Bacterial nutritional transfer to a hydrothermal vent shrimp

Endosymbiotic relationships between invertebrates and chemoautotrophic bacteria are regularly encountered in deep sea hydrothermal vents. Although most are regarded as nutritional endosymbioses, there has been little direct evidence of the bacteria-host interactions to date. Ponsard et al. (2013) investigated the bacteria-host interactions of the shrimp Rimicaris exoculata, which dominates several Mid-Atlantic ridge hydrothermal vent sites and possesses a large bacterial community in their enlarged gill chamber. They aimed to find out whether there was transfer across the gill integument of inorganic carbon fixed by the chemoautotrophic metabolisms of the bacterial epibionts and to do this the shrimp that were collected from a deep sea site were incubated in pressurized, temperature controlled vessels, with different isotope labeled molecules as well as different electron donors. They then looked at the concentrations of the different isotope-labeled molecules in different tissues of the shrimp as well as in the bacterial biofilms within the shrimp.

Ponsard et al. (2013) found that the bacterial mats of the branchiostegites and the mouthparts showed high incorporation rates of both ¹³C- and ¹⁴C-bicarbonate, with much lower rates in the digestive tract and abdominal muscles. The rate of incorporation was found to be higher when the shrimp were incubated in the presence of either iron or thiosulphate than when they were incubated in seawater alone, without the additional presence of an electron donor, however incorporation of the ¹³C- and ¹⁴C-bicarbonate occurred whatever the incubation conditions were. The digestive gland showed the lowest incorporation rates. When the shrimp were incubated with ¹⁴C-acetate the incorporation rates were once again highest in the mouthparts and inner bacterial biofilms, indicating that acetate is a suitable carbon source for the bacteria. Incorporation rates for ³H-lysine were much lower than for acetate, but were significant and suggest that the bacteria can also use lysine. This analytical data was confirmed by autoradiography (pictures taken of segments of the shrimp showing the radiolabeled areas).

The results of this study produce strong evidence for a mutualistic relationship between R. exoculata and their chemosynthetic epibiotic bacteria, although the fixation rates here are lower than for other chemosynthetic symbionts of other hydrothermal vent invertebrates, so it is suggested that the shrimp may also feed on molecules and bacteria from the chimney walls of the vents. They also provide evidence disproving a previous hypothesis that crustaceans cannot take up dissolved organic matter or dissolved organic carbon as the shrimp appear to be able to take up dissolved organic molecules such as acetate and lysine across their gill integument. Further research should focus on what the dominant energy (carbon) source of these bacteria is as well as whether they are capable of mixotrophic metabolism or whether a consortia of bacteria is involved depending on the prevailing conditions.

It was really interesting to read a study that actually used a deep sea organism in the lab, as previously there have been issues in maintaining them in a pressurized environment, however, I found the method for this review a little difficult to follow as it’s not a method I’m familiar with at all! I also thought it was really interesting to read of direct evidence for the bacterial transfer of nutrition to an organism, and not just the consumption of the bacteria by the shrimp.

Ponsard, J., Cambon-Bonavita, M., Zbinden, M., Lepoint, G., Joassin, A., Corbari, L., Shillito, B. et al. (2013) Inorganic carbon fixation by chemosynthetic ectosymbionts and nutritional transfers to the hydrothermal vent host shrimp Rimicaris exoculata. The ISME Journal. 7, 96-109