Unveiling the P-solubilizing potential of bacteria enriched from natural colonies of Red Sea Trichodesmium spp.

Trichodesmium, a genus of filamentous and diazotrophic cyanobacteria abundant in the Red Sea, has been extensively investigated. Its ability to form colonies is linked to overcoming nutrient limitations in the oligotrophic ocean. Particularly puff-shaped colonies have been demonstrated to exhibit a strong tendency to capture and centre dust particles. Although numerous studies agree that this particle-mining-strategy performed by puff-shaped colonies of Trichodesmium spp. is crucial to meeting their high Fe demands, other nutrients such as P could be obtained additionally. However, details behind the mechanisms employed remain, to date, elusive.Since Trichodesmium colonies are known to host numerous bacteria, the presented study explores the potential presence of P-solubilizing bacteria (PSB) residing within Trichodesmiums associated microbial community, their possible contribution to the host, and the implications for P cycling in marine ecosystems. Therefore, natural, puff-shaped Trichodesmium spp. colonies, collected in the Gulf of Aqaba, Eilat, IL were utilized as inoculum and added to a defined medium (YBCII) supplied with N (nitrate or ammonia) and C (acetate or glucose) sources. As P source, synthetic hydroxyapatite or a phosphate co-precipitated ferrihydrite was added. In total, 28 enrichment cultures were obtained, primarily consisting of bacterial families such as Rhodobacteraceae, Alteromonadaceae and Burkholderiaceae. Five enrichment cultures were chosen for further testing and repeatedly grown on hydroxyapatite, revealing their Nitrogen and P consumption/releasing abilities while expressing strong physical interactions with the mineral supplied. These enrichments mostly utilized acid-production pathways to attack the mineral. Our results suggest that PSB could play a pivotal role in maintaining P balance in the Red Sea, thus supporting growth of Trichodesmium spp. and other marine organisms. Hence, this study adds to our repertoire of metabolic interactions employed by microbes in their quest for nutrients in the ocean.