Characterisation of carbonic anhydrase in the symbiotic dinoflagellate Symbiodinium

Bobeszko, Teressa (2017) Characterisation of carbonic anhydrase in the symbiotic dinoflagellate Symbiodinium. PhD thesis, James Cook University.

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Photosynthesis by Symbiodinium plays a central role in the coral-algal symbiosis as the majority (around 95%) of the hosts' metabolic demand is derived from photosynthetically fixed carbon. Photosynthesis in Symbiodinium is augmented by the use of a carbon-concentrating mechansism (CCM), of which the enzyme carbonic anhydrase (CA) plays a significant role in the accumulation, transportation and interconversion of inorganic carbon (Ci) forms to ultimately provide CO₂ for the carbon-fixing enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Environmental changes associated with climate change, such as ocean acidification and warming, represent key threats to coral reef ecosystems and are the major causes of the decline and deterioration of coral reefs worldwide and have prompted a major research focus on how climate related stressors affect coral-algal symbioses. Given the hosts' dependency on the symbionts ability to perform photosynthesis, how climate change will affect Symbiodinium photosynthesis is therefore an area that needs to be investigated.

Current understanding of eukaryotic CCM expression is predominately derived from the green alga Chlamydomonas reinhardtii. While it has been demonstrated that Symbiodinium possess a CCM, the signals that trigger the expression of the CCM and subsequent genes involved have not been precisely defined in Symbiodinium. Therefore, the aims of this research were to use sequence tag data for Symbiodinium sp. Clade C3 to characterise the genes encoding CAs involved in the Symbiodinium CCM and to determine if Symbiodinium CAs were modified by external CO₂ concentrations as in other photosynthetic algae; to determine what the combined effects of elevated CO₂ and temperature were on Symbiodinium photosynthesis and CA expression; to examine varying light intensities on the regulation of CA; and to investigate possible long-term effects of CO₂ enrichment on the Symbiodinium transcriptome.

To achieve these aims a sequencing project was performed. Bioinformatic analyses including analysis of conserved amino acid residues of CA sequences and homology of translated CA protein sequences with other known CA genes from algae and higher plants was performed. Phylogenetic comparison of the translated protein sequences of CAs with CA sequences from a variety of organisms was also undertaken. Quantitative PCR was used for RNA transcript analysis of the identified CAs, Rubisco and phosphoglycolate phosphatase (PGPase) under the various environmental stressors and Illumina RNA-seq was used to investigate long-term CO₂ effects on the Symbiodinium transcriptome.

Two distinct β-CAs and one δ-CA protein were identified and characterised in this study. Both β-CAs were encoded as polyproteins and, were presumably localised to the cytosol while the δ-CA protein is likely localised to the plasma membrane. Phylogenetic analysis revealed that the dinoflagellate β-CAs form a novel group within this gene family, illustrating the diversity that exists within the β-CA class. Expression analysis of CAs in Symbiodinium sp. clade C1 under elevated CO₂ concentrations revealed that CAs are down-regulated by elevated CO₂ conditions as seen in other algae however, expression patterns differ between different phylotypes of Symbiodinium.

Exposure to combined elevated CO₂ and temperature illustrated that thermal stress was the main driver of changes in both transcript levels and physiological parameters of Symbiodinium sp. clade F, while CO₂ concentrations relevant to current through to projected future levels of CO₂ had little significant effect overall.

Transcript abundance of Symbiodinium CAs under varied light intensities was also examined. High-light environments caused both a decrease in Symbiodinium CA transcripts and photosynthetic efficiency. Lastly, the response of Symbiodinium clade F to long-term elevated CO₂ concentrations highlighted the transcriptome wide changes with elevated CO₂ significantly enhancing processes such as photosynthesis, energy and ATP metabolism and CA transcript abundance while processes such as transmembrane transport and protein phosphorylation were significantly downregulated.

The information resulting from this research therefore provides a basis for future investigations into the role and functioning of CAs in Symbiodinium; a means to compare the expression of CAs between stress tolerant and susceptible Symbiodinium species; and a platform to understand how Symbiodinium photosynthesis and therefore the coral-algal symbiosis may be affected by future climate change conditions.

Item ID: 53111
Item Type: Thesis (PhD)
Keywords: coral, acidification, bleaching, gene expression, dinoflagellates, Symbiodinium, symbiosis, temperature, climate change
Date Deposited: 12 Apr 2018 02:32
FoR Codes: 06 BIOLOGICAL SCIENCES > 0604 Genetics > 060405 Gene Expression (incl Microarray and other genome-wide approaches) @ 60%
06 BIOLOGICAL SCIENCES > 0607 Plant Biology > 060702 Plant Cell and Molecular Biology @ 40%
SEO Codes: 96 ENVIRONMENT > 9603 Climate and Climate Change > 960305 Ecosystem Adaptation to Climate Change @ 80%
96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 20%
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