| Summary: | PhD Thesis Since industrialisation global CO2 emissions have increased, and as a consequence oceanic pH is predicted to drop by 0.3-0.4 units before the end of the century - a process coined ‘ocean acidification’ (OA). There is significant interest therefore in how pH changes will affect the oceans’ biota and integral processes. This thesis investigates microbial community organisation and functioning in response to predicted end of century CO2 concentrations using an elevated CO2 (~750ppm), large volume (11,000 L) contained seawater mesocosm. This thesis utilises RNA stable isotope probing (SIP) technologies, in conjunction with quantitative reverse transcriptase PCR (RT-qPCR), to investigate the response of microbial communities to elevated CO2. This thesis finds little evidence of changes occurring in bacterial abundance or community composition with elevated CO2, under both phytoplankton pre-bloom/bloom and post-bloom conditions. It is proposed that they represent a community resistant to the changes imposed. In contrast, significant differences were observed between treatments for a number of key eukaryote community members. These findings were investigated in the context of functional change, using the uptake of two key substrates (bicarbonate and glucose) as analogues for photosynthesis and respiration respectively. Unlike community abundance, distinct changes in carbon assimilation were detected in dominant members of the picoplankton. In conclusion the data presented suggest that although current microbial communities hold the capacity to respond to elevated CO2, future responses will likely be taxa specific and controlled by wider community dynamics. Natural Environmental Research Council (NERC) grant number NE/C507937/1 as part of the post genomics and proteomics programme, and the Centre for Ecology and Hydrology (CEH) internal science budget.
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