Biosignatures Associated with Freshwater Microbialites

Abstract

Freshwater microbialites (i.e., lithifying microbial mats) are quite rare in northern latitudes of the North American continent, with two lakes (Pavilion and Kelly Lakes) of southeastern BC containing a morphological variety of such structures. We investigated Kelly Lake microbialites using carbon isotope systematics, phospholipid fatty acids (PLFAs) and quantitative PCR to obtain biosignatures associated with microbial metabolism. δ¹³C_DIC values (mean δ¹³C_DIC -4.9 ± 1.1‱, n = 8) were not in isotopic equilibrium with the atmosphere; however, they do indicate ¹³C-depleted inorganic carbon into Kelly Lake. The values of carbonates on microbialite surfaces (δ¹³C) fell within the range predicted for equilibrium precipitation from ambient lake water δ¹³C_DIC (-2.2 to -5.3‱). Deep microbialites (26 m) had an enriched δ¹³C_carb value of -0.3 ± 0.5‱, which is a signature of photoautotrophy. The deeper microbialites (>20 m) had higher biomass estimates (via PLFAs), and a greater relative abundance of cyanobacteria (measured by 16S copies via qPCR). The majority of PLFAs constituted monounsaturated and saturated PLFAs, which is consistent with gram-negative bacteria, including cyanobacteria. The central PLFA δ¹³C values were highly depleted (-9.3 to -15.7‱) relative to δ¹³C values of bulk organic matter, suggesting a predominance of photoautotrophy. A heterotrophic signature was also detected via the depleted iso- and anteiso-15:0 lipids (-3.2 to -5.2‱). Based on our carbonate isotopic biosignatures, PLFA, and qPCR measurements, photoautotrophy is enriched in the microbialites of Kelly Lake. This photoautotrophy enrichment is consistent with the microbialites of neighboring Pavilion Lake. This indication of photoautotrophy within Kelly Lake at its deepest depths raises new insights into the limits of measurable carbonate isotopic biosignatures under light and nutrient limitations.

Keywords: Kelly Lake; Pavilion Lake; biosignatures; microbialites.

Figure 1

The location of Kelly Lake and the transect along which microbialites were collected. The wells and Porcupine Creek from which water samples were taken are also shown. The four representative Kelly Lake microbialites samples from depths (11, 14, 20, 26 m) are listed on the right with the red scale bar (10 cm).

Figure 2

(a) Temperature; (b) Specific conductance; and (c) photosynthetically active radiation (PAR) profiles (logarithmic scale) of Kelly Lake with depth. All profiles were measured during the first week of August, except 2008, which was measured in the first week of July. Only PAR profiles for 2006 and 2007 are plotted in panel (c).

Figure 3

(a) Concentrations of PLFAs over microbialite depths (one-way ANOVA, p < 0.01); (b) Weight % TOC of microbialite surface material with depth (one-way ANOVA, p < 0.05). * Indicates significant differences between depths.

Figure 4

The mol % of each structurally defined PLFA class as a function of depth.

Figure 5

The δ¹³C offset of PLFAs from one representative sample at each depth relative to the δ¹³C value of the bulk organic carbon, set here as 0. Error bars represent one standard deviation from the mean based on triplicate analysis of the same sample. Most PLFA, except for i-15:0 and a-15:0, fall within or below the photoautotrophic synthesis range.

Figure 6

(a) Total bacterial 16S copy number per gram (dry weight) of microbialite shows no significant change with depth (one-way ANOVA); (b) The percent of cyanobacterial 16S copy number at each depth relative to 20 m. * Indicates a significant difference between 20 m and all other depths (one-way ANOVA, p < 0.05 for all).

Figure 7

Confocal images of microbialite surfaces in cross section from samples collected at (A) 11 m; (B) 14 m; (C) 20 m; (D) 26 m. The fluorescent overlay indicates the presence of cyanobacteria within voids located in the first few millimeters of carbonate minerals. Arrows indicate the orientation of microbialite growth. Note the greater fluorescence and therefore higher abundance of cyanobacteria at 20 m.

Figure 8

SEM back-scattered electron micrographs of microbialites from 20 m in cross section. (A) Coccoid cyanobacterial microcolony located less than 1 mm below the microbialite surface. (B) Void spaces within the carbonate are similar in shape and size to extant coccoid and filamentous species. (C) Cells encased in carbonate minerals (circled in white) suggest that in situ precipitation occurred around individual cells. (D) Secondary infilling of void spaces with carbonate minerals suggests active precipitation.

Figure 9

δ¹³C values of surface carbonates collected from microbialites found at depths of 11, 14, 20, and 26 m. The shaded area represents the predicted range of δ¹³C_carb values which would result from equilibrium precipitation, based on the most enriched and depleted surface δ¹³C_DIC values found in any of the summer sampling years. The solid line represents the mean predicted δ¹³C_carbonate value determined from surface water δ¹³C_DIC values.