Effects of color variation and physiological state on ascidian microbiomes

Microbial Distortion? Impacts of Delayed Preservation on Microbiome Diversity and Composition in a Marine Invertebrate

Field collections of marine invertebrates are often accompanied by delays in preservation, which may impact microbiome composition. Here, we tested the effects of delayed preservation and relaxation methods on microbiome diversity and composition in the colonial ascidian Trididemnum solidum using 16S rRNA amplicon sequencing. Replicate samples collected from Belizean reefs were either (1) immediately preserved in ethanol ("control"), (2) held in ambient seawater for 3 h before preservation ("SW"), or (3) held in ambient seawater with menthol (a common pre-preservation relaxation technique for ascidian identification) for 3 h before preservation ("SW + M"). All T. solidum microbiomes were different from ambient seawater bacterioplankton and dominated by the same microbial taxa, including the genera Thalassobaculum, Tistrella, and Synechocystis. However, the 3-h delay in sample preservation (SW) significantly reduced microbiome richness compared to controls (p = 0.028), while menthol treatment (SW + M) mitigated this diversity loss (p = 0.208). Microbial composition at the community level did not differ significantly for either delayed preservation method compared to controls (SW p = 0.054, SW + M p = 0.052). Taxon-level shifts were rare but did occur, most notably a bloom of the facultatively anaerobic gammaproteobacterium Catenococcus that was 37x (SW) and 197x (SW + M) more abundant in delayed preservations. After a 3-h preservation delay (SW), only 122 microbial taxa (1.85% of total) exhibited significantly differential abundances with controls, with menthol treatment (SW + M) reducing taxon-level shifts to 65 taxa (0.98%). Our results showed that brief delays in preservation did not significantly alter community-level microbiome composition and dominant taxa, with menthol exposure counteracting minor microbiome shifts associated with preservation delays.

Effects of color variation and physiological state on ascidian microbiomes

Ascidians, known for their color variation, host species-specific microbial symbiont communities. Some ascidians can also transition into a nonfiltering (resting) physiological state. Recent studies suggest that the microbial symbiont communities may vary across different physiological states and color morphs of the host. The colonial ascidian, Polyclinum constellatum, which exhibits several color morphs in the Caribbean Sea, periodically ceases its filtering activity. To investigate if color variation in P. constellatum is indicative of sibling speciation, we sequenced fragments of the ribosomal 18S rRNA and the mitochondrial cytochrome oxidase subunit I genes. Additionally, we sequenced a fragment of the 16S rRNA gene to characterize the microbial communities of two common color morphs (red and green) in colonies that were either actively filtering (active) or nonfiltering (resting). Phylogenetic analyses of both ascidian genes resulted in well-supported monophyletic clades encompassing all color variants of P. constellatum. Interestingly, no significant differences were observed among the microbial communities of the green and red morphs, suggesting that color variation in this species is a result of intraspecific variation. However, the host's physiological state significantly influenced the microbial community structure. Nonfiltering (resting) colonies hosted higher relative abundances of Kiloniella (Alphaproteobacteria) and Fangia (Gammaproteobacteria), while filtering colonies hosted more Reugeria (Alphaproteobacteria) and Endozoicomonas (Gammaproteobacteria). This study demonstrates that microbial symbiont communities serve as reliable indicators of the taxonomic state of their host and are strongly influenced by the host's feeding condition.