Organic matter exudation by Emiliania huxleyi under simulated future ocean conditions

[摘要] Emiliania huxleyi (strain B 92/11) was exposed to different nutrient supply, CO₂ andtemperature conditions in phosphorus controlled chemostats to investigateeffects on organic carbon exudation and partitioning between the pools ofparticulate organic carbon (POC) and dissolved organic carbon (DOC).¹⁴C incubation measurements for primary production (PP) andextracellular release (ER) were performed. Chemical analysis included the amountand composition of high molecular weight (>1 kDa) dissolved combinedcarbohydrates (HMW-dCCHO), particulate combined carbohydrates (pCCHO) andthe carbon content of transparent exopolymer particles (TEP-C). AppliedCO₂ and temperature conditions were 300, 550 and 900 μatmpCO₂ at 14 °C, and additionally 900 μatm pCO₂ at 18 °Csimulating a greenhouse ocean scenario.

Enhanced nutrient stress by reducing the dilution rate (D) from D = 0.3 d⁻¹to D = 0.1 d⁻¹ (D = μ) induced the strongest response in E. huxleyi. At μ = 0.3 d⁻¹, PPwas significantly higher at elevated CO₂ and temperature and DO¹⁴Cproduction correlated to PO¹⁴C production in all treatments, resultingin similar percentages of extracellular release (PER; (DO¹⁴Cproduction/PP) × 100) averaging 3.74 ± 0.94%. At μ = 0.1 d⁻¹,PO¹⁴C production decreased significantly, while exudation of DO¹⁴Cincreased. Thus, indicating a stronger partitioning from the particulate tothe dissolved pool. Maximum PER of 16.3 ± 2.3% were observed at μ = 0.1 d⁻¹at elevated CO₂ and temperature.

While cell densities remained constant within each treatment and throughoutthe experiment, concentrations of HMW-dCCHO, pCCHO and TEP were generallyhigher under enhanced nutrient stress. At μ = 0.3 d⁻¹, pCCHOconcentration increased significantly with elevated CO₂ andtemperature. At μ = 0.1 d⁻¹, the contribution (mol % C) of HMW-dCCHO toDOC was lower at elevated CO₂ and temperature while pCCHO and TEPconcentrations were higher. This was most pronounced under greenhouseconditions. Our findings suggest a stronger transformation of primaryproduced DOC into POC by coagulation of exudates under nutrient limitation.Our results further imply that elevated CO₂ and temperature willincrease exudation by E. huxleyi and may affect organic carbon partitioning in theocean due to an enhanced transfer of HMW-dCCHO to TEP by aggregationprocesses.

[学科分类] 地球化学与岩石