Thomas Schott

Sandy sediments of the coastal ocean are generally not considered a habitat for highly active sulfate-reducing microorganisms (SRM) due to fluctuations in oxygen concentrations and carbon availability. This study investigated the taxonomic composition and gene expression patterns of sulfate-reducing communities in both sandy, organic-poor sediments and muddy, organic-rich sediments in the mesohaline southern Baltic Sea. Our results revealed that a novel order-level lineage within the class Thermodesulfovibrionia (phylum Nitrospirota), named Arenidesulfomonadales, contributed significantly to sulfate reduction in the sandy sediments and supported sulfate reduction rates similar to those in the organic-rich, muddy sediments. In the sands, Thermodesulfovibrionia effectively competed with Desulfobacterales, which strongly dominated the sulfate-reducing communities in the organic-rich sediments. Metatranscriptomic data revealed that Desulfobacterales functioned as typical secondary fermenters in both sediment types and expressed nearly complete sets of genes for the catabolism of fermentation products such as lactate, propionate, and butyrate. In contrast, the gene expression profile of Thermodesulfovibrionia suggests that hydrogenotrophic sulfate reduction, coupled with carbon fixation via the Wood–Ljungdahl pathway and a reduced capacity to oxidize organic compounds, was a competitive metabolic strategy enabling sulfate reduction to proceed independently of fermentation-derived carbon. Overall, our results suggest that overlooked lineages of SRM may contribute to higher levels of sulfate reduction than previously assumed in the extensive areas of permeable sediments of coastal and shelf seas.

Carrot psyllid Trioza apicalis is a serious pest of cultivated carrot and also a vector of the plant pathogen ‘Candidatus Liberibacter solanacearum’ (Lso). To find out whether T. apicalis harbours other species of bacteria that might affect the Lso infection rate, the bacterial communities and metagenome in T. apicalis were studied. Lso haplotype C was detected in a third of the psyllids sampled, at different relative amounts. Surprisingly, T. apicalis was found to harbour only one secondary endosymbiont, a previously unknown species of gamma proteobacterium endosymbiont (Gpe), beside the primary endosymbiont ‘Candidatus Carsonella ruddii’ (CCr). The relative abundancies of these two endosymbionts were approximately equal. The genomes of CCr, Gpe and Lso were assembled from a T. apicalis metagenome sample. Based on the 16S rRNA gene, the closest relative of Gpe of T. apicalis could be a secondary endosymbiont of Trioza magnoliae. The 253.171 kb Gpe genome contains all the tRNA and rRNA genes and most of the protein-coding genes required for DNA replication, transcription and translation, but it lacks most of the genes for amino acid biosynthesis. Gpe has no genes encoding cell wall peptidoglycan synthesis, suggesting it has no cell wall, and could thus live as an intracellular endosymbiont. Like the CCr of other psyllids, CCr of T. apicalis retains a broad amino acid biosynthetic capacity, whilst lacking many genes required for DNA replication and repair and for transcription and translation. These findings suggest that these two endosymbionts of T. apicalis are complementary in their biosynthetic capabilities.