CO ₂ Uptake and Fixation by Endosymbiotic Chemoautotrophs from the Bivalve Solemya velum

Abstract

Chemoautotrophic symbioses, in which endosymbiotic bacteria are the major source of organic carbon for the host, are found in marine habitats where sulfide and oxygen coexist. The purpose of this study was to determine the influence of pH, alternate sulfur sources, and electron acceptors on carbon fixation and to investigate which form(s) of inorganic carbon is taken up and fixed by the gamma-proteobacterial endosymbionts of the protobranch bivalve Solemya velum. Symbiont-enriched suspensions were generated by homogenization of S. velum gills, followed by velocity centrifugation to pellet the symbiont cells. Carbon fixation was measured by incubating the cells with 14 C-labeled dissolved inorganic carbon. When oxygen was present, both sulfide and thiosulfate stimulated carbon fixation; however, elevated levels of either sulfide (>0.5 mM) or oxygen (1 mM) were inhibitory. In the absence of oxygen, nitrate did not enhance carbon fixation rates when sulfide was present. Symbionts fixed carbon most rapidly between pH 7.5 and 8.5. Under optimal pH, sulfide, and oxygen conditions, symbiont carbon fixation rates correlated with the concentrations of extracellular CO 2 and not with HCO 3 ؊ concentrations. The half-saturation constant for carbon fixation with respect to extracellular dissolved CO 2 was 28 ؎ 3 M, and the average maximal velocity was 50.8 ؎ 7.1 mol min ؊1 g of protein ؊1 . The reliance of S. velum symbionts on extracellular CO 2 is consistent with their intracellular lifestyle, since HCO 3 ؊ utilization would require protein-mediated transport across the bacteriocyte membrane, perisymbiont vacuole membrane, and symbiont outer and inner membranes. The use of CO 2 may be a general trait shared with many symbioses with an intracellular chemoautotrophic partner.

Impact of dissolved inorganic carbon concentrations and pH on growth of the chemolithoautotrophic epsilonproteobacterium Sulfurimonas gotlandica GD1T

“…GD1 was isolated from a relatively stable habitat. Scott and Cavanaugh (2007) confirm this conclusion with their studies about chemoautotrophic c-proteobacteria, living as endosymbionts in sulfidic/oxic interfaces. These Solemya velum symbionts have also a relatively narrow range of pH optimum (between pH 7.4 and 8.5), showing the same sharp decline in growth directly below and above these levels.…”

“…However, in the deeper anoxic zones of the central Baltic basins the DIC concentration is already around 2 mmol L À1 and the pH is 7.1 (Beldowski et al 2010;Schneider 2011) and acidification impacts are therefore assumed to be relatively small in the environment. Only few studies have examined the impact of DIC and pH on growth of chemolithoautotrophic bacteria, mostly with focus on carbon concentrating mechanisms, and using isolates derived from hydrothermal vent habitats (e.g., Dobrinski et al 2005;Scott and Cavanaugh 2007). Therefore, in this study we first investigated whether different DIC concentrations and pH values have an influence on growth of S. gotlandica GD1 T .…”

Inorganic carbon fixation by chemosynthetic ectosymbionts and nutritional transfers to the hydrothermal vent host-shrimp Rimicaris exoculata

“…Accordingly, Zbinden et al (2008) observed o30% degraded epibionts in re-pressurised shrimps. The carbon fixation rates determined here are much lower, however, than those obtained for other chemosynthetic symbionts (bacterial homogenates or endobacteria of bivalve gills), for example, those of Bathymodiolus thermophilus: 150 mmolC inc  gC sam À 1  h À 1 in SW (Nelson et al, 1995), or Solemya velum: 2500 mmolC inc  gC sam À 1  h À 1 in the presence of thiosulphate (Scott and Cavanaugh, 2007). Moreover, owing to the relatively low energy level of thiosulphate as compared with sulphide, the latter authors observed threefold higher carbon fixation rates with 0.2 mM Na 2 S (70 mmolC  g prot À 1  min À 1 ) than with 1 mM Na 2 S 2 O 3 (20 mmolC  g prot À 1  min À 1 ).…”

“…They thus strengthen the view that the relationship between bacteria and shrimp is mutualistic: the epibionts supply carbon compounds to the shrimp and the shrimp, thanks to its gill chamber flow and its swimming behaviour, offers them protection and a supply of chemical compounds, maintaining their position at the oxic/anoxic interface around active chimneys Segonzac et al, 1993). The R. exoculata bacterial epibiosis can thus be regarded as a true mutualistic trophic ectosymbiosis, similar to what is known about the chemosynthetic endosymbiosis of organisms such as R. pachyptila (Felbeck, 1981;Fisher et al, 1989), Calyptogena magnifica (Childress et al, 1991) and S. velum (Stewart and Cavanaugh, 2006;Scott and Cavanaugh, 2007). Finally, the view that bacterial products are assimilated across the shrimp integument rather than via the DT is strongly supported by: (1) the significant incorporation of both 14 C-acetate and 3 Hlysine after an incubation time as short as 1 h. This is insufficient for uptake by ingestion assimilation, which commonly takes several hours (ingestion assimilation of carbon from bicarbonate, for example, requires prior incorporation by the bacteria, followed by grazing of bacteria from the MP) (Chipps, 1998;Hoyt et al, 2000); (2) the high incorporation levels recorded in the gill chamber integument lining (OB, Gi), as opposed to (3) the much lower levels recorded in the DT (Figure 6).…”

Influence of increasing dissolved inorganic carbon concentrations and decreasing pH on chemolithoautrophic bacteria from oxic-sulfidic interfaces

“…Although the impact of ocean acidification on microbial communities has been examined in several studies, mainly with respect to the response to elevated pCO 2 , there occur direct and indirect effects simultaneously and we do not have a Figures consistent picture yet (see review by Liu et al, 2010). Only few studies have examined the impact of pH and DIC on growth of chemolithoautotrophic bacteria, mostly with focus on carbon concentrating mechanisms, and using isolates derived from hydrothermal vent habitats (Scott and Cavanaugh, 2007;Dobrinski et al, 2005). However, the effects of DIC and pH were generally not investigated separately, and representative organisms of pelagic anoxic zones were not available until now.…”