Transcript Sulfur cycling

Sulfur biogeochemistry
• 8 e- between stable redox
states
• Polymerizes, cyclizes
• Reduced, intermediate,
and oxidized solid forms
• Thousands of organic
sulfur forms (organosulfur
compounds; thiols have
an –SH group, thioethers
–C-S-C, thioesters and
sulfonates are oxidized S
forms, sulfoxides/sulfones
RS(=O)R’, RS(=O)2R,
thioketones, thioamides,
sulfonium ylides less
common)
Sulfur Cycle
Early earth ocean-atmosphere and S
Assimilatory vs. Dissimilatory
• S is an essential nutrient (key to amino
acids cysteine and methionine) and many
other cellular molecules, so all organisms
need an assimilatory pathway
• Many dissimilatory reactions due to
complicated intermedaite pathways
involving S redox chemistry- leads to idea
that S-utilizing organisms are the most
diverse group of microbes which metabolize
a single element
1 piece of sulfur oxidation pathways
Assimilatory pathways
• APS pathway – uptake of SO42- to APS
(Adenosine phosphosulfate) using an ATP
• APS then goes thorugh 1 of 2 paths:
– Forms PAPS (phosphoadenosine
phosphosulfate)
– Or forms organic thiosulfate derivative (G-S-SO3)
• These are furthur reduced to HS- to form
cysteine or other useful sulfur forms
• All of this COSTS ENERGY!
Dissimilatory SO42- reduction
• Biological Sulfate Reduction (BSR) and
Thermochemical Sulfate Reduction (TSR)
• At temperatures <150-200ºC the reduction
of SO42- by reduced organics is VERY slow
(though thermodynamically favorable) –
formation of sulfide at low T is thus
MICROBIAL
• ‘Mineralization’ process because H2S and
metals strongly interact – form sulfide
minerals – very low solubility!
Measuring rates of BSR
• Profiles and flux rates from gradients
• Culture-based incubations
• Radiolabeling using 35S-labeld sulfate
– Done quickly in sediments (reduce chance of
re-oxidation)
– Recovery of H2S produced can be difficult (if it
quickly goes into pyrite for example it is harder
to recover)
– However, this is the most accurate and
common technique
BSR and Carbon mineralization
• Carbon compound degradation to CO2
through BSR
– AT high sedimentation rates, BSR can
account for significant fraction of this
– At lower sedimentation rates, BSR is less
important
– WHY THE DIFFERENCE??
– In lake sediments this can be very different
than in marine sediments, WHY?
Where do sulfate-reducing bacteria
(SRB) hang out?
• Need anaerobic/microaerophilic
environment, enough SO42-, organics/ H2
• Reduced sediments
• Hydrothermal springs (deep sea, terrestrial)
• Cyanobacterial mats (where in the mats do
you think??)
• SRB inhabit widest range of conditions – T 0127, 0-28% NaCl
SRB Phylogeny
• Deep-branching, widely distributed across
tree of life (both archaeal and bacterial),
thermophilic
• Bacteria – mostly in d-proteobacteria, also
spore-formers, gram+, in nitrospira group
• Archaea – Archeoglobus T max=92ºC
• LGT of dissimilatory sulfur reductase (DSR)
gene supported across archaea, different
bacterial species
SRB Metabolism pathway
• SO42- import – costs energy, coupled to
transport of H+ of Na+
• ‘Activated’ by ATP sulfurylase  forms
APS, which is then reduced to sulfite
which is reduced to sulfide by the DSR
enzyme (a reductase)
• H2S is highly toxic (interacts strongly with
organics and metals)  rapidly excreted
from the cell
DSR substrate limitations
• Require smaller, less recalcitrant substrates
(anaerobes do not make radicals needed to degrade
bigger molecules into something useable)
• Grow best on simple substrates like acetate, but can
grow on a wide range of substrates, including some
xenobiotics and even PO33• Some are complete oxidizers, many incomplete –
(incomplete ones grow faster)
• H2 as an e- source, most are
chemolithoheterotrophic, a few known
chemolithoautotrophs…
SRB Diversity
• Over 100 different species known
• IN one study, 20 different species were
identified from a single sediment sample!
• For the same metabolism – what other
factors may play into which one(s) are
predominant at any point in time or
space??
Elemental sulfur
• S8 a product of sulfide oxidation, some
organisms store it intracellualry, also forms
abiotically on interaction of H2S with metals,
organics
• Elemental sulfur respiration coupled with H2 or
organic carbon oxidation (complete and
incomplete) found in many organisms
• Several identified species of the dproteobacterial clade that primarily metabolize
S8,
• Widespread archaeal metabolism –
Crenarcheota, Sulfolobus, Acidianus, othrs
Sulfide oxidation
• Abiotic pathways – sulfide reaction with
FeOOH or MnOOH is fast, reaction with
O2 slower, with NO3- slow too…
• Plenty of differences in the intermediates
of H2S oxidation depending on specific
chemistry and availability of oxidants too
Black Sea
Green Lake, NY
Green Lake Voltammetric
Profile
0
Oxygen
(dissolved)
5
Depth(m)
Hydrogen
Sulfide
• Voltammetric evidence for
significant role of polysulfides in
sulfide oxidation and elemental
sulfur reactions
10
Polysulfide
15
Elemental
Sulfur
20
25
0
0.1
0.2
Peak Current (A)
0.3
Sulfide Oxidizing Organisms
• Chemolithoautotrophs (and heterotrophs)
exist that can oxidize H2S and other
intermediates
– Many can also reduce elemental sulfur…
• Use O2 or NO3- as electron acceptor
• Most obligate or facultative aerobes, but
some are obligately microaerophilic (can’t
handle above a few tens of uM)
Intracellular S8
• Several S-oxidizers
can store S8 in
vacuoles
• Noteably Beggiatoa
and Thiothrix spp.
