Localized electron transfer rates and microelectrode-based enrichment of microbial communities within a phototrophic microbial mat

Abstract

Phototrophic microbial mats frequently exhibit sharp, light-dependent redox gradients that regulate microbial respiration on specific electron acceptors as a function of depth. In this work, a benthic phototrophic microbial mat from Hot Lake, a hypersaline, epsomitic lake located near Oroville in north-central Washington, was used to develop a microscale electrochemical method to study local electron transfer processes within the mat. To characterize the physicochemical variables influencing electron transfer, we initially quantified redox potential, pH, and dissolved oxygen gradients by depth in the mat under photic and aphotic conditions. We further demonstrated that power output of a mat fuel cell was light-dependent. To study local electron transfer processes, we deployed a microscale electrode (microelectrode) with tip size ~20 μm. To enrich a subset of microorganisms capable of interacting with the microelectrode, we anodically polarized the microelectrode at depth in the mat. Subsequently, to characterize the microelectrode-associated community and compare it to the neighboring mat community, we performed amplicon sequencing of the V1-V3 region of the 16S gene. Differences in Bray-Curtis beta diversity, illustrated by large changes in relative abundance at the phylum level, suggested successful enrichment of specific mat community members on the microelectrode surface. The microelectrode-associated community exhibited substantially reduced alpha diversity and elevated relative abundances of Prosthecochloris, Loktanella, Catellibacterium, other unclassified members of Rhodobacteraceae, Thiomicrospira, and Limnobacter, compared with the community at an equivalent depth in the mat. Our results suggest that local electron transfer to an anodically polarized microelectrode selected for a specific microbial population, with substantially more abundance and diversity of sulfur-oxidizing phylotypes compared with the neighboring mat community.

Keywords: electron transfer; hot lake; microbial mats; microelectrodes; sequence analysis; sulfur cycle.

FIGURE 1

Photo of open channel reactors containing microbial mat samples used in microprofiling (left). Stereomicroscope image of excised mat slice on its side showing stratification (right).

FIGURE 2

Side view of laboratory mat incubation experiments. (A) The microelectrode tip was positioned at depth inside the mat. The microelectrode was continuously polarized anodically against a Ag/AgCl reference electrode. (B) The anode of the mat fuel cell was placed directly below the mat and the cathode was placed in the bulk solution. The electrons are transferred from anode to cathode via an external circuit.

FIGURE 3

Redox potential (A) and pH (B) depth profiles entering the upper strata of the explanted microbial mats at two time points during a diel cycle. Zero indicates the approximate location of the mat’s surface.

FIGURE 4

Dissolved oxygen depth profiles entering the upper strata of the explanted microbial mats at two time points during a diel cycle.

FIGURE 5

(A) Power generation by the mat fuel cell showing oscillations in tandem with the diel cycle where maximum power occurred near the end of each light cycle and reached a minimum at the end of the dark cycle. (B) An example charge/discharge cycle. The shorter charging time corresponds to higher power generation.

FIGURE 6

Increase in current at a microelectrode tip placed 3 mm inside mat and polarized at +400 mV_Ag/AgCl.

FIGURE 7

Distribution of phyla for the microbial mat sample (A) and microelectrode tip sample (B).

FIGURE 8

Proposed pathway of bioelectrochemical cycling of sulfur species at the microelectrode tip.