A mutualistic bacterium rescues a green alga from an antagonist

Photosynthetic protists, known as microalgae, are key contributors to primary production on Earth. Since early in evolution, they coexist with bacteria in nature, and their mode of interaction shapes ecosystems. We have recently shown that the bacterium Pseudomonas protegens acts algicidal on the microalga Chlamydomonas reinhardtii. It secretes a cyclic lipopeptide and a polyyne that deflagellate, blind, and lyse the algae [P. Aiyar et al., Nat. Commun. 8, 1756 (2017) and V. Hotter et al., Proc. Natl. Acad. Sci. U.S.A. 118, e2107695118 (2021)]. Here, we report about the bacterium Mycetocola lacteus, which establishes a mutualistic relationship with C. reinhardtii and acts as a helper. While M. lacteus enhances algal growth, it receives methionine as needed organic sulfur and the vitamins B1, B3, and B5 from the algae. In tripartite cultures with the alga and the antagonistic bacterium P. protegens, M. lacteus aids the algae in surviving the bacterial attack. By combining synthetic natural product chemistry with high-resolution mass spectrometry and an algal Ca2+ reporter line, we found that M. lacteus rescues the alga from the antagonistic bacterium by cleaving the ester bond of the cyclic lipopeptide involved. The resulting linearized seco acid does not trigger a cytosolic Ca2+ homeostasis imbalance that leads to algal deflagellation. Thus, the algae remain motile, can swim away from the antagonistic bacteria and survive the attack. All three involved genera cooccur in nature. Remarkably, related species of Pseudomonas and Mycetocola also act antagonistically against C. reinhardtii or as helper bacteria in tripartite cultures.

Mycetocola spongiae sp. nov., isolated from deep-sea sponge Cacospongia mycofijiensis

A novel bacterial strain (MSC19^T) was isolated from a deep-sea sponge Cacospongia mycofijiensis collected in the Mariana Trench at a depth of 2681 m. The cells of the new isolate were Gram-stain-positive, non-motile, oxidase- and catalase-positive, rod-shaped and yellow-coloured. They could grow at 4-32 °C (optimum, 28 °C), pH 5.5-12 (optimum, pH 7.0) and with 0-12 % (w/v) NaCl (optimum, 4 %). The strain's 16S rRNA gene sequence showed 98.41 % similarity to that of Mycetocola saprophilus CM-01^T. Phylogenetic analysis further suggested that strain MSC19^T represents a new species within the genus Mycetocola. The total genome of MSC19^T was approximately 3 196 754 bp in size with a G+C content of 66.43 mol%. The average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) values among MSC19^T and other Mycetocola type strains were 70.35-75.37 % (ANIb), 83.79-84.73 % (ANIm) and 20.3-21.7 % (dDDH). The major fatty acids of MSC19^T were composed of anteiso-C_15 : 0, iso-C_16 : 0 and anteiso-C_17 : 0, and its predominant menaquinones were MK-10 and MK-9. The polar lipids of MSC19^T mainly consisted of diphosphatidylglycerol, phosphatidylglycerol and glycolipid. The diagnostic cell-wall diamino acid was lysine. Combined molecular, physiological, biochemical and chemotaxonomic analyses suggest that strain MSC19^T represents a novel species of the genus Mycetocola, for which the name Mycetocola spongiae sp. nov. is proposed. The type strain is MSC19^T (=MCCC 1K06265^T=KCTC 49701^T).

