Isotopic biosignatures in carbonate‐rich, cyanobacteria‐dominated microbial mats of the Cariboo Plateau, B.C.

Abstract

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 δ(13) CDIC values up to +6.0‰ above predicted carbon dioxide (CO2 ) equilibrium values, representing a biosignature of photosynthesis. Mat-associated δ(13) Ccarb 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 δ(13) C values reflected the balance between photosynthetic (13) C-enrichment and heterotrophic inputs of (13) C-depleted DIC. Mat microelectrode profiles identified oxic zones where δ(13) Ccarb was within 0.2‰ of surface DIC overlying anoxic zones associated with sulphate reduction where δ(13) Ccarb was depleted by up to 5‰ relative to surface DIC reflecting inputs of (13) C-depleted DIC. δ(13) C 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 δ(13) Corg values ranged from -18.7 ± 0.1 to -25.3 ± 1.0‰ with mean Δ(13) Cinorg-org values ranging from 21.1 to 24.2‰, consistent with non-CO2 -limited photosynthesis, suggesting that Precambrian δ(13) Corg 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.

Robust, high-productivity phototrophic carbon capture at high pH and alkalinity using natural microbial communities

“…Based on previous data [47, 48], benthic microbial mats from four soda lakes located on the Cariboo Plateau, British Columbia were chosen: Last Chance Lake (LCL-M), Probe Lake (PL-M), Deer Lake (DL-M) and Goodenough Lake (GEL-M). Mats were collected away from the shore (PL-M, DL-M and GEL-M) in sterile screw cap tubes.…”

Terrestrial microbialites provide constraints on the mesoproterozoic atmosphere

“…Modern Mats, Alkaline Pool, British Columbia, Brady et al, 2013Shark Bay, Pages et al, 2014Orakai Hot Springs, DesMarais, 1992Laws et al, 1995McCabe, 1985 Modern Microbial Mats Modern Single-Celled Aquatic Organisms area ratio (V/S) (Popp et al, 1989;Laws et al, 1995;Dupraz et al, 2009) (Figure 4). In a well-mixed water body, the δ 13 C org of individual aquatic photosynthesizers reflects isotopic discrimination associated with diffusion of CO 2 to the site of C fixation and the dissolved CO 2 is proportional to atmospheric pCO 2 .…”

“…Under conditions of low CO 2aq and pCO 2atm , δ 13 C of organic biomass shows significantly reduced carbon isotope discrimination, while under high CO 2 availability, carbon isotope discrimination approaches a theoretical maximum (Popp et al, 1989). Data from modern microbial mat carbonates formed in marine hypersaline, freshwater alkaline and hydrothermal systems were used to calculate carbonate isotope discrimination () as a function of variable [DIC] using published δ 13 C carbonate , δ 13 C mat and DIC data (Des Marais et al, 1992;Brady et al, 2013;Pagès et al, 2014). The value is calculated after Laws et al (1995), where ε = (1,000 (δ 13 C org −δ 13 C aq )/ (1,000 + δ 13 C org )).…”

“…For clarification, the δ 13 C of organic matter was compared relative to δ 13 C aq (not δ 13 C DIC ) to quantify ε. Figure 4 shows measured ε versus CO 2aq for modern microbial mats in alkaline pools in British Columbia (Brady et al, 2013), Orakei Hot Springs (Des Marais et al, 1992 and Shark Bay (Pagès et al, 2014), as well as data for free-floating single cell growth experiments (McCabe, 1985;Laws et al, 1995), and the pCO 2 that would be calculated for an atmosphere in equilibrium with the dissolved CO 2aq . Figure 4 shows measured ε versus CO 2aq for modern microbial mats in alkaline pools in British Columbia (Brady et al, 2013), Orakei Hot Springs (Des Marais et al, 1992 and Shark Bay (Pagès et al, 2014), as well as data for free-floating single cell growth experiments (McCabe, 1985;Laws et al, 1995), and the pCO 2 that would be calculated for an atmosphere in equilibrium with the dissolved CO 2aq .…”

“…The δ 13 C gas was calculated from the isotopic composition of associated carbonates using the empirical δ 13 C carb /δ 13 C aq fractionation factor determined by Deines et al (1974). Using this approach, the calculated atmospheric pCO 2 for waters associated with carbonate-bearing microbial mats in British Columbia (Brady et al, 2013) ranges from 93 to 413 ppm with a mean of 208 ppm. These data provide boundary conditions for examining the isotopic values recorded by CHC mat carbonate and organic matter.…”