Inorganic carbon acquisition by aquatic photolithoatrophs of the Dighty Burn, Angus, U.K.: uses and limitations of natural abundance measurements of carbon isotopes

SUMMARY The 13 C/ 12 C ratio (expressed as δ 13 C) of benrhic photolithotrophs. in the Dighn Water (= Burn) were measured fur comparison with that of the potential inorganic carhun sources. CO 2 and HCO 3 ‐ , in the Burn. The Burn water contains an average of 65.7 mmol m ‐3 CO 2 with δ 13 C of ‐14.7% and 1600 mmol m ‐3 HCO 3 ‐ with δ 13 C of ‐4.%. δ 13 C values of riparian vegetation were also measured as contributors, after respiration in the soil or the Burn, to the δ 13 C of inorganic carbon in the Burn. The potential range of differences in 13 C/1 2C between dissolved CO 2 and plant organic C is set by the intrinsic 13 c/ 12 C discrimination (α value) in CO 2 fixation by Rubisco. Main results and conclusions are. as follows, (i) A literature survey suggests that there is no convincing evidence that the α, Values (rate constant for 12 CO 2 fixation relative to that for 13 CO 2 fixation by Rubisco in the absence of CO 2 transport limitation) for the‘lower plants’in the Burn (diatoms, green and red algae, mosses) are significantly different from the well‐established α p values for the flowering plum enzyme. (ii) In confirmation of earlier work, the semi‐erect 'streamer’gametophytes of the red alga Lemanea mamillosa and the moss Fontinalis antipyetica have δ 13 C values which can only be interpreted in terms of diffusive CO 2 entry with minimal limitation of photosynthesis by CO‐ diffusion, (iii) The serui‐erect grren alga Cladophora glomerata and the flowering plant Ranunculus penicillatus ssp. pseudofluitons (formerly var. calcareus ) are‐ both able to use HCO 3 ‐ . Their δ 13 C values indicate that, if the HCO 3 ‐ ‐use system does not (as is likely) discriminate significantly between 13 C and 12 C, then a substantial fraction of the inorganic C made available to Rubisco must return to the medium, carrying 13 C‐inorganic C not fixed by Rubisco. (iv) Two sets of δ 13 C data from different hydrodynamic regimes distance from leading edge of a flat stone; different size of thalli) show that the attainable differences in situ in thickness of the diffusion boundary layer do not alter the fractional limitation of photosynthesis of Cladophora by external diffusion of inorganic C, considered with HCO 3 use. (vi) The entrusting red alga Hildenbrandia rivularis has a δ 13 C value suggestive of CO 2 as the inorganic C source, but not entirely ruling nut HCO 3 ‐ . Marine species of both Hildenbrundia and Cladophora have δ 13 C values which, even when corrected for source inorganic C δ 13 C values, are 10%, more positive than the freshwater species. (vii) Mats of pennate diatoms were shown by pH‐drift to by able to use HCO 3 ‐ ; the relatively high (i.e. not very negative) δ 12 C value of these mats could relate to a relatively‘non‐leaky’HCO 3 ‐ aequisition mechanism and/or to limitation by external diffusion (e.g. through the mat).