The transcriptomic response of the cold-water coral Desmophyllum dianthus to experimental changes in pH

• Main Author: Niedzwiedz, Sarina
• Format: Thesis
• Language: unknown
• Published: 2021
• Subjects: Ocean acidification
• Online Access: https://epic.awi.de/id/eprint/55262/; https://epic.awi.de/id/eprint/55262/1/MasterThesis_SarinaNiedzwiedz.pdf; https://hdl.handle.net/10013/epic.a58f8ad2-a8f5-42f3-b654-fb344c9ede9b; https://hdl.handle.net/

The seawater pH is influenced by the interaction of various natural physical and biological factors. Since the beginning of industrialisation, anthropogenic activities are also having a significant impact on the seawater pH, as the atmospheric increase of the carbon dioxide (CO2) concentration led to an enrichment of the ocean with CO2. The release of protons during the reaction of CO2 with water molecules results in a declining pH (ocean acidification; OA). Apart from the seawater pH, the aragonite saturation state (Ωarag) is commonly used to measure the OA, as it describes the ‘easiness to calcify’ and is therefore biologically relevant. With decreasing pH, the Ωarag is also decreasing and calcifying organisms have to invest more energy to maintain their calcium carbonate structures, most prominently in cold and deep waters. Therefore, scleractinian cold-water corals (CWCs), such as the cosmopolitan species Desmophyllum dianthus, were thought to be among the taxa most threatened by OA, as they are mainly restricted to water temperatures between 4–12 °C and water depths below 50 m. However, their reported occurrence in aragonite under-saturated waters indicates that they are able to mitigate the negative impacts of the low Ωarag. The aim of this study was to gain information on the regulations and mechanisms that allow the CWC D. dianthus to thrive under low-pH conditions. An understanding of the physiological and molecular processes affected by low-pH conditions will help to assess the development and future biogeographic distribution of D. dianthus. Therefore, corals acclimated to pH 8.0 were exposed for two weeks to low-pH conditions (pH 7.4), to assess their short-term acclimation potential to an experimental reduction of the pH conditions. Afterwards, the pH was turned to pH 8.0 for two months, to determine their recovery potential. Corals being exposed to pH 8.0 during the whole experiment served as control. Physiological and transcriptomic response parameters were measured at various sampling times throughout the experiment. The observed gene expression patterns were compared to field samples that grew under two different pH conditions (pH 7.5; pH 7.8). The results of this study suggest that D. dianthus is highly tolerant towards short-term changes in the seawater pH, in a range corresponding to the natural pH conditions. While no significant pH-dependent differences were detected on a physiological level, the dynamic regulation of the transcriptome indicates that the experimental pH-range was within the limits of phenotypic buffering. As a pH-dependent change was neither detected in the calcification rates nor in the expression of ion transporters, this suggests that the experimental changes of the seawater pH had no negative effect on calcification. However, the downregulation of genes coding for cytoskeletal elements (actin and tubulin) (Δ log2 fold change > 20) might be an indicator for a change in the crystal structure of the calcium carbonate skeleton. Further, no significant changes of the respiration rate during the two weeks of low-pH conditions could be observed, though the median respiration rate of the corals exposed to pH 7.4 was 15–38 % lower compared to the control. In combination with the immediate downregulation of genes after first reaching pH 7.4, this may be an indicator for metabolic suppression under acute low-pH stress. This is supported by the downregulation of the expression of genes belonging to the protein synthesising apparatus (histones, ribosomal subunits and elongation factor) (Δ log2 fold change > 20). A change in the feeding regime during the recovery phase showed a strong regulation on both physiological (60–85 % higher calcification-; 50–60% higher respiration rates) and transcriptomic (1195 differential regulated transcripts) level, indicating that the food availability influences the susceptibility of D. dianthus towards low-pH stress. Comparing the observed gene expression patterns of the pH exposure experiment with field samples, 38 % of all differentially expressed transcripts were found in the field samples, suggesting that the transcriptome of D. dianthus is highly variable and strongly depends on the prevailing abiotic conditions. Stress- and transcription-related genes (metalloproteinase and zinc fingers) (Δ log2 fold change = 3.5) were found upregulated in those samples growing under pH 7.5 in the field. However, as the field samples were influenced by a multitude of fluctuating environmental factors, the assignment of these genes as a pH-related response is difficult. In summary, this study demonstrated that D. dianthus has comprehensive mechanisms to withstand short-term pH fluctuations. To be able to draw conclusions about the development of D. dianthus, the observed metabolic suppression and its long-term effects on the corals fitness must be examined. In addition, the strong response towards the increased food supply must be noted, as the nutritional status may be influencing the corals sensitivity towards low-pH conditions.