Micronutrient supplements control metabolism and shape
microbiome composition in vitro
To investigate if micronutrients can modulate the microbiome composition
of the microcosms for top-down engineering in MEOR, our in vitroscreening system was used to track metabolism and community composition
under various micronutrient supplements. In vitro screening
focused on Well D because it was one of the two most responsive wells.
Moreover, Well G was heavily dominated by one genus
(Malaciobacter , class Campylobacteria) (Supplemental
Figure 1 ), with less microbial diversity to adapt to the micronutrient
conditions. Again, Well D was stimulated with molasses and supplemented
with a variety of micronutrients including chloride (NaCl and KCl),
phosphate (K2HPO3 and
KH2PO3), nitrate (KNO3),
molybdate (Na2MoO4), and a combination
of both nitrate and molybdate. Chloride salts were used as a negative
control while phosphate salts were tested as a phosphate source, which
have been added in other MEOR studies.9,31,36 To
reduce sulfate reduction, nitrate and molybdate salts were tested.
Nitrate is a competing electron acceptor in place of sulfate in several
sulfur-reducing bacteria,11,39–41 while molybdate has
been shown to inhibit H2S generation in different
aqueous and marine contexts.40–43
Gas and organic acid production varied strongly as a function of
micronutrient supplementation (Figure 2A, Supplemental Figure
3 ). Nitrate- and molybdate-supplemented communities from Well D
produced significant amounts of pressure. The cumulative effects of
combination treatments were additive, resulting in the most observed
pressure generation. Similarly, nitrate dropped the pH of the culture
the most (to a pH ~4) where the molybdate treatment had
a slightly smaller effect (Supplemental Figure 3B). These results
suggest that chloride and phosphorous do not alter microbial metabolism
in the microcosms and that nitrate best stimulated the production of
gases and acids for MEOR. There was some additional benefit to adding
molybdate in combination with nitrate as it generated the most gas and
one of the larger pH changes. Because only select microbes can convert
nitrate into N2 and/or ammonia, or use it as a nitrogen
source for synthesis of DNA and amino acids, supplemented nitrate will
confer a competitive advantage to specific microbes in an otherwise
nitrogen-limited environment such as an oil well. In turn, we expect
that the differences in the metabolic outputs (Figure 2A ) are
reflective of the differences in the microbial community composition as
a result of the added compounds.