TY - JOUR
T1 - Carbon isotope fractionation by photosynthetic aquatic microorganisms
T2 - Experiments with Synechococcus PCC7942, and a simple carbon flux model
AU - Erez, Jonathan
AU - Bouevitch, Anne
AU - Kaplan, Aaron
PY - 1998/6
Y1 - 1998/6
N2 - Stable carbon isotopes (12C and 13C) are widely used to trace biogeochemical processes in the global carbon cycle. Natural fractionation of carbon isotopes is mainly due to the discrimination of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) against 13C during photosynthesis. In marine and other aquatic microorganisms, this fractionation is lowered when the dissolved CO2 (CO(2(aq))) is decreasing, but the underlying mechanisms are poorly understood. Cultured Synechococcus PCC7942 showed maximum isotopic fractionations of -33‰ (in δ13C units) relative to the total inorganic carbon (C(i)) when CO(2(aq)) is above 30 μM. As the culture grew, pH increased, CO(2(aq)) was lower than 1 μM, and the C(i) concentrating mechanism was induced although the C(i) was above 3 mM. The isotopic fractionation was drastically reduced to values of -1 to -3 ‰ relative to C(i). A simple carbon isotope flux model suggests that during the first stages of the experiment the total uptake (F1) was roughly three- to four-fold greater than the photosynthetic net accumulation (F2). When the C(i) concentrating mechanism was induced, the leakage of CO2 from the cells declined, the cells started to utilize HCO3- and the F1/F2 ratio decreased to values close to 1. Based on this model the isotopic variability of oceanic phytoplankton suggests that the F1/F2 ratio may be above 3 in high latitudes and ~1.1 in equatorial waters, where the C(i) concentrating mechanism is probably induced. Attempts to reconstruct past atmospheric CO2 levels and paleoproductivity should take into account the effects of the C(i) concentrating mechanism on the isotopic fractionation of aquatic primary producers.
AB - Stable carbon isotopes (12C and 13C) are widely used to trace biogeochemical processes in the global carbon cycle. Natural fractionation of carbon isotopes is mainly due to the discrimination of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) against 13C during photosynthesis. In marine and other aquatic microorganisms, this fractionation is lowered when the dissolved CO2 (CO(2(aq))) is decreasing, but the underlying mechanisms are poorly understood. Cultured Synechococcus PCC7942 showed maximum isotopic fractionations of -33‰ (in δ13C units) relative to the total inorganic carbon (C(i)) when CO(2(aq)) is above 30 μM. As the culture grew, pH increased, CO(2(aq)) was lower than 1 μM, and the C(i) concentrating mechanism was induced although the C(i) was above 3 mM. The isotopic fractionation was drastically reduced to values of -1 to -3 ‰ relative to C(i). A simple carbon isotope flux model suggests that during the first stages of the experiment the total uptake (F1) was roughly three- to four-fold greater than the photosynthetic net accumulation (F2). When the C(i) concentrating mechanism was induced, the leakage of CO2 from the cells declined, the cells started to utilize HCO3- and the F1/F2 ratio decreased to values close to 1. Based on this model the isotopic variability of oceanic phytoplankton suggests that the F1/F2 ratio may be above 3 in high latitudes and ~1.1 in equatorial waters, where the C(i) concentrating mechanism is probably induced. Attempts to reconstruct past atmospheric CO2 levels and paleoproductivity should take into account the effects of the C(i) concentrating mechanism on the isotopic fractionation of aquatic primary producers.
KW - Carbon concentrating mechanism
KW - Carbon isotope fractionation
KW - CO
KW - Photosynthesis
UR - http://www.scopus.com/inward/record.url?scp=0032408569&partnerID=8YFLogxK
U2 - 10.1139/cjb-76-6-1109
DO - 10.1139/cjb-76-6-1109
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AN - SCOPUS:0032408569
SN - 0008-4026
VL - 76
SP - 1109
EP - 1118
JO - Canadian Journal of Botany
JF - Canadian Journal of Botany
IS - 6
ER -