Microalgae, a diverse group of single-celled organisms exhibiting versatile traits, find broad applications in industry. However, high production costs require further efforts to optimize their production and to enhance biomass yields. In the present study, co-occurrence of algae and methylobacteria was observed when naturally occurring microalgae biofilms were subjected to 16S rRNA gene fragment amplicon sequencing. This bacterial group is so far less explored than other microalgae-associated bacteria in terms of mutualistic relationships that might be exploitable for biotechnological applications. In order to assess the potential of four plant growth-promoting strains from the genus Methylobacterium for increased algae biomass production, co-cultivation experiments were conducted with three industrially relevant microalgae (Chlorella vulgaris, Scenedesmus vacuolatus, and Haematococcus lacustris). For S. vacuolatus and H. lacustris, a significant increase in algal biomass formation of 1.3-fold to up to 14fold was observed after 7 days of co-incubation. Visualization of mixed cultures using confocal laser scanning microscopy revealed a high abundance of methylobacteria in the phycosphere of H. lacustris and S. vacuolatus, visually attached to the algae's surface forming a biofilm-like assemblage. Genome analyses revealed that features attributable to enhanced algal growth include genes involved in the synthesis of vitamins, siderophores and plant hormones. Our results provide evidence for the constructability of novel symbiotic algae-bacteria relationships with inter-kingdom supportive capacities, underlining the potential of microbial consortia as promising tool for sustainable biotechnology and agriculture.

The suppressive influence of compost towards the soil-borne fungus Fusarium oxysporum f. sp. lycopersici on tomatoes with special emphasis on root morphological modifications was examined. Roots of inoculated and non-inoculated tomato plants, grown in three different substrates (control, 20 % compost and 40 % compost), were scanned and analyzed in order to identify their morphological traits. To examine a potential systemic effect of compost, the plants were placed in individual pots or in split-root systems. Obtained results showed that F. oxysporum f. sp. lycopersici caused decreased root lengths and reduced root weight, root surface area and root volume. Furthermore, an increase in fine root fraction and specific root length was observed in inoculated plants. Substrate containing 20 % compost had a clear stabilizing effect towards pathogenrelated changes of the root morphology and was best able to mitigate below-ground symptoms on plants cultivated in individual pots. The morphology of roots grown in split-root systems was not directly affected by the presence of F. oxysporum f. sp. lycopersici, but complex interactions between the pathogen and the substrate were observed, strongly depending on which substrate was inoculated. These results indicated that soil amendment with compost in moderate amounts contributes to plant health and could provide an alternative to peat based substrates in sustainable production systems.

Phyllosphere microbiota represents a substantial but hardly explored reservoir for disease resistance mechanisms. The goal of our study was to understand the link between grapevine cultivars susceptibility to Plasmopara viticola, one of the most devastating leaf pathogens in viticulture, and the phyllosphere microbiota. Therefore, we analyzed a 16S rRNA gene library for the dominant phyllosphere bacterial phyla Alphaproteobacteria of seven Vitis genotypes at different developmental stages, i.e., flowering and harvesting, via amplicon sequencing. Young leaves had significantly higher Alphaproteobacterial richness and diversity without significant host-specificity. In contrast, the microbial communities of mature leaves were structurally distinct in accordance with P. viticola resistance levels. This statistically significant link between mature bacterial phyllosphere communities and resistant phenotypes was corroborated by beta diversity metrics and network analysis. Beyond direct host-driven effects via the provision of microhabitats, we found evidence that plants recruit for specific bacterial taxa that were likely playing a fundamental role in mediating microbe-microbe interactions and structuring clusters within mature communities. Our results on grape-microbiota interaction provide insights for targeted biocontrol and breeding strategies.