No abstract
Elucidating the timescale of the evolution of Alphaproteobacteria, one of the most prevalent microbial lineages in marine and terrestrial ecosystems, is key to testing hypotheses on their co-evolution with eukaryotic hosts and Earth’s systems, which, however, is largely limited by the scarcity of bacterial fossils. Here, we incorporate eukaryotic fossils to date the divergence times of Alphaproteobacteria, based on the mitochondrial endosymbiosis that mitochondria evolved from an alphaproteobacterial lineage. We estimate that Alphaproteobacteria arose ~1900 million years (Ma) ago, followed by rapid divergence of their major clades. We show that the origin of Rickettsiales, an order of obligate intracellular bacteria whose hosts are mostly animals, predates the emergence of animals for ~700 Ma but coincides with that of eukaryotes. This, together with reconstruction of ancestral hosts, strongly suggests that early Rickettsiales lineages had established previously underappreciated interactions with unicellular eukaryotes. Moreover, the mitochondria-based approach displays higher robustness to uncertainties in calibrations compared with the traditional strategy using cyanobacterial fossils. Further, our analyses imply the potential of dating the (bacterial) tree of life based on endosymbiosis events, and suggest that previous applications using divergence times of the modern hosts of symbiotic bacteria to date bacterial evolution might need to be revisited.
The biochemical composition of dissolved organic phosphorus (DOP) in the ocean is dominated by phosphoesters (C-O-P and C-O-P-O-C bonds), which are hydrolyzed by a diverse group of alkaline phosphatases (PhoA, PhoD, PhoX), and by phosphonates (C-P bond), which are degraded by C-P lyases and hydrolases. We designed a bioinformatics pipeline and a statistical approach to recover and analyze the alkaline phosphatase and phosphonate utilization genes from a metagenomic database derived from water samples collected from 7 depths (between 10 and 4000 m) in the oligotrophic North Pacific Subtropical Gyre. The alkaline phosphatase genes phoD and phoX were more abundant than phoA in the euphotic zone (10-130 m) and in deep waters (500-4000 m). The C-P lyase genes were most abundant in the euphotic zone at 70 m and were rare in deep water (≥500 m) where phosphate concentrations were relatively high. These observations indicate that phosphonates are utilized primarily as a phosphorus source by bacterial C-P lyases; this is consistent with the observation that C-P lyase genes are part of the pho regulon which is expressed upon phosphorus limitation. In contrast, C-P hydrolase and alkaline phosphatase genes were often more abundant in deep waters, indicating that DOP serves mainly as a carbon and energy source in phosphaterich deep waters which are depleted in bioavailable dissolved organic matter (DOM). The observed differences in depth distributions and presumed functions of C-P lyase and hydrolase genes indicate variability in the chemical composition of phosphonates between the euphotic zone and deep waters. KEY WORDS: Phosphonate hydrolase · C-P lyase · Alkaline phosphatase · Microbial phosphorus metabolismResale or republication not permitted without written consent of the publisher
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