Imaging-based characterization of prokaryotic virocells at single-cell level

Viruses are assumed to be the most abundant entity on earth, when quantified as viral particles (virions). However, without metabolism these virions are of low ecological impact. Only encountering a potential host triggers their ecological role. For one decade, the virocell concept addresses the need to change the perspective of how to access the impact of viruses on ecosystems. The imbalance between properties of isolated viruses and the actual environmental diversity provided by microscopic and metagenomic studies on morphological and genomic traits of uncultivated viruses point out the need combine culture independent tools for virocell studies in microbial ecology.
This thesis aims to identify and characterize prokaryotic virocells at the single-cell level, using imaging-based techniques. We provide an insight into biochemical changes of prokaryotic virocells under laboratory conditions using Raman microspectroscopy. The analyses of single-cell spectra allowed reduction of multivariate data into a ratio for univariate virocell identification. The biomolecular assignment of observed spectral changes are consistent with characteristics of the virion composition and their maturation stages. In addition, we established a correlative light and electron microscopy approach, which allows the study of morphological traits of indigenous archaeal virocells identified based on metagenomic data. Examining individual environmental virocells in detail revealed their increase in cell size, putative viral particles on the cells surface and indices of the intracellular organization of virus replication. In a future perspective, further application and combination of the different approaches are discussed to point out how virocells and their impact on ecosystems can be accessed in situ using multimodal imaging approaches.
In summary, this thesis provides a substantial overview of imaging techniques and data processing to identify and characterize individual virocells and provides guidance for future experimental design to address different levels of the ecological impact of virocells throughout their lifecycle.



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