Elemental stoichiometry of the key calcifying marine phytoplankton Emiliania huxleyi under ocean climate change: A meta‐analysis

• Published in: Global change biology Vol. 29; no. 15; pp. 4259 - 4278
• Main Authors: Sheward, Rosie M., Liefer, Justin D., Irwin, Andrew J., Finkel, Zoe V.
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01.08.2023
• Subjects: Additives; Analysis; Biogeochemistry; C:N:P; Calcification; Chemical composition; Climate Change; coccolithophore; Ecological models; Ecosystem; elemental composition; Emiliania huxleyi; Environment models; Environmental changes; Environmental conditions; Environmental stress; Functional groups; Haptophyta; Irradiance; Marine ecosystems; Marine microorganisms; Meta-analysis; Microorganisms; Nutrient availability; Ocean-atmosphere system; Oceans and Seas; Phytoplankton; Phytoplankton - physiology; PIC:POC; Plankton; Redfield; Stoichiometry; Temperature; coccolithophore; Redfield; PIC:POC; C:N:P; elemental composition; phytoplankton; calcification; climate change
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/gcb.16807

The elemental composition of marine microorganisms (their C:N:P ratio, or stoichiometry) is central to understanding the biotic and biogeochemical processes underlying key marine ecosystem functions. Phytoplankton C:N:P is species specific and flexible to changing environmental conditions. However, bulk or fixed phytoplankton stoichiometry is usually assumed in biogeochemical and ecological models because more realistic, environmentally responsive C:N:P ratios have yet to be defined for key functional groups. Here, a comprehensive meta‐analysis of experimental laboratory data reveals the variable C:N:P stoichiometry of Emiliania huxleyi, a globally significant calcifying phytoplankton species. Mean C:N:P of E. huxleyi is 124C:16N:1P under control conditions (i.e. growth not limited by one or more environmental stressors) and shows a range of responses to changes in nutrient and light availability, temperature and pCO2. Macronutrient limitation caused strong shifts in stoichiometry, increasing N:P and C:P under P deficiency (by 305% and 493% respectively) and doubling C:N under N deficiency. Responses to light, temperature and pCO2 were mixed but typically shifted cellular elemental content and C:N:P stoichiometry by ca. 30% or less. Besides these independent effects, the interactive effects of multiple environmental changes on E. huxleyi stoichiometry under future ocean conditions could be additive, synergistic or antagonistic. To synthesise our meta‐analysis results, we explored how the cellular elemental content and C:N:P stoichiometry of E. huxleyi may respond to two hypothetical future ocean scenarios (increased temperature, irradiance and pCO2 combined with either N deficiency or P deficiency) if an additive effect is assumed. Both future scenarios indicate decreased calcification (which is predominantly sensitive to elevated pCO2), increased C:N, and up to fourfold shifts in C:P and N:P. Our results strongly suggest that climate change will significantly alter the role of E. huxleyi (and potentially other calcifying phytoplankton species) in marine biogeochemical processes. Marine biology and ocean biogeochemistry are closely linked to the elemental stoichiometry (C, N and P content) of marine microorganisms, which varies across (phyto)plankton groups and is responsive to changes in environmental conditions. Using a meta‐analysis of published data, we find that the stoichiometry of the coccolithophore species Emiliania huxleyi shows mixed responses to changes in nutrient and light availability, temperature and pCO2, ranging from no effect to greater than a fourfold change. These findings provide insights into the global success of E. huxleyi and suggest that climate change will substantially alter the role of E. huxleyi in marine biogeochemical processes.