Cave formation and stratified
analogues in central Italy
• Influx of sulfide-rich water accelerates cave
formation: H2S + 2 O2  SO42- + 2 H+
CaCO3 + H+  Ca2+ + HCO3Sxn-
HS-
S8
S4O62HSO3S2O32-
Microbial ecology and sulfur speciation
• Different microbial communities found in different
places --- related to BIG changes in S speciation!
3 different predominant mat types
Current (mA)
Sxn-
HS-
S8
HSO3-
S2O32-
Potential (V vs. Ag/AgCl)
White:
d-proteobacterial
mat
Red:
thiovulum mat
Green:
beggiotoa mat
thiovulum
mats
Above (green) and into biofilm (others)
HS-
Current (mA)
Sxn
S8
S2O32-
Potential (V vs. Ag/AgCl)
Scans above (green and into biofilm, red)
Current (mA)
beggiatoa
mats
HSS8
Potential (V vs. Ag/AgCl)
‘d-proteobacterial’
mats
Scans into white mat material
Current (mA)
Sxn-
HS-
S8
Potential (V vs. Ag/AgCl)
‘thiovulum’ mats, Pozzo di Cristale, Frassassi caves
Thiovulum mat profile data
Thiovulum Mat Profile
Pozzo di Cristale
100
90
~ 50 mm thick biofilm
Current (mA)
above
Depth from bottom (um)
80
70
60
elemental sulfur
50
sulfite
40
sulfide (uM)
30
20
10
0
0
20
40
Current (nA)
Potential (V vs. Ag/AgCl)
60
Current (mA)
Snottite
electrochemistry
Potential (V) vs Ag/AgCl
Potential (V vs. Ag/AgCl)
Sulfide peak location on Au/Hg microelectrode in Mini-Primrose
water over a range of pH values on HMDE
-0.7
y = -0.0702x - 0.2255
R2 = 0.9837
-0.65
-0.6
-0.55
-0.5
-0.45
-0.4
-0.35
-0.3
1
2
3
4
5
6
7
pH
pH varies 1-3 in these snottite streamers
S8 in biofilms at Frasassi
Images courtesy Jenn Macalady, Penn State
16s library of the
biofilms in
Frassassi
• New results
looking at
metagenomic data
has identified a
gene regulating
elemental sulfur
‘docking’
Courtesy Macalady lab, Penn State
Phototrophic S-oxidation
• Anoxygenic phototrophy using H2S, S8, S2O32- as
electron donors
• Organisms are common, in 5 major groups:
–
–
–
–
–
Purple sulfur bacteria
Purple nonsulfur bacteria
Green sulfur bacteria
Green nonsulfur bacteria
Heliobacteria
• These archaic groupings derived from ‘sulfur’
groups depositing visible S8, nonsulfur ones did not
– mistakenly thought they did not use reduced
sulfur as a result, and we still use the names…
Phototrophic Mats - Cyanos
Phototrophic Mat outside fracture
spring - Frassassi
0
-50
sulfide (uM)
Depth (m)
-100
elemental
sulfur
oxygen
-150
-200
-250
Anoxygenic photosynthetic
organisms oxidizing H2S across a
VERY sharp gradient!!
-300
approximate
top of mat
-350
0
Electrode tip
stuck bottom
20
40
Conc (nA)
60
Phototrophic mats - PSB
• Purple sulfur bacteria mats
0
-100
-200
Depth (microns)
– Respond to light level changes
in minutes  position in
sediment and water column
can vary significantly!
Purple sulfur bacteria mats
-300
-400
-500
-600
-700
-800
0
500
1000
1500
H2 S(aq) Concentration (M)
2000
Light Manipulation experiments
nA H2S
Cyanobacteria Light Manipulation Experiment
450
400
350
300
250
200
150
100
50
0
-80
-60
-40
-20
0
20
40
60
80
100 120
time (seconds)
Jacket on
Jacket off
Hat on Hat off
S-oxidizer phylogeny
• Anoxygenic photosynthesis development before
oxygenic photosynthesis?
– Geochemical record of the earth’s oceans?
– Photosystem less complicated
– Anoxygenic organisms more deeply branching
• Others argued based on pigment biosynthesis
pathways oxygenic photosynthesis is first
• Subsequent genetic analysis using genes related
to pigment biosynthesis showed anoxygenic
photosynthesis first (specifically, PSB) – but here
are some complications involving possible LGT…
Disproportionation
• Sulfur’s equivalence to fermentation –
intermediate oxidation state sulfur species
(elemental sulfur, thiosulfate, sulfite) split
into one more and one less oxidized
forms, ex:
– S2O32- + H2O  H2S + SO42-
S stable isotopes
• 4 stable isotopes of sulfur: 32S (95.04%), 33S
(0.749%), 34S (4.20%), 36S (0.0156%)
• Thermodynamic equilibrium for the fractionation of
S isotopes rarely obtained – observed
fractionations largely kinetic
• SRB fractionations (cultures) 3-46‰
– Rates, species/enzymes, substrates affect this
• S-disproportionation also results in large
fractionation (up to 37‰)
• SRB fractionations in nature up to >100+‰
• S-oxidation (biotic or abiotic) does not produce
much fractionation at all!