14Background: Halobacteriovorax are saltwater-adapted predatory bacteria that attack 15Gram-negative bacteria and therefore may play an important role in shaping microbial 16 communities. To understand the impact of Halobacteriovorax on ecosystems and develop 17 them as biocontrol agents, it is important to characterize variation in predation 18 phenotypes such as prey range and investigate the forces impacting Halobacteriovorax 19 genome evolution across different phylogenetic distances. 20 Results:We isolated H. marinus BE01 from an estuary in Rhode Island using Vibrio 21 from the same site as prey. Small, fast-moving attack phase BE01 cells attach to and 22 invade prey cells, consistent with the intraperiplasmic predation strategy of H. marinus 23 type strain SJ. BE01 is a prey generalist, forming plaques on Vibrio strains from the 24 estuary as well as Pseudomonas from soil and E. coli. Genome analysis revealed that 25 BE01 is very closely related to SJ, with extremely high conservation of gene order and 26 amino acid sequences. Despite this similarity, we identified two regions of gene content 27 difference that likely resulted from horizontal gene transfer. Analysis of modal codon 28 usage frequencies supports the hypothesis that these regions were acquired from bacteria 29 with different codon usage biases compared to Halobacteriovorax. In BE01, one of these 30 regions includes genes associated with mobile genetic elements, such as a transposase not 31 found in SJ and degraded remnants of an integrase occurring as a full-length gene in SJ. 32The corresponding region in SJ included unique mobile genetic element genes, such as a 33 site-specific recombinase and bacteriophage-related genes not found in BE01. Acquired 34 functions in BE01 include the dnd operon, which encodes a pathway for DNA 35 modification that may protect DNA from nucleases, and a suite of genes involved in 36 Background: 48Predation is an important force shaping microbial communities, which include microbial 49 species that prey on other microbes. Eukaryotic microbial predators have received the 50 majority of attention; however, bacterial predators are found in a wide range of 51 environments and attack bacteria and fungi [1]. Predatory bacteria such as Bdellovibrio 52 bacteriovorus attack animal and plant pathogens, which makes them a potential 53 biocontrol agent and an alternative to antibiotics [2, 3]. To further understand bacterial 54 predation and inform development of predatory bacteria as biocontrol agents, it is 55 important to characterize variation in predation phenotypes such as prey range and 56 examine evolution of predatory bacteria lineages at different scales. 57 58 . CC-BY 4.0 International license peer-reviewed) is the author/funder. It is made available under a The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/180265 doi: bioRxiv preprint first posted online 4 Halobacteriovorax is a genus of predatory bacteria belonging to the Deltaproteobacteria. 59 Similar to Bdellovi...

Predatory bacteria attack and digest other bacteria and therefore may play a role in shaping microbial communities. To investigate phenotypic and genotypic variation in saltwater-adapted predatory bacteria, we isolated Halobacteriovorax marinus BE01 from an estuary in Rhode Island, assayed whether it could attack different prey bacteria, and sequenced and analyzed its genome. We found that BE01 is a prey generalist, attacking bacteria from different phylogenetic groups and environments. Gene order and amino acid sequences are highly conserved between BE01 and the H. marinus type strain, SJ. By comparative genomics, we detected two regions of gene content difference that likely occurred via horizontal gene transfer events. Acquired genes encode functions such as modification of DNA, membrane synthesis and regulation of gene expression. Understanding genome evolution and variation in predation phenotypes among predatory bacteria will inform their development as biocontrol agents and clarify how they impact microbial communities.