title: The impact of irradiance on optimal and cellular nitrogen to phosphorus ratios in phytoplankton.
abstract: Phytoplankton acclimates to irradiance by regulating the cellular content of light-harvesting complexes, which are nitrogen (N) rich and phosphorus (P) poor. Irradiance is thus hypothesised to influence the cellular N : P ratio and the N : P defining the threshold between N and P limitation (the 'optimal' N : P). We tested this hypothesis by first addressing the response of the optimal N : P to irradiance in a controlled experiment with Chlamydomonas reinhardtii. Then, we did a meta-analysis of experimental data on optimal and cellular N : P ratios across light gradients to test the generality of an N : P to light response within species. In both the experiment and the meta-analysis, N : P ratios decreased with irradiance, indicating that factors affecting underwater irradiance, like depth and the composition of the water, may influence the relative N : P requirement. The effect of irradiance did not differ between optimal and cellular N : P ratios, but observations of optimal N : P were on average 2.8 times higher than observations of cellular N : P.
authors.affiliation: Section for aquatic biology and toxicology (AQUA), Department of Biosciences, University of Oslo, P.O. Box 1066, Blindern, 0316, Oslo, Norway.
authors.forename: Jan-Erik
authors.identifier: 0000-0001-8961-6474
authors.initials: JE
authors.lastname: Thrane
authors.affiliation: Section for aquatic biology and toxicology (AQUA), Department of Biosciences, University of Oslo, P.O. Box 1066, Blindern, 0316, Oslo, Norway.
authors.forename: Dag O
authors.initials: DO
authors.lastname: Hessen
authors.affiliation: Section for aquatic biology and toxicology (AQUA), Department of Biosciences, University of Oslo, P.O. Box 1066, Blindern, 0316, Oslo, Norway.
authors.forename: Tom
authors.initials: T
authors.lastname: Andersen
mesh_terms: D016825:Chlamydomonas reinhardtii / Q000378:metabolism / Q000528:radiation effects*; D008027:Light; D009584:Nitrogen / Q000378:metabolism*; D010758:Phosphorus / Q000378:metabolism*; D010839:Phytoplankton / Q000378:metabolism / Q000528:radiation effects*
chemical_list: D010758:Phosphorus; D009584:Nitrogen
keywords: Chlamydomonas reinhardtii; N : P ratio; Redfield ratio; ecological stoichiometry; irradiance; light; nitrogen; optimal N : P; phosphorus