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Effect of growth rate and CO 2 concentration on carbon isotopic fractionation by the marine diatom Phaeodactylum tricornutum
Author(s) -
Laws Edward A.,
Bidigare Robert R.,
Popp Brian N.
Publication year - 1997
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.4319/lo.1997.42.7.1552
Subject(s) - phaeodactylum tricornutum , bicarbonate , chemistry , carbon fixation , diatom , total inorganic carbon , fractionation , diffusion , carbon dioxide , environmental chemistry , photosynthesis , chromatography , botany , biochemistry , biology , thermodynamics , organic chemistry , physics
The carbon isotopic composition ( δ 13 C) of the marine diatom Phaeodactylum tricornutum was measured over a series of growth rates ( µ ) in a chemostat system in which both the δ 13 C and the concentration of aqueous CO 2 [CO 2 (aq)] were measured. CO 2 (aq) ranged from 0.64 to 35 µ mol kg −1 and growth rates from 0.5 to 1.4 d −1 . ε p , the biological fractionation factor associated with carbon fixation, was found to be a nonlinear function of µ /CO 2 (aq), contrary to the predictions of a model that assumes that CO 2 enters the cell by passive diffusion. The experimental results suggest that active uptake of bicarbonate does not account for the nonlinearity of the relationship and that inorganic carbon enters the cell as CO 2 . The data are very well described by theoretical model that assumes that P. tricornutum regulates the CO 2 concentration in its cytoplasm so as to minimize the energy required to concentrate CO 2 at the site of carboxylation. This is probably achieved by active uptake of CO 2 or by conversion of bicarbonate to CO 2 by an external carbonic anhydrase followed by transport of the CO 2 into the cell via either active transport or passive diffusion. Based on the model and data, µ /CO 2 (aq) = 0.225 × [(26.8 − ε p )/( ε p − 5.5)] kg d −1 µ mol −1 . This equation accounts for 92% of the variance in the µ /CO 2 (aq) data. The model has potential utility for estimating phytoplankton growth rates in field studies without incubations and has important implications for the estimation of ancient CO 2 (aq) from the δ 13 C of preserved organic compounds.