SUMMARYThe 13C/12C ratio (expressed as δ13C) of benrhic photolithotrophs. in the Dighn Water (= Burn) were measured fur comparison with that of the potential inorganic carhun sources. CO2 and HCO3‐, in the Burn. The Burn water contains an average of 65.7 mmol m‐3 CO2 with δ13C of ‐14.7% and 1600 mmol m‐3 HCO3‐ with δ13C of ‐4.%. δ13C values of riparian vegetation were also measured as contributors, after respiration in the soil or the Burn, to the δ13C of inorganic carbon in the Burn. The potential range of differences in 13C/12C between dissolved CO2 and plant organic C is set by the intrinsic 13c/12C discrimination (α value) in CO2 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 12CO2 fixation relative to that for 13CO2 fixation by Rubisco in the absence of CO2 transport limitation) for the‘lower plants’in the Burn (diatoms, green and red algae, mosses) are significantly different from the well‐established αp 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 δ13C values which can only be interpreted in terms of diffusive CO2 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 HCO3‐. Their δ13C values indicate that, if the HCO3‐ ‐use system does not (as is likely) discriminate significantly between 13C and 12C, then a substantial fraction of the inorganic C made available to Rubisco must return to the medium, carrying 13C‐inorganic C not fixed by Rubisco. (iv) Two sets of δ13C 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 HCO3 use. (vi) The entrusting red alga Hildenbrandia rivularis has a δ13C value suggestive of CO2 as the inorganic C source, but not entirely ruling nut HCO3‐. Marine species of both Hildenbrundia and Cladophora have δ13C values which, even when corrected for source inorganic C δ13C values, are 10%, more positive than the freshwater species. (vii) Mats of pennate diatoms were shown by pH‐drift to by able to use HCO3‐; the relatively high (i.e. not very negative) δ12C value of these mats could relate to a relatively‘non‐leaky’HCO3‐ aequisition mechanism and/or to limitation by external diffusion (e.g. through the mat).

RAVEN, J. A., JOHNSTON, A. M., NEWMAN, J. R., & SCRIMGEOUR, C. M. (1994). Inorganic carbon acquisition by aquatic photolithoatrophs of the Dighty Burn, Angus, U.K.: uses and limitations of natural abundance measurements of carbon isotopes. New Phytologist, 127(2), 271–286. Portico.