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Photosynthetic activity in carbonate-rich benthic microbial mats located in saline, alkaline lakes on the Cariboo Plateau, B.C. resulted in pCO2 below equilibrium and δ13CDIC values up to +6.0‰ above predicted carbon dioxide (CO2) equilibrium values, representing a biosignature of photosynthesis. Mat-associated δ13Ccarb values ranged from ˜4 to 8‰ within any individual lake, with observations of both enrichments (up to 3.8‰) and depletions (up to 11.6‰) relative to the concurrent dissolved inorganic carbon (DIC). Seasonal and annual variations in δ13C values reflected the balance between photosynthetic 13C-enrichment and heterotrophic inputs of 13C-depleted DIC. Mat microelectrode profiles identified oxic zones where δ13Ccarb was within 0.2‰ of surface DIC overlying anoxic zones associated with sulphate reduction where δ13Ccarb was depleted by up to 5‰ relative to surface DIC reflecting inputs of 13C-depleted DIC. δ13C values of sulphate reducing bacteria biomarker phospholipid fatty acids (PLFA) were depleted relative to the bulk organic matter by ˜4‰, consistent with heterotrophic synthesis, while the majority of PLFA had larger offsets consistent with autotrophy. Mean δ13Corg values ranged from −18.7 ± 0.1 to −25.3 ± 1.0‰ with mean Δ13Cinorg-org values ranging from 21.1 to 24.2‰, consistent with non-CO2-limited photosynthesis, suggesting that Precambrian δ13Corg values of ˜−26‰ do not necessitate higher atmospheric CO2 concentrations. Rather, it is likely that the high DIC and carbonate content of these systems provide a non-limiting carbon source allowing for expression of large photosynthetic offsets, in contrast to the smaller offsets observed in saline, organic-rich and hot spring microbial mats.