title: Unimodal size scaling of phytoplankton growth and the size dependence of nutrient uptake and use.
abstract: Phytoplankton size structure is key for the ecology and biogeochemistry of pelagic ecosystems, but the relationship between cell size and maximum growth rate (μ(max) ) is not yet well understood. We used cultures of 22 species of marine phytoplankton from five phyla, ranging from 0.1 to 10(6)  μm(3)  in cell volume (V(cell) ), to determine experimentally the size dependence of growth, metabolic rate, elemental stoichiometry and nutrient uptake. We show that both μ(max)  and carbon-specific photosynthesis peak at intermediate cell sizes. Maximum nitrogen uptake rate (V(maxN) ) scales isometrically with V(cell) , whereas nitrogen minimum quota scales as V(cell)             (0.84) . Large cells thus possess high ability to take up nitrogen, relative to their requirements, and large storage capacity, but their growth is limited by the conversion of nutrients into biomass. Small species show similar volume-specific V(maxN)  compared to their larger counterparts, but have higher nitrogen requirements. We suggest that the unimodal size scaling of phytoplankton growth arises from taxon-independent, size-related constraints in nutrient uptake, requirement and assimilation.
authors.affiliation: Departamento de Ecología y Biología Animal, Universidad de Vigo, 36210, Vigo, Spain. em@uvigo.es
authors.forename: Emilio
authors.initials: E
authors.lastname: Marañón
authors.forename: Pedro
authors.initials: P
authors.lastname: Cermeño
authors.forename: Daffne C
authors.initials: DC
authors.lastname: López-Sandoval
authors.forename: Tamara
authors.initials: T
authors.lastname: Rodríguez-Ramos
authors.forename: Cristina
authors.initials: C
authors.lastname: Sobrino
authors.forename: María
authors.initials: M
authors.lastname: Huete-Ortega
authors.forename: José María
authors.initials: JM
authors.lastname: Blanco
authors.forename: Jaime
authors.initials: J
authors.lastname: Rodríguez
mesh_terms: D002244:Carbon / Q000378:metabolism; D049108:Cell Enlargement; D009584:Nitrogen / Q000378:metabolism; D010839:Phytoplankton / Q000166:cytology / Q000254:growth & development* / Q000378:metabolism
chemical_list: D002244:Carbon; D009584:Nitrogen