Organic matter exudation by Emiliania huxleyi under simulated future ocean conditions

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). C-14 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 mu atm pCO(2) at 14 A degrees C, and additionally 900 mu atm pCO(2) at 18 A degrees 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 = mu) induced the strongest response in E. huxleyi. At mu = 0.3 d(-1), PP was significantly higher at elevated CO2 and temperature and (DOC)-C-14 production correlated to (POC)-C-14 production in all treatments, resulting in similar percentages of extracellular release (PER; ((DOC)-C-14 production/PP) x 100) averaging 3.74 +/- 0.94%. At mu = 0.1 d(-1), (POC)-C-14 production decreased significantly, while exudation of (DOC)-C-14 increased. Thus, indicating a stronger partitioning from the particulate to the dissolved pool. Maximum PER of 16.3 +/- 2.3% were observed at mu = 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 mu = 0.3 d(-1), pCCHO concentration increased significantly with elevated CO2 and temperature. At mu = 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.