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Abundance and diveristy of Alphaproteobacteria in the Southern Ocean: the dark side of SAR11
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Genetics, Limnology.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Alphaproteobacteria represented by lineages such as SAR11 and Roseobacter are ubiquitous and often dominant in marine bacterioplankton communities. The Southern Ocean is no exception even if annual extremes in light regime and autochthonous inputs of organic substrates present an environment that are in many ways very different from other oceanic regions. Using population mapping and community analysis of bacterioplankton during the highly dynamic summer season within the Ross and Amundsen Seas, in combination with experimental incubations under contrasting light regimes, we studied the impact of solar radiation and other environmental factors on individual lineages and populations within class Alphaproteobacteria. Quantitative population tracking by fluorescence in-situ hybridization was combined with pyrosequencing-derived 16S rRNA gene inventories to resolve the community beyond class and abundant lineages. Both experiments and depth-resolved distribution patterns confirm SAR11 as a major component of the bacterial community regardless of water mass and depth. However, the experiments revealed that SAR11 as a lineage was less competitive under solar-exposed conditions whereas the opposite response was observed for Roseobacter. Resolving the SAR11 linage into subclades, clear partitioning of groups between the different water masses and light regimes was observed. Also the diversity within the SAR11 lineage varied with significantly higher richness in the deeper, permanently dark water masses. Using this abundant marine bacterial lineage as a model, we could demonstrate clear separation of closely related bacterial populations between water masses and along environmental gradients of light exposure, oxygen availability, phytoplankton and nutrients. It is evident that such ecologically coherent populations can only be tracked at high phylogenetic resolution and that ecological and evolutionary mechanisms underpinning the observed phylogeographic patterns differ between water masses in the Southern Ocean. 

Keyword [en]
Southern Ocean, Alphaproteobacteria, SAR11, Roseobacter, community composition, solar radiation
National Category
Research subject
Biology with specialization in Evolutionary Genetics
URN: urn:nbn:se:uu:diva-245066OAI: oai:DiVA.org:uu-245066DiVA: diva2:790348
Available from: 2015-02-24 Created: 2015-02-24 Last updated: 2015-04-17
In thesis
1. Drivers of Population Dynamics in Bacterioplankton: Spotlight on Alphaproteobacteria and its dominant SAR11 Lineage
Open this publication in new window or tab >>Drivers of Population Dynamics in Bacterioplankton: Spotlight on Alphaproteobacteria and its dominant SAR11 Lineage
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Bacteria are mediators of biogeochemical cycles and are in this way vital for maintaining life on earth. Their distribution, abundance and functioning are driven by environmental heterogeneity and dynamic change in abiotic and biotic factors. Both, freshwater lakes and oceans play central roles in the global carbon cycle and bacteria in these systems perform many services for the ecosystems, such as the transfer of organic carbon from primary producers to higher trophic levels. With estimated relative abundances up to 50% of the total bacterioplankton, the Alphaproteobacteria lineage SAR11 is the most abundant group of aquatic bacteria. It is globally distributed and can be partitioned into multiple sub-clades, one of which is exclusive to freshwaters. Until recently, the distribution, abundance and ecological role of this freshwater SAR11 named LD12 was unknown. The aim of the thesis was to study the drivers and mechanisms that influence the dynamics of aquatic bacterial communities in general and the SAR11 and LD12 groups in particular. The thesis consists of environmental surveys of a mesotrophic Lake Erken and the western Southern Ocean, an experiment and a data-mining exercise to reveal the phylogenetic structure of the SAR11 lineage on various temporal and spatial scales. The analysis of a long-term bacterioplankton community survey in lake Erken provided insights about the dynamics of the entire bacterial community and the LD12 population over an annual cycle. The results demonstrate that LD12 can be an equally abundant member of freshwater communities as marine SAR11 in oceans. LD12 featured strong seasonality and was positively coupled to environmental conditions indicative for an oligotrophic lifestyle. LD12 as well as other dominant lake bacterioplankton also maintained stable populations throughout spatial and temporal varying environments, but at high phylogenetic resolution, habitat preferences were revealed, particularly in response to oxygen concentrations. The later was not the case in LD12 as a single ribotype dominated. This is in stark contrast to the habitat partitioning with light availability, depth and water masses observed for marine SAR11 subclades in the Southern Ocean. The global data-mining corroborated that LD12 as a group was much less diverse than SAR11 furthermore, suggesting that the marine-freshwater barrier acted as a population bottleneck. My work shows that bacterial populations can respond in very different ways to environmental drivers, highlight the importance of highly resolved temporal and spatial scales as well as the need for high phylogenetic resolutions to target ecologically coherent populations.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. 54 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1230
bacterial community dynamics, Alphaproteobacteria, SAR11, LD12, Southern Ocean, lakes
National Category
Research subject
Biology with specialization in Limnology
urn:nbn:se:uu:diva-245072 (URN)978-91-554-9172-7 (ISBN)
Public defence
2015-04-10, Fries-salen, Evolutionsbiologiskt Centrum (EBC), Norbyvägen 14, Uppsala, 10:00 (English)
Available from: 2015-03-19 Created: 2015-02-24 Last updated: 2015-04-17

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