CO<sub>2</sub> increases <sup>14</sup>C-primary production in an Arctic plankton community

Abstract

Responses to ocean acidification in plankton communities were studied during a CO₂-enrichment experiment in the Arctic Ocean, accomplished from June to July 2010 in Kongsfjorden, Svalbard (78°56, 2' N, 11°53, 6' E). Enclosed in 9 mesocosms (volume: 43.9–47.6 m³), plankton was exposed to CO₂ concentrations, ranging from glacial to projected mid-next-century levels. Fertilization with inorganic nutrients at day 13 of the experiment supported the accumulation of phytoplankton biomass, as indicated by two periods of high Chl <i>a</i> concentration. <br><br> This study tested for CO₂ sensitivities in primary production (PP) of particulate organic carbon (PP_POC) and of dissolved organic carbon (PP_DOC). Therefore, ¹⁴C-bottle incubations (24 h) of mesocosm samples were performed at 1 m depth receiving about 60% of incoming radiation. PP for all mesocosms averaged 8.06 ± 3.64 μmol C l⁻¹ d⁻¹ and was slightly higher than in the outside fjord system. Comparison between mesocosms revealed significantly higher PP_POC at elevated compared to low <i>p</i>CO₂ after nutrient addition. PP_DOC was significantly higher in CO₂ enriched mesocosms before as well as after nutrient addition, suggesting that CO₂ had a direct influence on DOC production. DOC concentrations inside the mesocosms increased before nutrient addition and more in high CO₂ mesocosms. After addition of nutrients, however, further DOC accumulation was negligible and not significantly different between treatments, indicating rapid utilization of freshly produced DOC. Bacterial biomass production (BP) was coupled to PP in all treatments, indicating that 3.5 ± 1.9% of PP, or 21.6 ± 12.5% of PP_DOC provided sufficient carbon for synthesis of bacterial biomass. The response of ¹⁴C-based PP rates to CO₂ enrichment was at odds with O₂-based net community production (NCP) rates that were also determined during this study, albeit at lower light level. We conclude that the enhanced release of labile DOC during autotrophic production at high CO₂ exceedingly stimulated activities of heterotrophic microorganisms. As a consequence, increased PP induced less NCP, as suggested earlier for carbon limited microbial systems in the Arctic

Long-term acclimation to elevated p CO 2 alters carbon metabolism and reduces growth in the Antarctic diatom Nitzschia lecointei

“…This imbalance generally occurs when nutrients are depleted and often results in increased DOC leakage from the cells. Carbon overconsumption may also occur as a response to increased pCO 2 [7,53]. Since nutrient levels were kept replete in this study, the increased DOC excretion is most likely a direct effect of elevated DIC levels.…”

“…For instance, Engel et al [53] reported on average 21-23% PP DOC of PP TOT in mesocosms with a natural Arctic phytoplankton community exposed to different pCO 2 treatments. Generally, DOC release in natural marine phytoplankton communities does not exceed 40% of the total primary production [10].…”

“…Although DOC excretion is a response of algal growth, DOC leakage does not promote growth. Increased cellular DOC excretion has also been correlated to combinations of environmental stressors [55] and carbon overconsumption [52,53]. Acclimation to high pCO 2 may have affected the gene expression in N. lecointei, and needs to be further investigated.…”

Dissolved organic matter (DOM) release by phytoplankton in the contemporary and future ocean

“…However, analysis of the TEP concentrations from the same experiment by Egge et al (2009) showed no difference in TEP production between the different pCO 2 treatments. Recently, experiments using mesocosms in Arctic waters showed enhanced production of DOC by phytoplankton grown under elevated CO 2 (Engel et al, 2013).…”

Temperature dependence of CO2-enhanced primary production in the European Arctic Ocean

“…Experimental research from the European Arctic suggests that increasing CO 2 concentrations enhance primary production in nutrient-replete conditions 16 , although this response is possibly species-specific owing to varying efficiencies of the mechanisms for concentrating cellular carbon 17 . However, the response to increased CO 2 when combined with warming may deviate from the expected additive effect.…”