Publications

4776

Summary Marine Thaumarchaeota are abundant ammonia‐oxidizers but have few representative laboratory‐cultured strains. We report the cultivation of Candidatus Nitrosomarinus catalina SPOT01, a novel strain that is less warm‐temperature tolerant than other cultivated Thaumarchaeota. Using metagenomic recruitment, strain SPOT01 comprises a major portion of Thaumarchaeota (4–54%) in temperate Pacific waters. Its complete 1.36 Mbp genome possesses several distinguishing features: putative phosphorothioation (PT) DNA modification genes; a region containing probable viral genes; and putative urea utilization genes. The PT modification genes and an adjacent putative restriction enzyme (RE) operon likely form a restriction modification (RM) system for defence from foreign DNA. PacBio sequencing showed >98% methylation at two motifs, and inferred PT guanine modification of 19% of possible TGCA sites. Metagenomic recruitment also reveals the putative virus region and PT modification and RE genes are present in 18–26%, 9–14% and <1.5% of natural populations at 150 m with ≥85% identity to strain SPOT01. The presence of multiple probable RM systems in a highly streamlined genome suggests a surprising importance for defence from foreign DNA for dilute populations that infrequently encounter viruses or other cells. This new strain provides new insights into the ecology, including viral interactions, of this important group of marine microbes.

ABSTRACT Deep-ocean regions beyond the reach of sunlight contain an estimated 615 Pg of dissolved organic matter (DOM), much of which persists for thousands of years. It is thought that bacteria oxidize DOM until it is too dilute or refractory to support microbial activity. We analyzed five single-amplified genomes (SAGs) from the abundant SAR202 clade of dark-ocean bacterioplankton and found they encode multiple families of paralogous enzymes involved in carbon catabolism, including several families of oxidative enzymes that we hypothesize participate in the degradation of cyclic alkanes. The five partial genomes encoded 152 flavin mononucleotide/F420-dependent monooxygenases (FMNOs), many of which are predicted to be type II Baeyer-Villiger monooxygenases (BVMOs) that catalyze oxygen insertion into semilabile alicyclic alkanes. The large number of oxidative enzymes, as well as other families of enzymes that appear to play complementary roles in catabolic pathways, suggests that SAR202 might catalyze final steps in the biological oxidation of relatively recalcitrant organic compounds to refractory compounds that persist. IMPORTANCE Carbon in the ocean is massively sequestered in a complex mixture of biologically refractory molecules that accumulate as the chemical end member of biological oxidation and diagenetic change. However, few details are known about the biochemical machinery of carbon sequestration in the deep ocean. Reconstruction of the metabolism of a deep-ocean microbial clade, SAR202, led to postulation of new biochemical pathways that may be the penultimate stages of DOM oxidation to refractory forms that persist. These pathways are tied to a proliferation of oxidative enzymes. This research illuminates dark-ocean biochemistry that is broadly consequential for reconstructing the global carbon cycle.

Huanglongbing (HLB) is a destructive disease of citrus caused by phloem-limited bacteria, namely ‘Candidatus Liberibacter asiaticus’ (Las), ‘Candidatus Liberibacter africanus’, and ‘Candidatus Liberibacter americanus’. Although there are no known HLB-resistant citrus species, studies have reported Poncirus trifoliata as being more tolerant. Assuming that callose deposition in the phloem of infected plants can inhibit translocation of photosynthetic products and cause starch accumulation, we compared callose deposition in petioles and starch accumulation in infected leaves of three genotypes (Citrus sinensis, C. sunki, and P. trifoliata) and 15 hybrids (C. sunki × P. trifoliata). Compared with the mock-inoculated plants, higher bacterial counts and greater accumulation of callose and starch were found in C. sinensis, C. sunki, and 10 of the hybrid plants. Lower titer and fewer metabolic changes due to Las infection were observed in P. trifoliata and in two Las-positive hybrids while three hybrids were Las-negative. Callose accumulation was linked to and correlated with genes involved in phloem functionality and starch accumulation was linked to up-regulation of genes involved in starch biosynthesis and repression of those related to starch breakdown. Lower expression of genes involved in phloem functionality in resistant and tolerant plants can partially explain the absence of distinct disease symptoms associated with starch accumulation that are usually observed in HLB-susceptible genotypes.