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Microbial oceanography of the Southern Ocean water masses

Sow, SLS ORCID: 0000-0001-5887-7049 2020 , 'Microbial oceanography of the Southern Ocean water masses', PhD thesis, University of Tasmania.

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Microorganisms from all three domains of life - Bacteria, Archaea and Eukarya are the
base of the marine food web and the key engines sustaining the marine nutrient budget
via both primary production and nutrient remineralization. Microbial biogeography and
ecology are closely tied to hydrography and the physical oceanographic processes
related to the global ocean circulation. Within the Southern Ocean, understanding of the
microbial biogeography is still at its infancy, particularly within the pelagic dark ocean
which is a large reservoir of both microbes and organic matter available for microbial
activity. The Southern Ocean is a region with pivotal influence on the global nutrient
circulation and climate but is also a hotspot for the impacts of climate change. As
microbial biogeography links both the causes and consequences of microbial interactions
with their environment, there is an urgent need to better understand the biogeographic
distribution of the Southern Ocean microbial community, the key players within this
This thesis explores the microbial community composition within the full water column
along several transects of the Southern Ocean. High-throughput tag sequencing of
microbial marker genes (16S and 18S rRNA genes) and bioinformatics analysis were
used to examine the relationship of the community with environmental and geographical
variables. The initial study considered the bacterial community from the Pacific and Indian
sectors of the Southern Ocean. This work investigated if the bacterial community
composition was strictly delineated by the hydrography of distinct water masses despite
being geographically distant and tested the hypothesis if uniform environments of the
abyssopelagic water masses promote a more homogenous microbiota. Extending
previous findings, bacterial biogeography was explained in part by water mass
hydrography, but also exhibited community composition variations at the family
taxonomic level between sectors. Deeper water masses harbored a remarkably high
bacterial beta-diversity across sites and was only weakly explained by water mass
hydrography. Depth and bacterial lifestyle were major considerations in the influence of
environmental factors on the Southern Ocean bacterial community composition.
Subsequent work involved an expanded scope to include a high resolution (0.5-1
latitudinal degree interval) microbial sampling and analysis from surface to depth of a
latitudinal transect within the South Pacific Ocean. Bacterial, archaeal and eukaryotic
taxonomic profiles were constructed for each of the 1045 samples. All the microbial
domains showed strong depth stratification but displayed varying patterns and intensities
of delineation by water mass hydrography. These samples were used to focus in on the
diversity of Phaeocystis, a ubiquitously distributed keystone phytoplankton with
fundamental contributions to the marine carbon and sulfur cycles. Previous Phaeocystis
studies have been centered primarily on its colonial forms, including massive blooms of
Phaeocystis during the austral spring-summer. Through analysis of 18S rRNA gene
sequences, this study showed that Phaeocystis was an abundant phytoplankton in high
latitude waters even in the late autumn, contributing up to 12% of the eukaryotic
sequences detected. Stable oceanographic fronts within surface waters were shown to
structure the Phaeocystis community which also exhibited patterns of low diversity in a
thriving community. P. globosa, a species commonly reported only within the northern
hemisphere, was detected within the Subantarctic to Subtropical as well as equatorial
upwelling regions.
Overall, this thesis has provided the first high vertical and spatial resolution genomics
survey of the Southern Ocean, filling in critical knowledge gaps of Southern Ocean
microbial oceanography, and represents an important first step towards a microbial atlas
of the Southern Ocean.

Item Type: Thesis - PhD
Authors/Creators:Sow, SLS
Keywords: Southern Ocean; prokaryotes; microbial eukaryotes; biogeography; genomics; microbiome; latitudinal distribution; vertical distribution
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Copyright 2019 the author

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