The burden of independence: Inorganic carbon utilization strategies of the sulphur chemoautotrophic hydrothermal vent isolate Thiomicrospira crunogena and the symbionts of hydrothermal vent and cold seep vestimentiferans

Riftia pachyptila symbionts, which have the lowest affinities for carbon dioxide, have half-saturation constants an order of magnitude smaller than the concentrations in the coelomic fluid and blood of the host (Table 1). Therefore, symbiont carbon fixation rates will be high even if the pool of carbon dioxide at the symbiont cell surface is substantially depleted with respect to the pool measured in the blood. The smaller values of K( 1/2 ) for carbon dioxide measured in R. piscesae symbionts, compared to R. pachyptila symbionts, mirror the generally lower internal inorganic carbon concentrations in R. piscesae (Table 1). As symbionts from these two hosts are strains of the same species (K. Nelson, pers.com.), this change in K( 1/2 ) may be either a response to host species, genetic differences between the two strains, or carbon dioxide availability. Thiomicrospira crunogena grown with low concentrations of inorganic carbon have the smallest half-saturation constants for carbon dioxide measured here: Given the large changes of affinity for inorganic carbon in response to inorganic carbon concentration during growth (Table 1), as well as the presence of carboxysomes in these cells, this high affinity may facilitate the growth of T. crunogena in an environment with greater variability in inorganic carbon abundance than that experienced by vestimentiferan symbionts. Although carbon dioxide concentrations can get quite high in the hydrothermal vent fluid bathing the R. pachyptila tubes from which T. crunogena was originally isolated (Table 1), this pool falls substantially during fluctuations in fluid flow that result in this microhabitat being dominated with alkaline, relatively carbon dioxide-poor bottom water. Under these circumstances, T. crunogena would require a versatility of inorganic carbon use that vestimentiferan symbionts, buffered from high-frequency environmental variability by their host, may not.