Inter-kingdom interactions shapes macroalga Ulva holobiont : studies on algal growth and morphogenesis promoting factors (phytohormones and thallusin) and algal associated microbial community

Ulva is a ubiquitous marine and freshwater macroalgae genus that belongs to the phylum Chlorophyta. Macroalgae and bacteria strongly interact through chemical queues. Ulva mutabilis forms with the bacteria, Roseovarius sp. and Maribacter sp. a robust community. Ulva attracts and supports bacterial growth, and, in return, the bacteria release algal growth and morphogenetic factors (AGMPFs). This work aims to investigate AGMPFs, and their effect on algal metabolism. The first step was characterized by the optimization of the protocol to obtain axenic Ulva germ cells and the standardization of the workflow for algal metabolomics. Phytohormones presence and concentrations were screened in different morphotypes of U. mutabilis and their activity was tested with axenic algae. Even though they did not affect Ulva's development, the concentration of salicylic acid was higher in the axenic than in presence of bacteria. The study of AGMPFs continued with their characterization in Ulva aquaculture and in coastal lagoon. With solid-phase extraction and targeted liquid chromatography it was possible to identify the morphogen thallusin in the phycosphere of Ulva. The final proof was obtained after the discovery that thallusin is produced and released into the growing medium by Maribacter sp. and the morphogen alone induces the differentiation of the primary rhizoid cell and a normal cell wall formation in Ulva.  With transcriptomics and metabolomics analyses it was possible to suggest that in the early phase of Ulva's development, thallusin regulates the transcription factor CCCH-type zinc finger protein, phosphoglycerate kinase and enolase enzymes, which affect the algal metabolism and likely future morphology. Lastly, the characterization of the microbiome associated with Ulva rigida of an industrial facility set the bases to study further the biotechnological application of thallusin and a synthetic microbiome in aquaculture.

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