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Leavitt, W.D.
[email protected]
18 Feb 2016 at Princeton
The inner lives of sulfate reducing bacteria as revealed by sulfur and hydrogen
isotopic phenotypes
Microbial sulfate reduction accounts for upwards of 90% of organic matter
remineralization in marine sediments [Jørgensen. 1982. Nature]. As such, tracking the
past and present activity of these organisms (both Bacterial and Archeal
representatives) is critical to constructing global carbon cycle models. Bacterial sulfate
reducers (BSRs) in particular are known to produce lipids strongly depleted in
deuterium relative to their growth water (2εlipid-H2O ~ -250‰) [Campbell et al. 2009.
GCA; Osburn. 2013; Dawson et al. 2015. Geobiology], along with product sulfide that is
depleted in the heavier isotopes of sulfur relative to reactant sulfate (2εSO4-H2S ~ 0 to 70‰). In recent studies it has been suggested that the magnitude of hydrogen
isotopic fractionation may relate to central energy metabolism [Zhang et al. 2009.
PNAS], while sulfur isotope fractionation (34εSO4-H2S) scales inversely with sulfate
reduction rate over a range of up to 70‰ [Chambers & Trudinger 1975. J. Can
Microbio.; Sim et al. 2011. GCA; Leavitt et al. 2013. PNAS]. Given that both isotope
systems may relate to cellular energy conservation, albeit assimilatory versus
dissimilatory
metabolic
reactions, we investigate
mmol S per L chemostat per day
0
2
4
6
8
10
12
continuous and batch
75 Primarily
75
equilibrium fractionation 0 to 30°C
culture conditions that may
Equilibrium
be recorded by both
60
60
systems.
45
45
in vitro chemostat ε = 17.3 (±1.5) ‰
34ε
An important aspect of
r-p
in vitro sediment ε = 17.3 (±3.8) ‰
energy metabolism in some
30
(‰) 30
sulfate reducing bacteria is
15 Primarily
15
the activity of electron
Kinetic
bifurcating
0
0
transhydrogenase [Price et
0.0
0.5
1.0
1.5
all published
al. 2014. Front Microbio]].
mmol S per L sediment per day
experiments
Recent work in aerobic
methylotrophs [Bradley, Leavitt et
al.
2014
unpub.]
implicates
-50
0.95
transhydrogenase (TH) activity as a
critical control on 2εlipid-H2O. In order
to test the role of TH’s in BSRs we
grew mutant strains of Desulfovibrio
-100
0.90
alaskensis strain G20 deficient in the
nfnAB-2 gene, encoding one of two
copies of electron-bifurcating TH
0.85
-150
[Kuehl et al. 2014. mBio]. These
strains
produce
lipids
with
perturbed 2εlipid-H2O, and the range
Y = (Y - Plateau)*e
and magnitude of the apparent
R =0.888; Adj. R =0.87
-200
0.80
fractionation scales strongly with
growth rate.
Intriguingly, this
0.00
0.05
0.10
0.15
0.20
relationship is non-linear and
μ = average growth rate (per hour)
inverse to growth rate and can be
fit with an identical function that has been applied to sulfur isotope fractionations as a
function of cell-specific sulfate reduction rates [Leavitt et al. 2013. PNAS; Bradley,
Leavitt et al. 2015 Geobiology]. Due to perturbed central energy metabolism, we
predict altered 34εSO4-H2S in these TH mutants relative to their parent wildtype.
I will discuss the implications and potential applications for understanding Hisotope fractionation during microbial fatty acid biosynthesis in BSRs and the critical
role of TH’s in anaerobic microbial metabolisms in general. Furthermore, applying duel
sulfur and hydrogen isotopic proxies, as tracers of past microbial metabolic states (e.g.
distinguishing energy limited versus replete conditions), may be possible.
34
-K*X) + Plateau
0
2
2
Y0 = 0.934 (CI95: 0.910 to 0.957)
Plateau = 0.818 (CI95: 0.774 to 0.862)
εfatty acid wght. mean / water (‰)
2
min
min
2
αfatty acid wght. mean / water (‰)
34