilization is of lesser importance, and certainly not of greater importance, than resource acquisition. Multinutrient models intended to describe competition need to recognize these interactions; the traditional quota model is poorly equipped to do so.

Estimating the carbon flux through main phytoplankton groups in the northwestern Mediterranean

“…On the basis of physical constraints, size can be considered a first proxy of physiological processes related to surface : volume ratios, such as diffusive nutrient uptake or incoming irradiance. However, active transport of substances (Flynn 2002), adaptation to different irradiance regimes (Moore et al 1998), and metabolic activity spent in nongrowing activities (Buitenhuis et al 1999) are processes with an impact on the final capacity to generate growth that are not necessarily linked to size.…”

Effects of nitrate : phosphate supply ratio and irradiance on the C : N : P stoichiometry ofChaetoceros muelleri

“…Do phytoplankton expend energy regulating the uptake of these elements irrespective of the environmental conditions or is there a 'modus operandi' within which little regulation is necessary? Evidence on luxury consumption (Elrifi & Turpin, 1985) and a recent model which shifts the emphasis to transport kinetics and efficiency of internal resource utilization (Flynn, 2002) support the suggestion of environmentally controlled regulation.…”

Monster potential meets potential monster: pros and cons of deploying genetically modified microalgae for biofuels production

“…There is far greater variability in these parameter values than for NC 0 and KQN, and they are recognized as important features in competition between microalgae [33,34]. Because of the mixed structural and energetic/regulatory functions of P (contrasting with the mainly structural functions of N), the value of KQP is much lower than KQN, and the resultant often strongly curvilinear relationship between P : C and growth rate indicates that the cells alter the efficiency of P usage as P becomes limiting [16,19].…”