New digestive symbiosis in the hydrothermal vent amphipoda Ventiella sulfuris
Ventiella sulfuris is the most abundant amphipod species inhabiting the Eastern Pacific Rise vent field, and is commonly found near population of the Pompeii worm (Alvinella pompejana)
Usually, species living on/near hydrothermal vents survive due to their symbiotic relationships, commonly with chemoautotrophic bacteria. These symbionts often aid their hosts by supplying them with nutrition, which they themselves obtain through sulphur or methane energy sources. For this reason the authors collected specimens of V. sulfuris with the objective of understanding and documenting the bacterial symbiosis occurring in the species (if it did occur), and to hypothesise how this amphipod gains nutrition.
Gut contents of V. sulfuris contained bacterial traces (empty walls or sheaths) and non-degraded Alvinella pompejana cuticle, suggesting that these amphipods feed on A. pompejana and/or their respective symbionts. A previously hypothesis suggested that V. sulfuris grazed directly on the epibiotic bacteria, found on the external surface of the Pompeii worm. This theory is supported by gut condense as they discovered no inner tissue from the worm, only surface tissue; alongside an abundance bacterial traces.
Light (LM) and electron microscopy (SEM and TEM) revealed the outer body, surface, gills and appendages of the host are free from micro-organisms; however the digestive system (the midgut and the hind gut) contains two major microbial communities which were observed in all organisms examined.
In amphipods the midgut and digestive gland are the major organs involved in digestion and absorption of nutrients. In this species electron microscopy revealed the posterior section of the midgut harboured a community of Gram-negative, single-cell, long rod-shaped bacteria. Some of these bacteria were in contact with the endodermal cell membrane, and high magnification revealed numerous membrane vesicles and debris between the bacterium. This is an unusual location, as this is a highly absorptive part of the digestive tract. Additionally, there was an absence of bacteria in the digestive gland.
In the hindgut, the authors observed that some cells exhibited numerous mitochondria that seemed larger than those found in other regions of the digestive system. Electron microscopy revealed densely packed epimural rod-shaped bacteria that (at high magnification) exhibited two morphotypes (electron-lucent short rods, and electron-dense thin-long rods). Both forms were tightly attached to the cuticle or citicular spines, and formed thick mats embedded in a dense organic matter.
16S rRNA gene diversity of all bacterial communities found in host were molecularly analysed to revealed 12 phylogenetic groups; the two most common of which included the Epsilonproteobacteria and the Firmicutes. Interestingly, many of the ID’ed bacterium were closely related to mammalian gut microflora. However some phyla, such as Epsilonproteobacteria are commonly associated with deep sea and/or hydrothermal vent symbiosis.
Sequencing analyses of 16S rRNA genes revealed that the bacteria found in the digestive system of V. sulfuris belong to six phyla: Epsilonproteobacteria, Firmicutes and Cytophaga– Flavobacter–Bacteroides, Gammaproteobacteria, Betaproteobacteria and Alphaproteobacteria. Most of these sequences are close to symbiotic bacteria of hydrothermal vent organisms, and bacteria involved in digestive symbiosis.
Due to a low sample size and an unsuitable fixation of specimens, bacterial DNA was partly degraded and therefore FISH (a technique that could have successfully correlated bacteria phylogenetic affiliation to their location) the authors were unable to specifically ID the two bacterial groups that were discovered in the gut. They did speculate that the midgut community was probably Epsilonproteobacteria, and the foregut phyla were likely to be Alphaproteobacteria, Firmicutes and/or CFB.
Overall an interesting study, but a real shame they couldn't pinpoint the exact phyla/species of the symbionts. However, they are one of the first studies to investigate this symbiotic relationship and for that credit is due.
Corbari, L., Durand, L., Cambon-Bonavita, M. A., Gaill, F., & Compère, P. (2012). New digestive symbiosis in the hydrothermal vent amphipoda Ventiella sulfuris. Comptes rendus biologies, 335(2), 142-154.