Abstract
Abstract. Microcosm experiments to assess the bacterioplankton's response to phytoplankton-derived organic matter obtained under current and future ocean CO2 levels were performed. Surface seawater enriched with inorganic nutrients was bubbled for 8 days with air (current CO2 scenario) or with a 1000 ppm CO2 air mixture (future CO2 scenario) under solar radiation. The organic matter produced under the current and future CO2 scenarios was subsequently used as an inoculum. Triplicate 12 L flasks filled with 1.2 µm of filtered natural seawater enriched with the organic matter inocula were incubated in the dark for 8 days under CO2 conditions simulating current and future CO2 scenarios, to study the bacterial response. The acidification of the media increased bacterial respiration at the beginning of the experiment, while the addition of the organic matter produced under future levels of CO2 was related to changes in bacterial production and abundance. This resulted in a 67 % increase in the integrated bacterial respiration under future CO2 conditions compared to present CO2 conditions and 41 % higher integrated bacterial abundance with the addition of the acidified organic matter compared to samples with the addition of non acidified organic matter. This study demonstrates that the increase in atmospheric CO2 levels can impact bacterioplankton metabolism directly, by changes in the respiration rate, and indirectly, by changes on the organic matter, which affected bacterial production and abundance.
Highlights
The increase in fossil fuel burning, cement production and deforestation together with changes in land use have resulted in an accumulation of atmospheric CO2 at levels never seen before in the past 2 million years (Caldeira and Wickettt, 2008; Le Quéré et al, 2015)
During the first incubation, aimed at obtaining the organic matter inocula under current and future CO2 conditions, the LC treatment pressure of CO2 (pCO2) values were close to the atmospheric equilibrium, with values ranging from 419 ± 21 ppm CO2 on day 0 to 226 ± 38 ppm CO2 on day 3
Black and grey circles correspond to the pCO2 mean ± SD (n = 3; ppm CO2) in HC and LC treatments, respectively. (b) Temporal evolution of the dissolved organic carbon (DOC) concentration during this first incubation
Summary
The increase in fossil fuel burning, cement production and deforestation together with changes in land use have resulted in an accumulation of atmospheric CO2 at levels never seen before in the past 2 million years (Caldeira and Wickettt, 2008; Le Quéré et al, 2015). Atmospheric gases can freely diffuse into the ocean surface, which has already absorbed about 30 % of the emitted anthropogenic CO2, perturbing the carbonate system and decreasing ocean pH in a process known as ocean acidification (Sabine et al, 2004; IPPC, 2014). Recent meta-analysis studies revealed decreased survival, growth, development and an abundance of a broad range of marine organisms, the magnitude of these responses varies between taxonomic groups, including variation within similar species (Kroeker et al, 2013). Other authors have demonstrated that ocean acidification can increase growth, primary production and N2 fixation rates in some phytoplankton species
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