Abstract
Symbiotic interactions between ascidians (sea-squirts) and microbes are poorly understood. Here we characterized the cyanobacteria in the tissues of 8 distinct didemnid taxa from shallow-water marine habitats in the Bahamas Islands by sequencing a fragment of the cyanobacterial 16S rRNA gene and the entire 16S–23S rRNA internal transcribed spacer region (ITS) and by examining symbiont morphology with transmission electron (TEM) and confocal microscopy (CM). As described previously for other species, Trididemnum spp. mostly contained symbionts associated with the Prochloron-Synechocystis group. However, sequence analysis of the symbionts in Lissoclinum revealed two unique clades. The first contained a novel cyanobacterial clade, while the second clade was closely associated with Acaryochloris marina. CM revealed the presence of chlorophyll d (chl d) and phycobiliproteins (PBPs) within these symbiont cells, as is characteristic of Acaryochloris species. The presence of symbionts was also observed by TEM inside the tunic of both the adult and larvae of L. fragile, indicating vertical transmission to progeny. Based on molecular phylogenetic and microscopic analyses, Candidatus Acaryochloris bahamiensis nov. sp. is proposed for this symbiotic cyanobacterium. Our results support the hypothesis that photosymbiont communities in ascidians are structured by host phylogeny, but in some cases, also by sampling location.
Highlights
Symbioses between cyanobacteria and marine invertebrates are common, especially in sponges and ascidians
The taxonomic status of T. cyanophorum has been argued in the past, with some authors suggesting that it may be a synonym of T. solidum [30]
We have found 12% sequence variability between T. solidum and T. cyanophorum and some morphological differences in terms of color and colony shape (Figure 1), suggesting that both species names are valid despite the lack of morphological differences among zooids
Summary
Symbioses between cyanobacteria and marine invertebrates are common, especially in sponges (reviewed in [1]) and ascidians (reviewed in [2]). The majority of ascidian-microbe studies have focused on species within the ascidian family Didemnidae (Aplousobranchia), which often establish symbiotic relationships with unicellular cyanobacteria from the genera Prochloron (Prochlorales) and Synechocystis (Chroococcales). The type species of these genera are Prochloron didemni, first found in Didemnum spp. from Baja California [9,10], and Synechocystis trididemni, found in the Caribbean ascidian Trididemnum cyanophorum [11]. The cell morphology of both cyanobacterial species is very similar [11], and molecular phylogenetic analyses revealed that they had evolved from a common cyanobacterial ancestor [12,13]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
