The Distributional Dynamics of South Australian Aquatic Bacterio- and Virioplankton

Author: Lisa Dann

Dann, Lisa, 2015 The Distributional Dynamics of South Australian Aquatic Bacterio- and Virioplankton, Flinders University, School of Biological Sciences

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Aquatic microbial communities, such as virio- and bacterioplankton, are important for ecosystem function as they play critical roles in biogeochemical processes, such as the cycling of carbon, phosphorus and nitrogen. These microbial communities are often observed by means of bulk phase sampling whereby relatively large volume samples are extrapolated over larger volumes or areas and considered representative of microbial activity and abundance. However, it is over micrometre to centimetre scales that crucial microbial interactions and processes occur, with microbial activity and abundance changing significantly over such scales. This patchiness indicates bulk phase sampling is not representative of biomass distributions as it implies homogeneity below the sampling scale or volume. This thesis investigates the extent of this patchiness in aquatic environments.

This thesis explores the distribution and composition of virio- and bacterioplankton communities within South Australian water sources, with particular focus given to the Murray River, Australia. Specifically, the microscale (mm) and large scale (km) spatial dynamics of virio- and bacterioplankton populations will be investigated via flow cytometric enumeration and taxonomic identification via primer-based sequencing.

The results of this thesis indicate microscale patchiness is higher than previously reported. This patchiness was observed as heterogeneity between samples from the same site in large scale (km) studies, with increased patchiness and diversity downstream of a small rural town (Chapter 2). This heterogeneity between samples from the same site indicated microscale patchiness may impact taxonomic diversity profiles more so than taxonomic patterns over larger distances, here 3.3 km (Chapter 2). When testing this patchiness on the microscale, at the sub-centimetre scale of marine interface microenvironments, virio- and bacterioplankton abundance varied 45- and 2500-fold per centimetre, respectively (Chapter 3). This increased patchiness indicates resource competition and the likelihood of viral infection are higher in the small volumes important for individual cell encounters than bulk measurements (Chapter 3). Investigations of this patchiness within a freshwater system showed virio- and bacterioplankton abundance varied up to 107-and 80.5-fold per centimetre (Chapter 4). This indicates significant microbial patchiness extends to scales that may directly impact small populations or individual microbial interactions in freshwater systems.

Taxonomic analysis of these microbial patches revealed hotspots and coldspots, which contained microenvironments distinct from the taxonomy of the background community (Chapter 5). This indicates heterogeneous genus richness and composition exists within a river system and therefore suggests small scale fluid parcels persist together long enough to build up genera numbers. This would enable distinctly different proximate taxonomic microenvironments to form, which will therefore have a significant impact on microbial biogeochemical processes. Furthermore, taxonomic analysis of 1 µl subsamples from hotspot, coldspot and background regions revealed heterogeneity, which was most extreme between hotspot subsamples (Chapter 6). Hotspot subsample heterogeneity revealed two distinct taxonomic patterns: overall increases in genera common to most subsamples or the dominance of specific individual genera. Therefore reiterating microscale microbial hotspots represent discrete microenvironments, which will have important implications for nutrient exchange and cellular interactions and hence overall system function.

The findings of this thesis therefore indicate large scale measurements are not an accurate assessment of the conditions under which microbial dynamics exist in freshwater and marine systems. The high variability reported in this thesis indicates few microbes experience the ‘average’ concentrations frequently measured and that bulk phase sampling underestimates absolute concentration and misses the resources, competition and viral exposure gradients that microbes experience. Therefore bulk phase sampling does not represent microbial processes within aquatic systems, highlighting the need to sample at the microscale to accurately understand globally important microbial biogeochemical processes.

Keywords: Bacterioplankton, virioplankton, aquatic environments, microscale, hotspots, coldspots, patchiness, Murray River

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2015
School: School of Biological Sciences
Supervisor: Jim Mitchell