Abstract
The production and release of extracellular vesicles (EVs) is a common process occurring in various types of bacteria. However, little is known regarding the functions of EVs derived from marine bacteria. We observed that during cell growth, Sediminicola sp. YIK13, a proteorhodopsin (PR)‐containing marine flavobacterium, produces EVs (S13EVs). Transmission electron microscopy showed that Sediminicola sp. YIK13 released two spherical vesicle types, with mono‐ and/or bi‐layered membranes, in the culture. Interestingly, the S13EVs have an orange pigment, indicating the presence of putative carotenoid and PR pigments ascribed to the parental cells. The S13EVs demonstrated the same PR‐derived absorption peak spectrum and light‐induced proton pump activity as the parental cells. Western blot (immunoblot) analysis of the S13EVs revealed the presence of PR. We confirmed the 16S rRNA gene, pro gene, and genes required for chromophore retinal synthesis, namely blh and crtI, in the DNA packaged into these vesicles. In addition, by metagenomic sequencing, we found microbial rhodopsin‐related genes in vesicles derived from natural aquatic environments. Our results suggest that EVs as well potentially pursue horizontal gene transfer of diverse microbial rhodopsin genes in marine ecosystems.
Highlights
Extracellular vesicle (EV) production is a ubiquitous process in all domains of life, namely Eukarya, Bacteria, and Archaea (Deatherage & Cookson, 2012)
We observed S13EV production using the qNano method after 6 hr of culture growth, reaching a maxi‐ mum of 4.5 × 1010 ml−1 when parental cell growth stabilized at 4.9 × 109 ml−1
These micrographic images revealed numerous lumen struc‐ tures protruding from the surface of the parental cell, which eventually migrated into the surrounding milieu as free EVs
Summary
Extracellular vesicle (EV) production is a ubiquitous process in all domains of life, namely Eukarya, Bacteria, and Archaea (Deatherage & Cookson, 2012). Gram‐negative bacteria release cell‐associated constituents, including proteins, lipoproteins, lipopolysaccharides, and genetic materials, through EVs, which are known as outer membrane vesicles (OMVs) (Kulp & Kuehn, 2010; Manning & Kuehn, 2013). Very little is known about the functions and presence of EVs produced by marine bacteria. We investigated the presence of diverse microbial rhodopsin genes in EVs derived from natural environments, based on shotgun sequenc‐ ing‐generated metagenomes. To the best of our knowledge, no pre‐ vious study has identified these genes in EVs. it is important to understand the role that EVs containing PR genes play in natural environments, as well as the functions of the PR gene and protein within the EVs
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