| Summary: | <p>Abstract</p> <p>Background</p> <p>The green crab <it>Carcinus maenas </it>is known for its high acclimation potential to varying environmental abiotic conditions. A high ability for ion and acid-base regulation is mainly based on an efficient regulation apparatus located in gill epithelia. However, at present it is neither known which ion transport proteins play a key role in the acid-base compensation response nor how gill epithelia respond to elevated seawater <it>p</it>CO<sub>2 </sub>as predicted for the future. In order to promote our understanding of the responses of green crab acid-base regulatory epithelia to high <it>p</it>CO<sub>2</sub>, Baltic Sea green crabs were exposed to a <it>p</it>CO<sub>2 </sub>of 400 Pa. Gills were screened for differentially expressed gene transcripts using a 4,462-feature microarray and quantitative real-time PCR.</p> <p>Results</p> <p>Crabs responded mainly through fine scale adjustment of gene expression to elevated <it>p</it>CO<sub>2</sub>. However, 2% of all investigated transcripts were significantly regulated 1.3 to 2.2-fold upon one-week exposure to CO<sub>2 </sub>stress. Most of the genes known to code for proteins involved in osmo- and acid-base regulation, as well as cellular stress response, were were not impacted by elevated <it>p</it>CO<sub>2</sub>. However, after one week of exposure, significant changes were detected in a calcium-activated chloride channel, a hyperpolarization activated nucleotide-gated potassium channel, a tetraspanin, and an integrin. Furthermore, a putative syntaxin-binding protein, a protein of the transmembrane 9 superfamily, and a Cl<sup>-</sup>/HCO<sub>3</sub><sup>- </sup>exchanger of the SLC 4 family were differentially regulated. These genes were also affected in a previously published hypoosmotic acclimation response study.</p> <p>Conclusions</p> <p>The moderate, but specific response of <it>C. maenas </it>gill gene expression indicates that (1) seawater acidification does not act as a strong stressor on the cellular level in gill epithelia; (2) the response to hypercapnia is to some degree comparable to a hypoosmotic acclimation response; (3) the specialization of each of the posterior gill arches might go beyond what has been demonstrated up to date; and (4) a re-configuration of gill epithelia might occur in response to hypercapnia.</p>
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