ELEVATED ATMOSPHERIC CARBON DIOXIDE INCREASES ORGANIC CARBON FIXATION BY EMILIANIA HUXLEYI (HAPTOPHYTA), UNDER NUTRIENT‐LIMITED HIGH‐LIGHT CONDITIONS1
ABSTRACT Phytoplankton play a key role in determining the partitioning of CO2 between the atmosphere and the ocean on seasonal, interannual, and millennial time scales. The magnitude of biological draw-down of atmospheric CO2 and C storage in the oceans is affected by concurrent changes in other environmental factors, like nutrient supply. Furthermore, variations in carbon-to-nitrogen (C:N) and carbon-to-phosphorus (C:P) assimilation ratios modify the oceanic CO2 storage capacity. Here we show that increased atmospheric CO2 concentration enhances CO2 fixation into organic matter by a noncalcifying strain of Emiliania huxleyi (Lohmann) Hay & Mohler only under certain conditions, namely high light and nutrient limitation. Enhanced organic matter production was accompanied by marked deviations of the C:N:P ratio from the canonical stoichiometry of marine particulate matter of 106:16:1 (C:N:P) known as the Redfield ratio. Increased cell organic carbon content, C:N, and C:P were observed at high light when growth was either nitrogen or phosphorus limited. Elevated CO2 led to further increases in the particulate C:N and C:P ratios. Enhanced CO2 uptake by phytoplankton such as E. huxleyi, in response to elevated atmospheric CO2, could increase carbon storage in the nitrogen-limited regions of the oceans and thus act as a negative feedback on rising atmospheric CO2 levels.
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ABSTRACT: Although aquatic ecosystems are a major carbon reservoir, how their carbon dynamics will respond to increasing concentrations of atmospheric CO2 is not well understood. The availability of essential nutrients has the potential to modify carbon fluxes under elevated CO2 by altering carbon processing and storage in the biota. Here, we describe a semi-continuous culture experiment with natural phytoplankton and bacteria assemblages designed to investigate (1) how carbon dynamics in aquatic ecosystems respond to continuously elevated atmospheric CO2, and (2) whether carbon fluxes resulting from elevated CO2 are modified by changes in inorganic nitrogen and phosphorus availability. Our results showed that elevated CO2 led to significant increases in photosynthetic carbon uptake, despite a decrease in the algal chlorophyll a concentrations and no significant change in total algal biovolume. This enhancement of inorganic carbon uptake was accompanied by a significant increase in dissolved organic carbon (DOC) production. Concurrent increases in the C/N and C/P ratios of dissolved organic matter also suggested that DOC stability increased. Nutrient availability, especially nitrogen availability, had strong effects on inorganic carbon uptake and biomass carbon pools. In contrast, CO2-enhanced DOC production was not significantly affected by varying concentrations of inorganic nitrogen and phosphorus. Our study underscores the importance of DOC as a potential carbon sink in aquatic ecosystems. The observed responses to changes in CO2 and nutrient availability could have important implications for long-term carbon cycling in aquatic ecosystems, and highlight the potential buffering capacity of aquatic ecosystems to future environmental change.Biogeochemistry 04/2014; 118(1-3). DOI:10.1007/s10533-013-9904-7 · 3.73 Impact Factor
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ABSTRACT: Although rising CO2 concentrations are thought to promote the growth and alter the carbon : nutrient stoichiometry of primary producers, several studies have reported conflicting results. To reconcile these contrasting results, we tested the following hypotheses: rising CO2 levels (1) will increase phytoplankton biomass more at high nutrient loads than at low nutrient loads, but (2) will increase their carbon : nutrient stoichiometry more at low than at high nutrient loads. We formulated a mathematical model to predict dynamic changes in phytoplankton population density, elemental stoichiometry and inorganic carbon chemistry in response to rising CO2 . The model was tested in chemostat experiments with the freshwater cyanobacterium Microcystis aeruginosa. The model predictions and experimental results confirmed the hypotheses. Our findings provide a novel theoretical framework to understand and predict effects of rising CO2 concentrations on primary producers and their nutritional quality as food for herbivores under different nutrient conditions.Ecology Letters 05/2014; DOI:10.1111/ele.12298 · 13.04 Impact Factor
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ABSTRACT: Effects of CO2 concentration on elemental composition of the coccolithophore Emiliania huxleyi were studied in phosphorus-limited, continuous cultures that were acclimated to experimental conditions for 30 days prior to the first sampling. We determined phytoplankton and bacterial cell numbers, nutrients, particulate components like organic carbon (POC), inorganic carbon (PIC), total nitrogen (PN), organic phosphorus (POP), transparent exopolymer particles (TEP), as well as dissolved organic carbon (DOC) and nitrogen (DON), in addition to carbonate system parameters at CO2 levels of 180, 380 and 750 μatm. No significant difference between treatments was observed for any of the measured variables during repeated sampling over a 14-day period. We considered several factors, i.e. light, nutrients, carbon overconsumption and transient vs. steady-state growth that might lead to these results. We suggest that the absence of a clear CO2 effect during this study does not necessarily imply the absence of an effect in nature. Instead, the sensitivity of the cell towards environmental stressors such as CO2 may vary depending on whether growth conditions are transient or sufficiently stable to allow for optimal allocation of energy and resources. We tested this idea on previously published data sets where PIC and POC divided by the corresponding cell abundance of E. huxleyi at various pCO2 levels and growth rates were available.Marine Ecology Progress Series 07/2014; 507. DOI:10.3354/meps10824 · 2.64 Impact Factor