Tracing carbon assimilation in endosymbiotic deep-sea hydrothermal vent Mytilid fatty acids by ¹³C-fingerprinting

Abstract. Bathymodiolus azoricus mussels thrive at Mid-Atlantic Ridge hydrothermal vents, where part of their energy requirements are met via an endosymbiotic association with chemolithotrophic and methanotrophic bacteria. In an effort to describe phenotypic characteristics of the two bacterial endosymbionts and to assess their ability to assimilate CO₂, CH₄ and multi-carbon compounds, we performed experiments in aquaria using ¹³C-labeled NaHCO₃ (in the presence of H₂S), CH₄ or amino-acids and traced the incorporation of ¹³C into total and phospholipid fatty acids (tFA and PLFA, respectively). 14:0; 15:0; 16:0; 16:1(n − 7)c+t; 18:1(n − 13)c+t and (n − 7)c+t; 20:1(n − 7); 20:2(n − 9,15); 18:3(n − 7) and (n − 5,10,13) PLFA were labeled in the presence of H¹³CO₃⁻ (+H₂S) and ¹³CH₄, while the 12:0 compound became labeled only in the presence of H¹³CO₃⁻ (+H₂S). In contrast, the 17:0; 18:0; 16:1(n − 9); 16:1(n − 8) and (n − 6); 18:1(n − 8); and 18:2(n − 7) PLFA were only labeled in the presence of ¹³CH₄. Some of these symbiont-specific fatty acids also appeared to be labeled in mussel gill tFA when incubated with ¹³C-enriched amino acids, and so were mussel-specific fatty acids such as 22:2(n − 7,15). Our results provide experimental evidence for the potential of specific fatty acid markers to distinguish between the two endosymbiotic bacteria, shedding new light on C₁ and multi-carbon compound metabolic pathways in B. azoricus and its symbionts.