'No room at the inn!': apparently not (in Vestimentiferan worms)...

Vestimentiferan worms can be found in abundance around hydrothermal vents and cold seeps. Lacking a digestive system, the tubeworm houses an abundance of endosymbionts which provide the host tubeworm with a nutritive supply. Endosymbionts are located intracellularly in an organ called the trophosome. Until recently, it was assumed that only one type of endosymbiont was found in symbiosis with the vestimentiferan worm; a sulfer-oxidising gammaproteobacteria. The assortment of endosymbionts found is subject to spatial and temporal variation, as well as inter- and intra-species variation. Whilst previous analysis hinted at the presence of more than one endosymbiont type, convincing evidence was lacking. Work carried out by Zimmerman et al. (2014) aims to fill this gap and address whether there is indeed, more than one type of endosymbiont present in vestimentiferan worms.

Zimmerman et al. (2014) undertook this work at Marsili Seamount in the Western Mediterranean Sea, an active venting region. Using cytochrome oxidase I genes (COI), the tube worms were identified as L. anaximandri (H1-H4). Identification based on morphology was also undertaken. 542 16S rRNA sequences underwent partial sequencing; this revealed the presence of two distinct gammaproteobacterial 16S rRNA phylotypes (named A and B). Full sequencing of representative clones of phylotypes A and B was also undertaken. Differences between the sequences of A and B were observed consistently at 28 positions. Phylotype A was most phylogenetically alike to endosymbionts found in the Mediterranean, whereas phylotype B was most phylogenetically alike to endosymbionts found in the Pacific.

Inside the trophosomes the endosymbionts are arranged in lobules as in other tube worms. However unlike other tube worms, the central lobule was filled with cocci-shaped endosymbionts  as opposed to the rod-shaped endosymbionts more typically found. The morphology and allometry of the endosymbionts changed according to their proximity to the axial blood vessel of the host worm. The above characteristics were determined using catalysed reporter deposition fluorescence in situ Hybridisation (CARD-FISH) and light microscopy.

Fig 1. CARD-FISH overview of a transverse section through the trophosome of

                                         tubeworm containing symbiont type A (red) and B (yellow). 

In order to determine whether the 16S rRNA phylotypes A and B are actually two separate symbionts, rather than two rRNA operons in the same symbiont genome, CARD-FISH probes were designed to target each phylotype; combined with the gammaproteobacteria probe this confirmed the presence of two symbionts belonging to the gammaproteobacteria. Most lobules had only one of the two symbionts present that said, some lobules did have a combination of both symbionts. The respective abundance of each of these endosymbionts was subject to variability; between individuals (of the same species) and individuals themselves. The endosymbiont composition varied on the antero-posterior axis of individuals; with symbiont B present in greater abundance anteriorly, and symbiont B flourishing posteriorly. (Fig 1) 

The metabolic potential of each of these symbionts was investigated. Each symbiont appears to have differing metabolic roles (as revealed by molecular analysis of metabolic genes – variants in key genes for sulfer oxidation and carbon fixation were noted) which are specific to each symbiont.

Analysis of endosymbiont variation in accordance with the physical environment was also undertaken. Water depth played a significant role in altering the endosymbiont consortia present.

As with all areas of scientific investigation, more research is essential. Our understanding of these symbiotic relationships could still be considered to be in their infancy. That said, new methods (and detailed analysis) are allowing these relationships to be dissected and understood to a greater extent. This investigation provides one of the first, convincing accounts of the presence of a dual-symbiosis in vestimentiferan worms. 

Zimmermann, J., Lott, C., Weber, M., Ramette, A., Bright, M., Dubilier, N. and Petersen, J. M. (2014), Dual symbiosis with co-occurring sulfur-oxidizing symbionts in vestimentiferan tubeworms from a Mediterranean hydrothermal vent. Environmental Microbiology.  doi: 10.1111/1462-2920.12427



Jack