Preprint Article Version 1 This version is not peer-reviewed

Granular Biofilms: Formation, Function, Application, and New Trends as Model Microbial Communities

Version 1 : Received: 5 June 2019 / Approved: 7 June 2019 / Online: 7 June 2019 (03:48:36 CEST)

How to cite: Trego, A.C.; Mills, S.; Collins, G. Granular Biofilms: Formation, Function, Application, and New Trends as Model Microbial Communities. Preprints 2019, 2019060053 (doi: 10.20944/preprints201906.0053.v1). Trego, A.C.; Mills, S.; Collins, G. Granular Biofilms: Formation, Function, Application, and New Trends as Model Microbial Communities. Preprints 2019, 2019060053 (doi: 10.20944/preprints201906.0053.v1).

Abstract

As the global demand for water increases, so does the quantity of wastewater requiring treatment. Due to a relatively low carbon footprint, compared with conventional wastewater treatment technologies, anaerobic digestion (AD) was identified in the 1970s as a forerunner in the push for sustainability, when interest in sustainable technologies and renewable energy sources was first sparked. AD technology development ultimately resulted in the discovery of the ‘anaerobic granule’. It is a spontaneously-forming bio-aggregate of microbial cells capable of digesting pollutants and producing methane-rich biogas as a renewable source of bioenergy. The high settling velocity of such granules meant that AD systems could be operated as high-rate treatment processes, because the active, relatively-slow-growing, pollutant-removing biomass would be retained inside, and not washed out of, even bioreactors operated at extremely high volumetric loading rates. In the intervening years the emergence of the anaerobic ammonium oxidising (anammox) granule, aerobic granule, hydrogenic granule, oxygenic photogranule, and many other functionally-specialised granules, has opened new opportunities in wastewater treatment biotechnology. Whilst environmental engineering based around wastewater treatment is still a growing field of research and implementation, the granule (in all forms) is starting to catch the attention of microbial ecologists. It is a self-immobilised biofilm, with many of the properties of ‘conventional’ biofilms formed in Nature. However, as a single entity, a granule represents an entire community of microorganisms, competing or functioning cooperatively or in syntrophy. Together, inside a bioreactor, granules perform side-by-side arguably representing a meta-organism. Granules are gaining traction as the perfect samples for high-throughput studies on fundamental ecological concepts. Granular biofilms can be used to test hypotheses around drivers of diversity, community assembly, biofilm formation and maturation, community expansion and succession, community stress response, among others. This review outlines the history of three of the most influential types of granules: the anaerobic (methanogenic), aerobic and anammox granule. The main biochemical processes found in each type; their primary characteristics; and the typical makeup of the microbial community underpinning the processes are compared. Finally, the adoption of granules as the perfect ‘playground’ for experiments in microbial ecology is reviewed.

Subject Areas

Anammox; biofilms; granulation; methanogens; microbial ecology; sludge granules; wastewater

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