Ecology, Evolution, Behavior and Systematics

Abstract Actinobacteria of the acI lineage are the most abundant microbes in freshwater systems, but there are so far no pure living cultures of these organisms, possibly because of metabolic dependencies on other microbes. This, in turn, has hampered an in-depth assessment of the genomic basis for their success in the environment. Here we present genomes from 16 axenic cultures of acI Actinobacteria. The isolates were not only of minute cell size, but also among the most streamlined free-living microbes, with extremely small genome sizes (1.2–1.4 Mbp) and low genomic GC content. Genome reduction in these bacteria might have led to auxotrophy for various vitamins, amino acids and reduced sulphur sources, thus creating dependencies to co-occurring organisms (the ‘Black Queen’ hypothesis). Genome analyses, moreover, revealed a surprising degree of inter- and intraspecific diversity in metabolic pathways, especially of carbohydrate transport and metabolism, and mainly encoded in genomic islands. The striking genotype microdiversification of acI Actinobacteria might explain their global success in highly dynamic freshwater environments with complex seasonal patterns of allochthonous and autochthonous carbon sources. We propose a new order within Actinobacteria (‘Candidatus Nanopelagicales’) with two new genera (‘Candidatus Nanopelagicus’ and ‘Candidatus Planktophila’) and nine new species.

Secondary metabolites, which are small-molecule organic compounds produced by living organisms, provide or inspire drugs for many different diseases. These natural products have evolved over millions of years to provide a survival benefit to the producing organism and often display potent biological activity with important therapeutic applications. For instance, defensive compounds in the environment may be cytotoxic to eukaryotic cells, a property exploitable for cancer treatment. Here, we describe the genome of an uncultured symbiotic bacterium that makes such a cytotoxic metabolite. This symbiont is losing genes that do not endow a selective advantage in a hospitable host environment. Secondary metabolism genes, however, are repeated multiple times in the genome, directly demonstrating their selective advantage. This finding shows the strength of selective forces in symbiotic relationships and suggests that uncultured bacteria in such relationships should be targeted for drug discovery efforts.

Summary Microorganisms are main drivers of the sulfur, nitrogen and carbon biogeochemical cycles. These elemental cycles are interconnected by the activity of different guilds in sediments or wastewater treatment systems. Here, we investigated a nitrate‐reducing microbial community in a laboratory‐scale bioreactor model that closely mimicked estuary or brackish sediment conditions. The bioreactor simultaneously consumed sulfide, methane and ammonium at the expense of nitrate. Ammonium oxidation occurred solely by the activity of anammox bacteria identified as Candidatus Scalindua brodae and Ca . Kuenenia stuttgartiensis. Fifty‐three percent of methane oxidation was catalyzed by archaea affiliated to Ca . Methanoperedens and 47% by Ca . Methylomirabilis bacteria. Sulfide oxidation was mainly shared between two proteobacterial groups. Interestingly, competition for nitrate did not lead to exclusion of one particular group. Metagenomic analysis showed that the most abundant taxonomic group was distantly related to Thermodesulfovibrio sp. (87–89% 16S rRNA gene identity, 52–54% average amino acid identity), representing a new family within the Nitrospirae phylum. A high quality draft genome of the new species was recovered, and analysis showed high metabolic versatility. Related microbial groups are found in diverse environments with sulfur, nitrogen and methane cycling, indicating that these novel Nitrospirae bacteria might contribute to biogeochemical cycling in natural habitats.

Abstract Asian citrus psyllid D iaphorina citri is the vector of the citrus H uanglongbing ( HLB ) associated bacterial agent ‘ C andidatus L iberibacter asiaticus’ ( C L as). The molecular interactions between C L as and D . citri remain unclear. In the present study, protein profiles of mitochondrial, microsomal and cytosolic fractions from uninfected and C Las‐infected adult D . citri are investigated using two‐dimensional gel electrophoresis. The comparative analysis reveals a total of 18, 24 and 20 protein spots that are unique or differentially expressed in mitochondrial, microsomal and cytosolic proteins fractions respectively. These proteins are successfully identified by mass spectrometry. Among the 62 identified proteins, 30 are up‐regulated, whereas 32 are down‐regulated. These proteins include important components in energy metabolism such as ATP synthase, ATP ase, ATP / ADP carrier protein, etc.; host stress responses such as heat shock proteins; host detoxification processes (i.e., cytochrome P 450 and glutathione S ‐transferase); and the cytoskeleton (such as actin, tubulin, myosin and tropomyosin). These data suggest that, after C L as infection, several proteins of D . citri , especially energy metabolism and protein biosynthesis, are altered, and extensive host defence responses are induced. In conclusion, the present study reports proteomic information that is helpful in understanding the vector–pathogen relationship between C L as and D . citri , and could be used to identify potential targets for limiting the spread of C L as, as well as to provide new insights into HLB management.

Nitrospira -like bacteria are among the most diverse and widespread nitrifiers in natural ecosystems and the dominant nitrite oxidizers in wastewater treatment plants (WWTPs). The recent discovery of comammox-like Nitrospira strains, capable of complete oxidation of ammonia to nitrate, raises new questions about specific traits responsible for the functional versatility and adaptation of this genus to a variety of environments. The availability of new Nitrospira genome sequences from both nitrite-oxidizing and comammox bacteria offers a way to analyze traits in different Nitrospira functional groups. Our comparative genomics analysis provided new insights into the adaptation of Nitrospira strains to specific lifestyles and environmental niches.