| Summary: | The ocean’s pH has decreased by 0.1 units in the last two centuries due to anthropogenic CO2 emissions and it is predicted will continue to decrease by 0.3 units during the next century. One key ecosystem process that may be altered by such a decrease is the microbial oxidation of ammonia, the first step of nitrification. At low pH, the equilibrium concentration shifts towards ammonium rather than ammonia and therefore ammonia oxidation by microorganisms can be inhibited. Previous studies have demonstrated an inhibitory effect of a low seawater pH on ammonia oxidation in the seawater column. Such effect on ammonia oxidation in coastal sediment, however, is not well understood. The relevance of studying the potential effect of acidified seawater on the oxidation of ammonia in coastal sediment is that nitrate, the end product of nitrification, is the second most preferred electron acceptor used by microorganisms to decompose organic matter. Nitrate is also an essential source of nitrogen for primary producers. I established a facility of recirculating seawater to study the effect of an experimental pH decrease of 0.3 units on the oxidation of ammonia in two contrasting types of coastal sediment, sandy and muddy sediment. My objectives were to investigate (1) the assemblage structure of ammonia-oxidising archaea and bacteria; and (2) the gene expression of amoA, the gene for the enzyme that catalyses ammonia oxidation. I also investigated the effect of the seawater pH decrease on the pH of the muddy sediment pore water. Overall, my study was inconclusive. I was able, however, to demonstrate that the seawater pH decrease altered the pore water pH in muddy sediment. I found enhanced pore water pH diel variations at the upper oxic zone, which were attributed to intensified respiration and photosynthesis of diatoms stimulated by the supply of CO2. This suggested that the diatom’s CO2-growth stimulation might play an important role in the effect of the future acidified ocean on the sediment’s biogeochemistry. I also ...
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