Abstract
Abstract. Climate-driven changes in environmental conditions have significant and complex effects on marine ecosystems. Variability in phytoplankton elements and biochemicals can be important for global ocean biogeochemistry and ecological functions, while there is currently limited understanding on how elements and biochemicals respond to the changing environments in key coccolithophore species such as Emiliania huxleyi. We investigated responses of elemental stoichiometry and fatty acids (FAs) in a strain of E. huxleyi under three temperatures (12, 18 and 24 ∘C), three N : P supply ratios (molar ratios 10:1, 24:1 and 63:1) and two pCO2 levels (560 and 2400 µatm). Overall, C : N : P stoichiometry showed the most pronounced response to N : P supply ratios, with high ratios of particulate organic carbon vs. particulate organic nitrogen (POC : PON) and low ratios of PON vs. particulate organic phosphorus (PON : POP) in low-N media, and high POC : POP and PON : POP in low-P media. The ratio of particulate inorganic carbon vs. POC (PIC : POC) and polyunsaturated fatty acid proportions strongly responded to temperature and pCO2, both being lower under high pCO2 and higher with warming. We observed synergistic interactions between warming and nutrient deficiency (and high pCO2) on elemental cellular contents and docosahexaenoic acid (DHA) proportion in most cases, indicating the enhanced effect of warming under nutrient deficiency (and high pCO2). Our results suggest differential sensitivity of elements and FAs to the changes in temperature, nutrient availability and pCO2 in E. huxleyi, which is to some extent unique compared to non-calcifying algal classes. Thus, simultaneous changes of elements and FAs should be considered when predicting future roles of E. huxleyi in the biotic-mediated connection between biogeochemical cycles, ecological functions and climate change.
Highlights
Climate change and intensive anthropogenic pressures have pronounced and diverse effects on marine ecosystems
Our results indicate that C : N : P stoichiometry in E. huxleyi largely reflected the changes in N : P supply ratios, across different temperatures and pCO2 levels
Our study showed that the increase in PIC : particulate organic nitrogen (POC) at high temperatures was driven by a markedly increased cellular PIC content (28 %) and a decreased cellular POC content (−8 %) (Tables 1, 2), consistent with the responses of PIC : POC to warming in other E. huxleyi strains such as the strain PML B92/11 (Sett et al, 2014) and the strain CCMP3266 from the Tasman Sea (Matson et al, 2016)
Summary
Climate change and intensive anthropogenic pressures have pronounced and diverse effects on marine ecosystems. Physical and chemical properties in marine ecosystems are changing simultaneously such as the concurrent shifts in temperature, CO2 and oxygen concentrations, and nutrient availability (Boyd et al, 2015). These changes have altered trophic interactions in both bottom-up and top-down directions and resulted in changes in community structure of different trophic levels and ecosystem functions (Doney et al, 2012). Coccolithophores are a key phytoplankton group in the ocean because of their production of calcified scales called coccoliths They are important photosynthetic producers of organic matter (causing a drawdown of CO2 in the surface layer) and play predominant roles in the production and export of calcium carbonate to deeper layers (causing a net release of CO2 into the atmosphere)
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