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

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 13 C-labeled NaHCO3 (in the presence of H2 S), CH4 or amino-acids and traced the incorporation of 13 C 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 H13 CO3- (+H2 S) and 13 CH4 , while the 12:0 compound became labeled only in the presence of H13 CO3- (+H2 S). 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 13 CH4 . Some of these symbiont-specific fatty acids also appeared to be labeled in mussel gill tFA when incubated with 13 C-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.