High variability in community structure and activity among individual colonies of Trichodesmium sp.

Trichodesmium spp. are globally important marine bloom-forming, nitrogen-fixing, filamentous cyanobacteria. Trichodesmium often forms colonies of several hundred filaments that are colonized by bacteria from the surrounding water. Studies of Trichodesmium blooms often apply classical and molecular tools on bulk water samples with high concentrations of colonies. Following recent studies by us and others that revealed variability in communities colonizing particulate organic matter in aquatic systems, we investigated the heterogeneity in community composition and transcriptional activity of 70 Trichodesmium colonies collected during an autumn bloom in the Gulf of Eilat (Aqaba), Red Sea, Israel. Using combined single-colony metagenomics and metatranscriptomics, we could show that colonies consist of three main strains of Trichodesmium closely related to Trichodesmium thiebautii with T. erythraeum occasionally present as well. The ratio between the strains varied between the different colonies. The associated microbial community varied, with over 70 % dissimilarity between colonies and no specific pattern. Network analysis revealed that aside from a core community of cyanobacteria, almost no other Bacteria, Archaea, and eukaryotic microorganisms, were part of a stable core community. The activity of Trichodesmium in individual colonies was assessed by the change in reads mapped to annotated genes when normalized to the housekeeping gene rpoB considering that gene copy number per genome remains fixed and variability is mostly driven by changes in expression. While housekeeping genes showed little variability in expression, functional genes involved in nitrogen fixation, phosphorus uptake, iron uptake, as well as photosynthesis, were variable between colonies. These results match data from different physiological assays conducted on individual colonies, revealing that despite thriving in the same water mass, different colonies experience a diverse microenvironment. Their localized response to this variability cannot be deconvoluted from bulk analyses, highlighting the need to complement bulk studies with the analysis of individual colonies. Furthermore, comparing the expression of over 900 genes, revealed the three strains exhibit distinct profiles, suggesting they may occupy slightly different niches, and thus, rather than competing, contribute to the overall successful survival of the species under varying environmental conditions.

Primary Presenter: Mina Bizic, Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) (mina.bizic@igb-berlin.de)

Authors:

Danny Ionescu, Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) / Technical University of Berlin  (danny.ionescu@igb-berlin.de)

Siyuan Wang, Hebrew University of Jerusalem  (siyuan.wang@mail.huji.ac.il)

Futing Zhang, Hebrew University of Jerusalem  (futing.zhang@mail.huji.ac.il)

Yeala Shaked, Hebrew University of Jerusalem  (yeala.shaked@mail.huji.ac.il)

Mina Bizic, Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) / Technical University of Berlin  (mina.bizic@igb-berlin.de)