Response of Thalassiosira oceanica and natural microbial communities to ocean acidification : a meta-omics comparison from unialgal cultures to mesocosms

The production of anthropogenic carbon dioxide (CO2) via fossil fuels combustion increases yearly and the consequences on the environment are still to some extend unknown. The most direct effect of increased atmospheric CO2 is its increased absorption by the oceans. The result of this absorption is the complex dissociation of CO2 in seawater which ultimately creates an increase in bicarbonate 〖"(HCO" 〗_"3" ^"-" ")" and in proton (H+) concentration while it decreases carbonate ion 〖"(CO" 〗_"3" ^"2-" ")" concentration. The increase of H+ concretely decreases the oceans’ surface pH and is termed ocean acidification (OA). This doctoral dissertation investigated the effect of OA on Thalassiosira oceanica, an ecologically relevant diatom highly resistant to iron limitation, as well as on a natural Arctic microbial community. It further compared the results of unialgal cultures to mesocosms. High-throughput sequencing was used to investigate how the transcriptome of unialgal cultures of T. oceanica were affected by elevated pCO2 and how it will affect the global gene expression in this species. Multiple genes were differentially expressed as a function of CO2; however, the genes previously identified in Emiliania huxleyi as influenced by elevated pCO2 were not significantly affected in T. oceanica. Calcifying organisms like E. huxleyi are known for their sensitivity to OA and in contrast elevated CO2 might be neutral or beneficial for diatoms. This type of sequencing was also used with two experiments conducted as part of a large-scale in situ costal pelagic mesocosms study part of the EPOCA 2010 Arctic campaign. These studies constitute the first combined metagenomic - metatranscriptomic community characterization of an Arctic fjord microbial community. The 16S rRNA amplicon sequencing was used to define the effect of OA on the community structure and indicated that CO2 was not one of the main structuring variables whereas time, nutrient addition, water origin were. This study further established that the effect of CO2 was significant on only 15 rare taxa. To characterize the metabolic diversity of the microbial community and the expression of functional genes, metagenome and metatranscriptome information was obtained by high-throughput sequencing of mRNA from one control and one elevated pCO2 mesocosms at five time points throughout the duration of the experiment. Phylogenetic analysis indicated that the overall microbial community structure was, as previously established (16S analysis), not strongly influenced by CO2 treatments but varied significantly over time. The analysis of the metatranscriptome showed that only a small number of species and functional genes were affected significantly by pCO2. The overall results of the mesocosms study demonstrate that the effects of OA on the Svalbard’s Arctic microbial community structure and metabolisms are restricted to low-abundance species and specialized functional genes like for example DMSP breakdown or acid resistance. The results presented in this dissertation increase the knowledge on the effect of OA on the diatom T. oceanica as well as the Svalbard’s Arctic microbial community. It also highlights the importance of high-sequencing analyses for establishment of the effect of climate changes on our oceans.


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