Optimal nitrogen-to-phosphorus stoichiometry of phytoplankton

Abstract

Redfield noted the similarity between the average nitrogen-to-phosphorus ratio in plankton (N:P = 16 by atoms) and in deep oceanic waters (N:P = 15; refs 1, 2). He argued that this was neither a coincidence, nor the result of the plankton adapting to the oceanic stoichiometry, but rather that phytoplankton adjust the N:P stoichiometry of the ocean to meet their requirements through nitrogen fixation, an idea supported by recent modelling studies. But what determines the N:P requirements of phytoplankton? Here we use a stoichiometrically explicit model of phytoplankton physiology and resource competition to derive from first principles the optimal phytoplankton stoichiometry under diverse ecological scenarios. Competitive equilibrium favours greater allocation to P-poor resource-acquisition machinery and therefore a higher N:P ratio; exponential growth favours greater allocation to P-rich assembly machinery and therefore a lower N:P ratio. P-limited environments favour slightly less allocation to assembly than N-limited or light-limited environments. The model predicts that optimal N:P ratios will vary from 8.2 to 45.0, depending on the ecological conditions. Our results show that the canonical Redfield N:P ratio of 16 is not a universal biochemical optimum, but instead represents an average of species-specific N:P ratios.

Phytoplankton niche generation by interspecific stoichiometric variation

“…Individual species' stoichiometric coefficients differ from one another, and those of one species differ in time and space [Geider and La Roche, 2002;Anderson and Pondaven, 2003;Klausmeier et al, 2004]. Combining the findings of this study with evidence from data supports the need for going beyond the Redfield Ratio as a common stoichiometry in models with multiple phytoplankton compartments.…”

Hindsight in the relative abundance, metabolic potential and genome dynamics of uncultivated marine archaea from comparative metagenomic analyses of bathypelagic plankton of different oceanic regions

“…If this is indeed the case and both archaeal groups were actually planktonic and free-living, a second, alternative hypothesis to explain the observed differences in genomic GC content could be that Group I Crenarchaeota had shorter generation times, behaving more like r-strategists or 'bloomers', as compared to Group II Euryarchaeota, which would have longer generation times relative to Group I Crenarchaeota, sustaining growth when resources are low and behaving as K-strategists or 'survivalists'. These different lifestyle strategies are indeed related to the N/P ratios in the cell in marine pelagic ecosystems (Klausmeier et al, 2004;Arrigo, 2005). Having low GC genomes is a way to save N, a limiting element in oligotrophic oceanic waters (AT having seven N atoms, one N atom less than GC pairs, making a significant difference at the whole genome scale).…”

Mapping the distribution, biomass and tissue nutrient levels of a marine benthic cyanobacteria bloom (Lyngbya majuscula)

“…One obvious explanation for higher N : P ratios in nitrogen-fixing species is that autotrophs are able to fix nitrogen from the atmosphere, potentially allowing accumulation of nutrient reserves (Geider and La Roche 2002). Another reason hypothesised by Klausmeier et al (2004) is that the energetically costly process of nitrogen-fixation leads to less carbon being incorporated per unit of light energy absorbed. This leads to an increase in allocation to light-harvesting proteins and chloroplasts that are rich in nitrogen and poor in phosphorus, and a decrease in allocation to assembly machinery (ribosomes) which have an N : P ratio closer to the Redfield ratio.…”