Bioinformatic discovery of type 11 secretion system (T11SS) cargo across the Proteobacteria

Type 11 secretion systems (T11SS) are broadly distributed amongst Proteobacteria, with more than 3,000 T11SS family outer membrane proteins (OMPs) comprising ten major sequence similarity network clusters. Of these, only seven, all from animal-associated cluster 1, have been experimentally verified as secretins of cargo, including adhesins, haemophores and metal-binding proteins. To identify novel cargo of a more diverse set of T11SS, we identified gene families co-occurring in gene neighbourhoods with either cluster 1 or marine microbe-associated cluster 3 T11SS OMP genes. We developed bioinformatic controls to ensure that perceived co-occurrences are specific to T11SS, and not general to OMPs. We found that both cluster 1 and cluster 3 T11SS OMPs frequently co-occur with single-carbon metabolism and nucleotide synthesis pathways, but that only cluster 1 T11SS OMPs had significant co-occurrence with metal and haem pathways, as well as with mobile genetic islands, potentially indicating the diversified function of this cluster. Cluster 1 T11SS co-occurrences included 2,556 predicted cargo proteins, unified by the presence of a C-terminal β-barrel domain, which fall into 141 predicted UniRef50 clusters and approximately ten different architectures: four similar to known cargo and six uncharacterized types. We experimentally demonstrate T11SS-dependent secretion of an uncharacterized cargo type with homology to plasmin-sensitive protein. Unexpectedly, genes encoding marine cluster 3 T11SS OMPs only rarely co-occurred with the C-terminal β-barrel domain and instead frequently co-occurred with DUF1194-containing genes. Overall, our results show that with sufficiently large-scale and controlled genomic data, T11SS-dependent cargo proteins can be accurately predicted.

Bacterial communities vary across populations and tissue type in red mangroves (Rhizophora mangle, Rhizophoraceae) along an expanding front

Plant-associated microbial communities may be important sources of functional diversity and genetic variation that influence host evolution. Bacteria provide benefits for their hosts, yet in most plant systems we know little about their taxonomic composition or variation across tissues and host range. Red Mangrove (Rhizophora mangle L.) is a vital coastal plant species that is currently expanding poleward and with it, perhaps, its microbiome. We explored variability in bacterial communities across tissues, individuals, and populations. We collected samples from six sample types from 5 to 10 individuals at each of three populations and used 16S rRNA gene (iTag) sequencing to describe their bacterial communities. Core community members and dominant bacterial classes were determined for each sample type. Pairwise PERMANOVA of Bray-Curtis dissimilarity and Indicator Species Analysis revealed significant differences in bacterial communities between sample types and populations. We described the previously unexplored microbiome of the reproductive tissues of R. mangle. Populations and most sample types were associated with distinct communities. Bacterial communities associated with R. mangle are influenced by host geography and sample type. Our study provides a foundation for future work exploring the functional roles of these microbes and their relevance to biogeochemical cycling.

Metaproteomics reveals unique metabolic niches of dominant bacterial groups in response to rapid regime shifts during a mixed dinoflagellate bloom

The dynamics of bacterial composition and metabolic activity during a distinct phytoplankton bloom have been reported. However, there is limited information on the bacterial community response to drastic environmental changes caused by species succession during a mixed-species bloom. This study investigated active bacterial groups and metabolic activity during a mixed bloom formed by dinoflagellates Prorocentrum obtusidens and Karenia mikimotoi using a metaproteomic approach. Bacterial community structure and dominant bacterial groups varied rapidly with the bloom regime shifts caused by species succession. Pseudoalteromonas and Vibrio dominated the bacterial community in the P. obtusidens-dominated regime, while Alteromonas, Cytophaga-Flavobacteria-Bacteroides (CFB) group, and marine Roseobacter clade (MRC) were the major contributors in other regimes, with the most abundant taxa being Alteromonas in the K. mikimotoi-dominated regime and the CFB group in the dissipation regime. Specific metabolic niches and unique substrate specificity of different bacterial groups enabled them to dominate and thrive in different bloom regimes. High metabolic plasticity in signal response, substrate utilization, motility, and adhesion are essential for bacteria to respond to drastic bloom regime shift, and the predominance of specific bacteria under unique bloom regimes may be the result of long-term coevolution between bacteria and bloom-forming phytoplankton species.

Complete Genome Sequence of the Silicimonas algicola Type Strain, a Representative of the Marine Roseobacter Group Isolated from the Cell Surface of the Marine Diatom Thalassiosira delicatula

Silicimonas algicola strain KC90B^T is an alphaproteobacterium of the Roseobacter clade that was isolated from a culture of the marine diatom Thalassiosira delicatula. Here, we report the complete genome sequence of this type strain, which is 4,351,658 bp in size with 4,272 coding sequences and an average G+C content of 65.2%.

Silicimonas algicola strain KC90B^T is an alphaproteobacterium of the Roseobacter clade that was isolated from a culture of the marine diatom Thalassiosira delicatula. Here, we report the complete genome sequence of this type strain, which is 4,351,658 bp in size with 4,272 coding sequences and an average G+C content of 65.2%.

Bacterial Epibiotic Communities of Ubiquitous and Abundant Marine Diatoms Are Distinct in Short- and Long-Term Associations

Interactions between phytoplankton and bacteria play a central role in mediating biogeochemical cycling and food web structure in the ocean. The cosmopolitan diatoms Thalassiosira and Chaetoceros often dominate phytoplankton communities in marine systems. Past studies of diatom-bacterial associations have employed community-level methods and culture-based or natural diatom populations. Although bacterial assemblages attached to individual diatoms represents tight associations little is known on their makeup or interactions. Here, we examined the epibiotic bacteria of 436 Thalassiosira and 329 Chaetoceros single cells isolated from natural samples and collection cultures, regarded here as short- and long-term associations, respectively. Epibiotic microbiota of single diatom hosts was analyzed by cultivation and by cloning-sequencing of 16S rRNA genes obtained from whole-genome amplification products. The prevalence of epibiotic bacteria was higher in cultures and dependent of the host species. Culture approaches demonstrated that both diatoms carry distinct bacterial communities in short- and long-term associations. Bacterial epibonts, commonly associated with phytoplankton, were repeatedly isolated from cells of diatom collection cultures but were not recovered from environmental cells. Our results suggest that in controlled laboratory culture conditions bacterial–diatom and bacterial–bacterial interactions select for a simplified, but specific, epibiotic microbiota shaped and adapted for long-term associations.