The Effects of pCO2 on Bacterioplankton-Mediated Carbon Cycling
The concentration of atmospheric carbon dioxide (CO2) is increasing at extraordinary rates (e.g. Le Quéré et al. 2016). Effectively mitigating the impacts of increasing atmospheric CO2 on climate change, the ocean has absorbed roughly 30 % of the anthropogenic CO2 produced since the Industrial Revolution (e.g. Doney et al. 2009). However, increased levels of CO2 in the surface ocean may have lasting implications for marine biogeochemical cycles (e.g. Riebesell et al. 2013). In addition to gradual increases in concentrations of CO2 in the surface ocean through rising atmospheric CO2, mixing of deep water upwards leads to the injection of elevated partial pressures of CO2 (pCO2) into the surface ocean, exposing some areas of the surface ocean to transient pulses of elevated pCO2 equivalent to those projected for the year 2100 (Feely et al. 2008, Hofmann et al. 2011). This broad range of exposure to elevated pCO2, from ephemeral pulses to gradual increases, highlights the necessity to understand the impacts of pCO2 on marine biogeochemical processes on a variety of timescales.
Heterotrophic bacterioplankton play a key role in the biogeochemical cycling of carbon in the ocean through the consumption and remineralization of dissolved organic carbon (DOC). The physical mixing of DOC that accumulates in the surface ocean into the mesopelagic represents ~ 20 % of global annual organic carbon export (Hansell and Carlson 2015), making DOC export an important pathway in the biological carbon pump. Export of DOC to ocean depths removes this carbon from interaction with the atmosphere on a variety of timescales. Processes that remove or reduce the accumulation of DOC in the surface ocean can decrease the amount of DOC available for export and ultimately lessen the effectiveness of DOC export as a sink of carbon in the ocean. As the primary consumers of DOC, heterotrophic bacterioplankton can reduce the amount and rate of DOC accumulation in the surface ocean. Thus, factors that affect the ability of bacterioplankton to consume DOC can affect DOC accumulation and have implications for DOC export potential.
In Chapter I, I present results from seawater culture experiments that were designed to assess the effects of pCO2 on bacterioplankton consumption of DOC. Results from these experiments provide evidence that short-term exposure to elevated pCO2 can enhance the rate of removal of photosynthetically-derived surface DOC by natural bacterioplankton communities. To evaluate potential physiological and metabolic mechanisms responsible for these enhanced rates of DOC removal by marine bacterioplankton, I present results from a metagenomic analysis in Chapter II. These results suggest that elevated pCO2 can alter the taxonomic composition and metabolic potential of natural bacterioplankton communities. Collectively, Chapters I and II contribute to a growing understanding of the effects of elevated pCO2 on bacterioplankton-mediated carbon cycling in the surface ocean. Chapter III provides the first high-resolution evaluation of key physical and biogeochemical variables controlling carbon dynamics in the oligotrophic waters surrounding the islands of Moorea and Tahiti, French Polynesia, providing the context needed to predict how short-term increases in pCO2 may alter carbon-cycling in oligotrophic gyre ecosystems.
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