Organic matter exudation by Emiliania huxleyi under simulated future ocean conditions

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

Enhanced nutrient stress by reducing the dilution rate (D) from D = 0.3 d⁻¹ to D = 0.1 d⁻¹ (D = μ) induced the strongest response in E. huxleyi. At μ = 0.3 d⁻¹, PP was significantly higher at elevated CO₂ and temperature and DO¹⁴C production correlated to PO¹⁴C production in all treatments, resulting in similar percentages of extracellular release (PER; (DO¹⁴C production/PP) × 100) averaging 3.74 ± 0.94%. At μ = 0.1 d⁻¹, PO¹⁴C production decreased significantly, while exudation of DO¹⁴C increased. Thus, indicating a stronger partitioning from the particulate to the dissolved pool. Maximum PER of 16.3 ± 2.3% were observed at μ = 0.1 d⁻¹ at elevated CO₂ and temperature.

While cell densities remained constant within each treatment and throughout the experiment, concentrations of HMW-dCCHO, pCCHO and TEP were generally higher under enhanced nutrient stress. At μ = 0.3 d⁻¹, pCCHO concentration increased significantly with elevated CO₂ and temperature. At μ = 0.1 d⁻¹, the contribution (mol % C) of HMW-dCCHO to DOC was lower at elevated CO₂ and temperature while pCCHO and TEP concentrations were higher. This was most pronounced under greenhouse conditions. Our findings suggest a stronger transformation of primary produced DOC into POC by coagulation of exudates under nutrient limitation. Our results further imply that elevated CO₂ and temperature will increase exudation by E. huxleyi and may affect organic carbon partitioning in the ocean due to an enhanced transfer of HMW-dCCHO to TEP by aggregation processes.