Sea Cucumber Intestinal Regeneration Reveals Deterministic Assembly of the Gut Microbiome

The gut microbiome has far-reaching effects on host organism health, so understanding the processes that underlie microbial community assembly in the developing gut is a current research priority. Here, a holothurian (also known as sea cucumber; phylum Echinodermata) host is explored as a promising model system for studying the assembly of the gut microbiome. Holothurians have a unique capacity for evisceration (expulsion of the internal organs), followed by rapid regeneration of the gut, decoupling host ontogeny from gut tissue development and permitting experimental manipulation of the gut microbiome in mature host individuals. Here, evisceration was induced in the sea cucumber Sclerodactyla briareus, and regenerating stomach and intestine microbiomes were characterized before and on days 0, 13, 17, and 20 after evisceration using Illumina sequencing of 16S rRNA genes. Regenerating stomach and intestine tissues had microbial communities significantly different from those of mature tissues, with much higher alpha diversity and evenness of taxa in regenerating tissues. Despite immersion in a diverse pool of sediment and seawater microbes in flowthrough seawater aquaria, regenerating gut microbiomes differed at each stage of regeneration and displayed a highly similar community structure among replicates, providing evidence for deterministic host selection of a specific microbial consortium. Moreover, regenerating gut tissues acquired a microbiome that likely conferred energetic and immune advantages to the sea cucumber host, including microbes that can fix carbon and degrade invading pathogens.IMPORTANCE The gut microbiome is pertinent to many aspects of animal health, and there is a great need for natural but tractable experimental systems to examine the processes shaping gut microbiome assembly. Here, the holothurian (sea cucumber) Sclerodactyla briareus was explored as an experimental system to study microbial colonization in the gut, as S. briareus individuals have the ability to completely eviscerate and rapidly regenerate their digestive organs. After induced evisceration, microbial community assembly was characterized over 20 days in regenerating animals. This study demonstrated that colonization of the sea cucumber gut was deterministic; despite immersion in a diverse consortium of environmental microbes, a specific subset of microbes proliferated in the gut, including taxa that likely conferred energetic and immune advantages to the host. Sea cucumbers have the potential to revolutionize our understanding of gut microbiome assembly, as rapid and repeatable gut tissue regeneration provides a promising and tractable experimental system.