Environmental and Biological Influences on Carbonate Precipitation Within Hot Spring Microbial Mats in Little Hot Creek, CA.

Dylan T Wilmeth,Megan L Dillon,William M Berelson,Blake W Stamps,John R Spear,Olivia Paradis,Bradley S Stevenson,Sharon L Grim,Hope A Johnson,Heather S Nunn,Frank A Corsetti

doi:10.3389/fmicb.2018.01464

Abstract

Microbial mats are found in a variety of modern environments, with evidence for their presence as old as the Archean. There is much debate about the rates and conditions of processes that eventually lithify and preserve mats as microbialites. Here, we apply novel tracer experiments to quantify both mat biomass addition and the formation of CaCO3. Microbial mats from Little Hot Creek (LHC), California, contain calcium carbonate that formed within multiple mat layers, and thus constitute a good test case to investigate the relationship between the rate of microbial mat growth and carbonate precipitation. The laminated LHC mats were divided into four layers via color and fabric, and waters within and above the mat were collected to determine their carbonate saturation states. Samples of the microbial mat were also collected for 16S rRNA analysis of microbial communities in each layer. Rates of carbonate precipitation and carbon fixation were measured in the laboratory by incubating homogenized samples from each mat layer with δ13C-labeled HCO3- for 24 h. Comparing these rates with those from experimental controls, poisoned with NaN3 and HgCl2, allowed for differences in biogenic and abiogenic precipitation to be determined. Carbon fixation rates were highest in the top layer of the mat (0.17% new organic carbon/day), which also contained the most phototrophs. Isotope-labeled carbonate was precipitated in all four layers of living and poisoned mat samples. In the top layer, the precipitation rate in living mat samples was negligible although abiotic precipitation occurred. In contrast, the bottom three layers exhibited biologically enhanced carbonate precipitation. The lack of correlation between rates of carbon fixation and biogenic carbonate precipitation suggests that processes other than autotrophy may play more significant roles in the preservation of mats as microbialites.

Highlights

Microbial mats have been preserved within the rock record over 3.5 billion years (Walter et al, 1980; Noffke et al, 2006; Schopf, 2006; Bosak et al, 2009)
This study examines the effects of microbial metabolism and environmental factors on nascent carbonate precipitation within microbial mats from Little Hot Creek (LHC), California by focusing on the rate of carbon fixation into biomass and concomitant carbonate precipitation
The LHC mat sampled for biomass and precipitation experiments was a surficial mat ∼30 m from source waters, extending 50 cm from the shoreline into creek flow (Figure 1D)

Summary

Introduction

Microbial mats have been preserved within the rock record over 3.5 billion years (Walter et al, 1980; Noffke et al, 2006; Schopf, 2006; Bosak et al, 2009). The metabolic activity within microbial mats and the physicochemical changes in the surrounding environment can influence mineral saturation states and induce mineral precipitation (Grotzinger and Knoll, 1999). Mineralization within mats that is independent of living microbial activity is defined as abiogenic precipitation (see “biologically influenced precipitation” in Frankel and Bazylinski, 2003; Dupraz et al, 2009). Three factors can influence the precipitation of calcium carbonate within microbial mats, whether biogenic or abiogenic in nature: the concentrations of CO32− ions, Ca2+ ions, and surface chemistry or nucleation centers (Dupraz et al, 2009). The third factor relates to the potential for locations within microbial mats to serve as nuclei for carbonate minerals to form (Arp et al, 1999a; Dupraz et al, 2009)

Methods

Results

Conclusion