| Summary: | ABSTRACT. Deep-sea sponge grounds are underexplored ecosystems that provide numerous goods and services to the functioning of the deep-sea. This study assessed the prokaryotic diversity in embryos, recruits, and adults of Craniella zetlandica and Craniella infrequens , common and abundant representatives of deep-sea sponge grounds in the North Atlantic. Our results reveal that symbiont transmission in the two Craniella sponge species likely occurs vertically, as highly similar microbial consortia have been identified in adults, embryos, and recruits. Moreover, transmission electron microscopy revealed high abundances of sponge-associated microorganisms, among which Chloroflexi ( SAR202 ) were identified as common representatives by amplicon sequencing and fluorescence in situ hybridization (FISH). Equal diversity metrices, a similar overall prokaryotic community composition and a distinct dominance of the phylum Chloroflexi within all life stages are the key findings of our analyses. Information such as presented here provide understanding on the recruitment of deep-sea sponge holobionts which is needed to develop integrated management tools of such vulnerable marine ecosystems. FUNDING. This study was funded by the European Union's Horizon 2020 Research and Innovation Program under Grant Agreement No. 679849 (the SponGES project). This document reflects only the authors' view and the Executive Agency for Small and Medium-sized Enterprises (EASME) is not responsible for any use that may be made of the information it contains. ACKNOWLEDGEMENTS. We express our gratitude to HR, coordinator of the H2020-SponGES project on deep-sea sponge grounds of the North Atlantic, deep-sea sponge taxonomist, explorer, mentor, and friend. We thank the crews and scientific parties of RV G.O.Sars cruises "GS2017110" and "GS2018108". On these cruises the ROV AEgir-team (University of Bergen) enabled sampling of precious samples. Ina Clefsen and Andrea Hethke provided valuable support in the lab back on land. We also thank Heikki ...
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