31.2.5 Got Arsenic?
We first isolated two new species of halo-alkaliphilic bacilli from the lake’s bottom
sediments that proved able to grow via dissimilatory reduction of arsenate (Switzer
Blum et al. 1998). This whetted our appetites, so that by the following year, we
successfully designed and deployed a radiotracer experiment to measure in situ
arsenate respiration in Mono Lake’s anoxic bottom waters. We measured rates
ranging from 1–5 μmoles/L/d, which when integrated and calculated as a sink for
electrons wound up mineralizing 8–14% of annual primary production, with another
40% contributed by sulfate-reduction. This was a major revelation for us, because it
illustrated that respiratory arsenate reduction has quantitative significance as an
electron acceptor for carbon mineralization in an actual ecosystem and not just as
a curious microbiological sidebar (Oremland et al. 2000). Our experimentally
determined rates were borne out when independently calculated via lake-wide
mass balance considerations (Hollibaugh et al. 2005), thereby solidifying the importance of arsenic biogeochemistry in this peculiar ecosystem.
Since arsenate respiration was occurring in the lake’s anoxic water column, we set
out to try to cultivate some of the arsenotrophs from that location (Hoeft et al. 2002,
2004) including arsenite-oxidizers (Oremland et al. 2002). The result of these efforts,
largely accomplished by Shelley Hoeft McCaan, was the isolation of the facultatively chemoautotrophic, facultatively anaerobic arsenite-oxidizer, Alkalilimnicola
ehrlichii strain MLHE-1 (Hoeft et al. 2007), a very flexible γ-Proteobacterium.
Strain MLHE-1 is a bacterium that proved crucial to our uncovering further mysteries of arsenic metabolism based on the discovery of its “reverse” functioning
arsenate reductase (ArrAB) homolog, an anaerobic arsenite oxidase we designated
ArxAB in further collaborations with John Stolz at Duquesne University and Chad
Saltikov at University of California Santa Cruz (e.g., Richey et al. 2009; Zargar et al.
2010, 2012). Shelley also isolated the difficult-to-grow δ-Proteobacterium strain
MLMS-1which, we eventually realized, disdained organic substrates and used
sulfide as its electron donor to achieve growth via arsenate reduction. Strain
MLMS-1 showed little inclination in doing much of anything else of interest with
its time, except a perhaps a disproportionation of thioarsenate (Hoeft et al. 2004;
Planar-Friederich et al. 2015).
Curiosity drove Larry and me to once again explore the hot springs on Paoha
Island, but this time actually stepping ashore to see if there was anything of interest
growing within them. There clearly was, as these springs (~45
C) hosted
red-colored biofilms as coatings on the rocks and pebbles over which the spring
water flowed. The biofilms were thin (~1 mm) and could easily be removed by
gently scraping the surfaces with a soft toothbrush. We were able to obtain a lot of
sample in this manner, which we stored within anaerobic serum bottles held in a
refrigerator. We later discovered that they remained blood-red in color and still
proved viable even after several months of such dark storage. The hot spring waters
were anoxic and contained dissolved methane, sulfide, and arsenite in abundance, all
of which were possible electron donors that could support anoxygenic
676
R. S. Oremland
We first isolated two new species of halo-alkaliphilic bacilli from the lake’s bottom
sediments that proved able to grow via dissimilatory reduction of arsenate (Switzer
Blum et al. 1998). This whetted our appetites, so that by the following year, we
successfully designed and deployed a radiotracer experiment to measure in situ
arsenate respiration in Mono Lake’s anoxic bottom waters. We measured rates
ranging from 1–5 μmoles/L/d, which when integrated and calculated as a sink for
electrons wound up mineralizing 8–14% of annual primary production, with another
40% contributed by sulfate-reduction. This was a major revelation for us, because it
illustrated that respiratory arsenate reduction has quantitative significance as an
electron acceptor for carbon mineralization in an actual ecosystem and not just as
a curious microbiological sidebar (Oremland et al. 2000). Our experimentally
determined rates were borne out when independently calculated via lake-wide
mass balance considerations (Hollibaugh et al. 2005), thereby solidifying the importance of arsenic biogeochemistry in this peculiar ecosystem.
Since arsenate respiration was occurring in the lake’s anoxic water column, we set
out to try to cultivate some of the arsenotrophs from that location (Hoeft et al. 2002,
2004) including arsenite-oxidizers (Oremland et al. 2002). The result of these efforts,
largely accomplished by Shelley Hoeft McCaan, was the isolation of the facultatively chemoautotrophic, facultatively anaerobic arsenite-oxidizer, Alkalilimnicola
ehrlichii strain MLHE-1 (Hoeft et al. 2007), a very flexible γ-Proteobacterium.
Strain MLHE-1 is a bacterium that proved crucial to our uncovering further mysteries of arsenic metabolism based on the discovery of its “reverse” functioning
arsenate reductase (ArrAB) homolog, an anaerobic arsenite oxidase we designated
ArxAB in further collaborations with John Stolz at Duquesne University and Chad
Saltikov at University of California Santa Cruz (e.g., Richey et al. 2009; Zargar et al.
2010, 2012). Shelley also isolated the difficult-to-grow δ-Proteobacterium strain
MLMS-1which, we eventually realized, disdained organic substrates and used
sulfide as its electron donor to achieve growth via arsenate reduction. Strain
MLMS-1 showed little inclination in doing much of anything else of interest with
its time, except a perhaps a disproportionation of thioarsenate (Hoeft et al. 2004;
Planar-Friederich et al. 2015).
Curiosity drove Larry and me to once again explore the hot springs on Paoha
Island, but this time actually stepping ashore to see if there was anything of interest
growing within them. There clearly was, as these springs (~45
C) hosted
red-colored biofilms as coatings on the rocks and pebbles over which the spring
water flowed. The biofilms were thin (~1 mm) and could easily be removed by
gently scraping the surfaces with a soft toothbrush. We were able to obtain a lot of
sample in this manner, which we stored within anaerobic serum bottles held in a
refrigerator. We later discovered that they remained blood-red in color and still
proved viable even after several months of such dark storage. The hot spring waters
were anoxic and contained dissolved methane, sulfide, and arsenite in abundance, all
of which were possible electron donors that could support anoxygenic
676
R. S. Oremland