Abstract
Microbial extracellular enzymatic activities are a rate-limiting step in the cycling of organic matter in the oceans. Heterotrophic microbes use extracellular enzymes to extract the nutrients they require for life, initiating key pathways in the global biogeochemical cycle. Global biogeochemical cycles maintain climate stability and drive marine productivity. A comprehensive understanding of microbial extracellular enzymes and how they modulate these cycles is essential for stewardship of the planet. Cell-free extracellular enzymes make up a significant component of the microbial extracellular enzyme pool in the oceans, and have only recently been identified. Cell-free extracellular enzymes are dissolved in the water column, and remain active for long periods post-release. The potential for temporal and/or spatial decoupling from the producing microbial community raises new questions about how these cell-free extracellular enzyme pools function, and which factors control them in the marine environment. In this thesis the activities of a broad range of extracellular enzyme groups, along with their cell-free proportions, were assessed through time in coastal and open ocean environments in the waters off the Otago coast, in Southern New Zealand. The aim of this thesis was to understand cell-free enzymes and their ecological roles in the Southern Pacific Ocean. To achieve this, seasonal extracellular enzyme activities were assessed through time in a long-term study in the Southern Pacific Ocean coastal marine environment. In addition, alkaline phosphatase and the understudied phosphodiesterase activities were determined across the subtropical and subantartic water masses of the Southland Front, including mesopelagic waters. The conservation of key members of the phosphate (Pho) regulon in microbial communities across these water masses was investigated using a phosphatase database analysis. The long-term coastal sampling program revealed significant negative correlations between cell-free proportions and sea temperature/solar radiation levels for all four extracellular enzyme groups tested, reflecting a winter pattern previously reported in the Baltic Sea. Substantial cell-free fractions were also found year round for all extracellular enzymes assayed, but the proportion present was enzyme group specific. When comparing the epi- and mesopelagic subantarctic waters, alkaline phosphatase and phosphodiesterase were found to exhibit similar hydrolysis rates. However, the ratio of alkaline phosphatase to phosphodiesterase ranged between 0.38 and 5.42, indicating potential differences in the availability and/or utilisation of phosphomonoester and phosphodiester pools. Cell-free extracellular alkaline phosphatase activities were shown to be consistently high across a range of diverse marine environments (ranging from 65 to 100%), irrespective of the inorganic phosphate concentrations. Associated incubation experiments revealed long residence times for cell-free alkaline phosphatases post release as previously reported. Together, these results provide support that indicates cell-free alkaline phosphatases with long residence times in the marine environment, are a reasonable explanation for the reported paradox of high potential alkaline phosphatase activities at high end-product inorganic phosphate concentrations in the ocean. The Pho-regulon conservation within microbial communities of the Southern Pacific, indicated PhoA and PhoX did not significantly differ from each other, casting doubt on the claim PhoX is more widely distributed in marine bacteria than the classic PhoA. It was also found that in environments frequently low in inorganic phosphate, such as the subtropical watermass, PhoX, (considered a maker of phosphate stress), was more highly conserved during periods of low phosphate concentration. Collectively, in this thesis, seasonal winter patterns for cell-free enzymes were confirmed in a globally relevant coastal ecosystem; phophodiesterase was shown to be as important as the frequently measured alkaline phosphatase in the hydrolysis of DOP; a known paradox from the literature was resolved by observations and experiments on cell-free extracellular alkaline phosphatase enzymes; and light was shed on the conservation of key members of the Pho-regulon in microbial communities of the Southland Front.