Effects of elevated pCO\\(_2\\) on the productivity of marine microbes and the remineralisation of nutrients in coastal Antarctic waters

High-latitude oceans are anticipated to be some of the first regions affected by ocean acidification. Marine microbes are the base of the food web and support the wealth of life in Antarctica. They are also a critical link in biogeochemical processes, such as the cycling of nutrients and carbon. Despite this, the effect of ocean acidification on natural communities of Antarctic marine microbes is poorly understood. This thesis sets out to address this lack of scientific knowledge of how the base of the Antarctic food web, both as individual taxa and communities, will respond to elevated CO\\(_2\\). Much of the results of this thesis are derived from an ocean acidification study performed on an early spring, coastal marine microbial community from Prydz Bay, Antarctica. Such studies are currently rare in Antarctic waters and can provide valuable insights into how future changes in CO\\(_2\\) will affect the marine microbial community. In this study, the microbial community was exposed to increasing \\(f\\)CO\\(_2\\) levels from ambient (343 ˜í¬¿atm) to 1641 ˜í¬¿atm in 650 L minicosms. Measurements of abundance and primary and bacterial productivity were taken to determine the effect of CO\\(_2\\) on different community groups. Photophysiological measurements were also performed to identify possible mechanisms driving changes in the phytoplankton community. The limits for CO\\(_2\\) tolerance were broad, likely due to the naturally variable environment this community inhabits. However, there were thresholds to this CO\\(_2\\) tolerance that elicited responses by different community groups. An important tipping point was identified in the phytoplankton community's ability to cope with the energetic requirements of maintaining efficient productivity under high CO\\(_2\\). These results highlighted the strong interplay between enrichment of CO\\(_2\\) enhancing physiology and metabolic costs imposed by increased H\\(^+\\). In addition, elevated CO\\(_2\\) slowed the growth of heterotrophic nanoflagellates, releasing their prey (picophytoplankton and prokaryotes). Thus, increasing CO\\(_2\\) has the potential to change the composition of Antarctic microbial communities by altering interactions among trophic levels. A diatom was isolated from the community-level study and exposed to \\(f\\)CO\\(_2\\) levels from 276 to 1063 ˜í¬¿atm in a unialgal culture study to determine taxon-specific CO\\(_2\\) sensitivities. Comparing these results with those reported for this species in the community-level study assessed the utility of unialgal studies for predicting the sensitivity of Antarctic phytoplankton taxa to elevated \\(f\\)CO\\(_2\\). A difference in growth response between the two studies confirmed that factors other than CO\\(_2\\) affected this species when it is part of a natural community. This research showed that ocean acidification altered microbial productivity, trophodynamics and biogeochemistry in Antarctic coastal waters. Changes in phytoplankton community production and predator-prey interactions with ocean acidification could have a significant effect on the food web and biogeochemistry in the Southern Ocean. In addition, while culture studies are useful for evaluating mechanisms of CO\\(_2\\)-induced tolerance and stress, such studies proved to be of limited value for predicting responses in nature as they fail to include interactions among species and trophic levels.