Bacterial Brown Pit, a New Disease of Edible Mushrooms Caused by Mycetocola sp

From September to December 2018, commercial button mushroom (Agaricus bisporus) farms in central Iran were surveyed to monitor the causal agent(s) of browning and blotch symptoms on mushroom caps. In addition to dozens of pseudomonads (i.e., Pseudomonas tolaasii and Pseudomonas reactans), six slow-growing gram-positive bacterial strains were isolated from blotched mushroom caps. These bacteria presented as creamy white, circular, smooth, nonfluorescent, and shiny colonies with whole margins resembling members of Microbacteriaceae (Actinobacteria). All of the actinobacterial strains were aggressively pathogenic on cut cap surface of two edible mushrooms (i.e., A. bisporus and Pleurotus eryngii), inducing brown pit symptoms 48 h postinoculation. The strains did not induce symptoms on the vegetables tested (i.e., carrot, cucumber, and potato), and they did not affect the growth of mycelium of tested plant-pathogenic fungi (i.e., Acremonium sp., Fusarium spp., and Phytopythium sp.). Phylogeny of 16S ribosomal RNA and multilocus sequence analysis of six housekeeping genes (i.e., atpD, dnaK, gyrB, ppK, recA, and rpoB) revealed that the bacterial strains belong to the actinobacterial genus Mycetocola spp., whereas the species status of the strains remains undetermined. Mushroom-associated Mycetocola species were previously reported to be capable of detoxifying tolaasin, a toxin produced by P. tolaasii, whereas the strains isolated in this study did not show tolaasin detoxification activities. Altogether, this is the first report of a mushroom disease caused by an actinobacterial species, and "bacterial brown pit" was assigned as the common name of the disease.

Mycetocola zhujimingii sp. nov., isolated from faeces of Tibetan antelopes (Pantholops hodgsonii)

Strains 449^T and 622 are both aerobic, Gram-stain-positive, short, rod-shaped bacilli that were recently isolated from the faeces of Tibetan antelopes on the Qinghai-Tibet Plateau in China. Their 16S rRNA gene sequences were most similar to those of Mycetocola zhadangensis ZD1-4^T(97.9-98.0 %) and Mycetocola miduiensis CGMCC 1.11101^T(97.3-97.4 %). Phylogenetic analysis of the 16S rRNA gene sequences further suggested that strains 449^T and 622 represent a new lineage within the genus Mycetocola. The G+C content of strain 449^T is 64.9 mol%. Optimal growth was achieved at pH 7.0 and 28 °C. Cells contained anteiso-C15 : 0 as the major cellular fatty acid, MK-10 and MK-11 as predominant menaquinones, diphosphatidylglycerol and phosphatidylglycerol as major polar lipids, and lysine as the diagnostic diamino acid. The DNA-DNA hybridization values of strains 449^T and 622 were below the 70 % cut-off with respect to known strains of the genus Mycetocola. Based on these genotypic, phenotypic and biochemical characteristics, it seems rational to conclude that strains 449^T and 622 belong to the genus Mycetocola and thus represent a novel species, for which the name Mycetocola zhujimingii sp. nov. is proposed. The type strain is 449^T (=CGMCC 1.16372^T=DSM 106173^T).

Multilocus sequence analysis of homologous recombination and diversity in Arthrobacter sensu lato named species and glacier-inhabiting strains

Members of the bacterial genus Arthrobacter sensu lato are Gram-positive actinomycetes distributed worldwide and found in numerous environments including soil, water, glacier ice, and sewage. Homologous recombination is an important driving force in bacterial evolution, but its impact on Arthrobacter sensu lato evolution is poorly understood. We evaluated homologous recombination among 41 Arthrobacter sensu lato named species, using multilocus sequence analysis (MLSA). A high level of recombination was found, associated with strong diversification and a reticulate evolutionary pattern of Arthrobacter sensu lato. We also collected a total of 31 cold-adapted Arthrobacter sensu lato strains from two cold glaciers located in northwest China and two temperate glaciers in southwest China, and evaluated their diversity and population structure by MLSA. The glacier strains displayed high diversity, but rates of recombination among the four glacier groups were quite low, indicating that barriers to homologous recombination formed in the past among the populations on different glaciers. Our findings indicate that historical glaciation events shaped the contemporary distributions, taxonomic relationships, and phylogeographic patterns of Arthrobacter sensu lato species on glaciers.