Shifts in methanogen community structure and function associated with long‐term manipulation of sulfate and salinity in a hypersaline microbial mat

Abstract

Methanogenesis was characterized in hypersaline microbial mats from Guerrero Negro, Baja California Sur, Mexico both in situ and after long-term manipulation in a greenhouse environment. Substrate addition experiments indicate methanogenesis to occur primarily through the catabolic demethylation of non-competitive substrates, under field conditions. However, evidence for the coexistence of other metabolic guilds of methanogens was obtained during a previous manipulation of sulfate concentrations. To fully characterize methanogenesis in these mats, in the absence of competition for reducing equivalents with sulfate-reducing microorganisms, we maintained microbial mats for longer than 1 year under conditions of lowered sulfate and salinity levels. The goal of this study was to assess whether observed differences in methane production during sulfate and salinity manipulation were accompanied by shifts in the composition of methanogen communities. Culture-independent techniques targeting methyl coenzyme M reductase genes (mcrA) were used to assess the dynamics of methanogen assemblages. Clone libraries from mats sampled in situ or maintained at field-like conditions in the greenhouse were exclusively composed of sequences related to methylotrophic members of the Methanosarcinales. Increases in pore water methane concentrations under conditions of low sulfate correlated with an observed increase in the abundance of putatively hydrogenotrophic mcrA, related to Methanomicrobiales. Geochemical and molecular data provide evidence of a significant shift in the metabolic pathway of methanogenesis from a methylotroph-dominated system in high-sulfate environments to a mixed community of methylotrophic and hydrogenotrophic methanogens under low sulfate conditions.

Diversity and Stratification of Archaea in a Hypersaline Microbial Mat

“…Recognizable methanogens represent only a small proportion of the archaea detected, but it is possible that some of the GN archaea with no characterized close relatives also conduct methanogenesis. However, the relative rarity of methanogens as observed by sequences in the mat is consistent with data from studies that showed that the pond 4 mat produces only small amounts of methane under field conditions (3,47). The depth distribution of some of the archaeal phylogenetic clades tends to be narrowly defined despite the fact that these same clades have relatively large ICDs, which might predict broad biochemical potential and thereby wider distribution in the mat.…”

“…More than a million metric tons of biomass (ϳ17 km 2 by ϳ6 cm by 1.2 gm/cm 3 ) covers the floor of hypersaline pond 4 in the form of a laminated photosynthetic microbial mat that is 4 to 6 cm thick. The subjective, macroscopic appearance of the pond 4 mat is stable from year to year, and the mat has been a subject of numerous microbiological and biogeochemical studies (3,8,15,19,24,29,30,38,47,49,56). Chemical measurements of the mat (8,29) show that the top 2 to 3 mm forms an oxic zone in daylight due to oxygenic photosynthesis.…”

“…The pond 4 mat is distinct from the crystallizer environment, covered by brine ϳ1 m deep with only ϳ80‰ salinity, Ͻϳ3ϫ seawater. Studies of archaea in the pond 4 mat have usually been related to methanogenesis, which is generally considered a minor metabolic theme of the mat (3,47). Consequently, the aims of this study were to describe the archaeal diversity in the pond 4 mat and to determine how that diversity is distributed with respect to depth in the mat and previously measured chemical gradients (16,28,29).…”

Trimethylamine and Organic Matter Additions Reverse Substrate Limitation Effects on the δ 13 C Values of Methane Produced in Hypersaline Microbial Mats

“…If competition for any glycine betaine in the medium occurred, then this may have lessened the amount of TMA (derived from glycine betaine) in the environment, reducing the amount of methane produced from TMA and allowing for the grow-in of CO 2 reducing methanogens. Hydrogenotrophic (CO 2 reducing) methanogens are either not active or active to a small extent in the Guerrero Negro ponds and yet will increase in number when conditions become favorable (34,35). At the Atacama sites, preliminary data from both Salar de Llamara and El Tatio suggest that the most abundant methanogens are members of the orders Methanomicrobiales and Methanobacteriales (B. Valenzuela et al, unpublished data).…”

A salinity and sulfate manipulation of hypersaline microbial mats reveals stasis in the cyanobacterial community structure

“…Over the course of a year, salinity and sulfate concentrations were lowered significantly. In these experimental mat ecosystems, the effects of these manipulations on carbon cycling, including methanogenesis, have been previously explored (Kelley et al, 2006;Smith et al, 2008). We hypothesized that the experimental manipulations of salinity and sulfate concentrations would also significantly impact the cyanobacterial community structure.…”