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A δ 13 C‐based carbon flux model for the hydrothermal vent chemoautotrophic symbiosis Riftia pachyptila predicts sizeable CO 2 gradients at the host–symbiont interface
Author(s) -
Scott Kathleen M.
Publication year - 2003
Publication title -
environmental microbiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.954
H-Index - 188
eISSN - 1462-2920
pISSN - 1462-2912
DOI - 10.1046/j.1462-2920.2003.00416.x
Subject(s) - biology , hydrothermal vent , carbon fixation , symbiosis , autotroph , biomass (ecology) , hydrothermal circulation , carbon fibers , ecology , paleontology , carbon dioxide , bacteria , materials science , composite number , composite material
Summary The chemoautotrophic symbiosis Riftia pachyptila has extremely 13 C‐enriched δ 13 C values. Neither isotopic discrimination by the RubisCO enzyme of their bacterial endosymbionts, nor the δ 13 C value of CO 2 at their hydrothermal vent habitat, suffice to explain biomass δ 13 C values in this organism, which range from − 9 to − 16‰. However, these 13 C‐enriched δ 13 C values are consistent with the presence of 13 C‐enriched CO 2 within the symbiont cytoplasm. Such a 13 C‐enriched pool of CO 2 is expected when the rate of CO 2 fixation by RubisCO, which fixes 12 CO 2 more rapidly than 13 CO 2 , approaches the rate of exchange between intracellular and extracellular CO 2 pools. Rapid CO 2 fixation rates will also generate concentration gradients between these two pools. In order to estimate the size of these concentration gradients, an equation was derived, which describes the δ 13 C of tubeworm biomass in terms of the size of the CO 2 gradient between the hydrothermal vent environment and the symbiont cytoplasm. Using mass balance equations for CO 2 exchange and fixation by the symbionts and the tubeworm host, this model predicts that a CO 2 concentration gradient of up to 17‐fold between the symbiont cytoplasm and the environment is sufficient to explain even the most 13 C‐enriched R. pachyptila biomass. This model illustrates how both physical and enzymatic factors can act to influence the δ 13 C of intracellular CO 2 , which, in turn, highlights the danger of assigning a carbon fixation pathway to an autotroph based solely on its biomass δ 13 C value.