Abstract
Ocean acidification (OA) is expected to negatively affect coral reefs, however little is known about how OA will change the coral-algal symbiosis on which reefs ultimately depend. This study investigated whether there would be differences in coral Symbiodinium types in response to OA, potentially improving coral performance. We used denaturing gradient gel electrophoresis (DGGE) of the internal transcribed spacer 2 (ITS2) region of ribosomal DNA to investigate the dominant types of Symbiodinium associating with six species of scleractinian coral that were exposed to elevated partial pressures of carbon dioxide (pCO2) in situ from settlement and throughout their lives. The study was conducted at three naturally occurring volcanic CO2 seeps (pCO2 ∼500 to 900 ppm, pHTotal 7.8 – 7.9) and adjacent control areas (pCO2 ∼390 ppm, pHTotal ∼8.0 – 8.05) in Papua New Guinea. The Symbiodinium associated with corals living in an extreme seep site (pCO2 >1000 ppm) were also examined. Ten clade C types and three clade D types dominated the 443 coral samples. Symbiodinium types strongly contrasted between coral species, however, no differences were observed due to CO2 exposure. Within five species, 85 – 95% of samples exhibited the same Symbiodinium type across all sites, with remaining rare types having no patterns attributable to CO2 exposure. The sixth species of coral displayed site specific differences in Symbiodinium types, unrelated to CO2 exposure. Symbiodinium types from the coral inhabiting the extreme CO2 seep site were found commonly throughout the moderate seeps and control areas. Our finding that symbiotic associations did not change in response to CO2 exposure suggest that, within the six coral hosts, none of the investigated 13 clade C and D Symbiodinium types had a selective advantage at high pCO2. Acclimatisation through changing symbiotic association therefore does not seem to be an option for Indo-Pacific corals to deal with future OA.
Highlights
Present atmospheric carbon dioxide (CO2) levels have surpassed 390 ppm, the highest they have been in at least two million years [1]
Four profiles were identical across coral species (Am1 and Pc3, Pm1 and Pc1, Pm3 and Pc2 as well as Gf2, Sh3 and Fp2) and the identity of these Symbiodinium communities was confirmed with the sequence data
This study shows that the dominant Symbiodinium community in scleractinian corals did not change despite a life-time exposure to elevated concentrations of CO2 around volcanic CO2 seeps
Summary
Present atmospheric carbon dioxide (CO2) levels have surpassed 390 ppm, the highest they have been in at least two million years [1]. Coral reefs are the most diverse marine ecosystems on our planet, primarily owing to the physical framework constructed by scleractinian corals as they secrete their calcium carbonate skeleton [12] This process is made possible through a symbiotic relationship formed between the coral cnidarian host and singlecelled photosynthetic dinoflagellates of the genus Symbiodinium [13]. Dinoflagellates, including Symbiodinium, are unique amongst eukaryotes in that they utilise type II ribulose biphosphate carboxylase/oxygenase (RuBisCO) during the onset of carbon fixation [15] This enzyme has a much lower affinity with inorganic carbon than RuBisCo I [16,17], leaving it undersaturated with CO2 under present-day pCO2 levels despite the apparent ability to use bicarbonate (HCO32) and the existence of a carbon concentrating mechanism (CCM) [18]. As pCO2 increases under OA, both CO2 and HCO32 substrates for photosynthesis will become more abundant
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