Lecture 10 - Sulfur cycling.ppt, reduction

sulfur cycle ppt for undergrad student

1. 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)

2. Sulfur Cycle

3. Early earth ocean-atmosphere and S

4. 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

5. 1 piece of sulfur oxidation pathways

6. 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-S-
O3)
•These are furthur reduced to HS-to form
cysteine or other useful sulfur forms
•All of this COSTS ENERGY!

7. Dissimilatory SO
4
2-
reduction
•Biological Sulfate Reduction (BSR) and
Thermochemical Sulfate Reduction (TSR)
•At temperatures <150-200ºC the reduction
of SO
4
2-
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!

8. Measuring rates of BSR
•Profiles and flux rates from gradients
•Culture-based incubations
•Radiolabeling using
35
S-labeld sulfate
–Done quickly in sediments (reduce chance of
re-oxidation)
–Recovery of H
2S 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

9. BSR and Carbon mineralization
•Carbon compound degradation to CO
2
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?

10. Where do sulfate-reducing bacteria
(SRB) hang out?
•Need anaerobic/microaerophilic
environment, enough SO
4
2-
, organics/ H
2
•Reduced sediments
•Hydrothermal springs (deep sea, terrestrial)
•Cyanobacterial mats (where in the mats do
you think??)
•SRB inhabit widest range of conditions –T 0-
127, 0-28% NaCl

11. 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 –ArcheoglobusT max=92ºC
•LGT of dissimilatory sulfur reductase (DSR)
gene supported across archaea, different
bacterial species

12. SRB Metabolism pathway
•SO
4
2-
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)
•H
2S is highly toxic (interacts strongly with
organics and metals) rapidly excreted
from the cell

14. 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 PO
3
3-
•Some are complete oxidizers, many incomplete –
(incomplete ones grow faster)
•H
2as an e
-
source, most are
chemolithoheterotrophic, a few known
chemolithoautotrophs…

15. 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??

16. Elemental sulfur
•S
8a product of sulfide oxidation, some
organisms store it intracellualry, also forms
abiotically on interaction of H2S with metals,
organics
•Elemental sulfur respiration coupled with H
2or
organic carbon oxidation (complete and
incomplete) found in many organisms
•Several identified species of the d-
proteobacterial clade that primarily metabolize
S
8,
•Widespread archaeal metabolism –
Crenarcheota, Sulfolobus, Acidianus, othrs

17. 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 H
2S oxidation depending on specific
chemistry and availability of oxidants too

18. Black Sea

19. Green Lake, NY
•Voltammetric evidence for
significant role of polysulfides in
sulfide oxidation and elemental
sulfur reactionsGreen Lake Voltammetric
Profile
0
5
10
15
20
25
0 0.1 0.2 0.3
Peak Current (A)
Depth(m)
Oxygen
(dissolved)
Hydrogen
Sulfide
Polysulfide
Elemental
Sulfur

20. Sulfide Oxidizing Organisms
•Chemolithoautotrophs (and heterotrophs)
exist that can oxidize H2S and other
intermediates
–Many can also reduce elemental sulfur…
•Use O
2or NO
3
-
as electron acceptor
•Most obligate or facultative aerobes, but
some are obligately microaerophilic (can’t
handle above a few tens of uM)

21. Intracellular S
8
•Several S-oxidizers
can store S
8in
vacuoles
•Noteably Beggiatoa
and Thiothrixspp.

22. Cave formation and stratified
analogues in central Italy
•Influx of sulfide-rich water accelerates cave
formation: H
2S + 2 O
2SO
4
2-
+ 2 H
+
CaCO
3+ H
+
Ca
2+
+ HCO
3
-
S
8
S
x
n-HS
-
S
2O
3
2-
HSO
3
-
S
4O
6
2-

23. Microbial ecology and sulfur speciation
•Different microbial communities found in different
places ---related to BIG changes in S speciation!
3 different predominant mat types
Potential (V vs. Ag/AgCl)
Current (
m
A)
White:
d-proteobacterial
mat
Red:
thiovulum mat
Green:
beggiotoa mat
S
8
S
x
n-
HS
-
S
2O
3
2-
HSO
3
-

24. ‘d-proteobacterial’
mats
Potential (V vs. Ag/AgCl)
Current (
m
A)
Scans into white mat material
S
x
n-
Potential (V vs. Ag/AgCl)
Current (
m
A)
Scans above (green and into biofilm, red)
Potential (V vs. Ag/AgCl)
Current (
m
A)
Above (green) and into biofilm (others)
beggiatoa
mats
thiovulum
mats
S
8
S
8
S
8
S
x
n
-
HS
-
HS
-
HS
-
S
2O
3
2-

25. ‘thiovulum’ mats, Pozzo di Cristale, Frassassi caves

26. Thiovulum mat profile data
~ 50 mm thick biofilmThiovulum Mat Profile
Pozzo di Cristale
0
10
20
30
40
50
60
70
80
90
100
0 20 40 60
Current (nA)
Depth from bottom (um)
elemental sulfur
sulfite
sulfide (uM)
Potential (V vs. Ag/AgCl)
Current (
m
A)
above

27. Snottite
electrochemistry
Potential (V) vs Ag/AgCl
Current (
m
A)Sulfide peak location on Au/Hg microelectrode in Mini-Primrose
water over a range of pH values on HMDE
y = -0.0702x - 0.2255
R
2
= 0.9837
-0.7
-0.65
-0.6
-0.55
-0.5
-0.45
-0.4
-0.35
-0.3
1 2 3 4 5 6 7
pH
Potential (V vs. Ag/AgCl)
pH varies 1-3 in these snottite streamers

28. S
8in biofilms at Frasassi
Images courtesy Jenn Macalady, Penn State

29. Courtesy Macalady lab, Penn State
16s library of the
biofilms in
Frassassi
•New results
looking at
metagenomic data
has identified a
gene regulating
elemental sulfur
‘docking’

30. Phototrophic S-oxidation
•Anoxygenic phototrophy using H
2S, S
8, S
2O
3
2-
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…

31. Phototrophic Mats -CyanosPhototrophic Mat outside fracture
spring - Frassassi
-350
-300
-250
-200
-150
-100
-50
0
0 20 40 60
Conc (nA)
Depth (

m)
sulfide (uM)
elemental
sulfur
oxygen
approximate
top of mat
Anoxygenic photosynthetic
organisms oxidizing H
2S across a
VERY sharp gradient!!
Electrode tip
stuck bottom

32. Phototrophic mats -PSB
•Purple sulfur bacteria mats
–Respond to light level changes
in minutes position in
sediment and water column
can vary significantly!Purple sulfur bacteria mats
-800
-700
-600
-500
-400
-300
-200
-100
0
0 500 100015002000
H
2S
(aq) Concentration (M)
Depth (microns)

33. Light Manipulation experimentsCyanobacteria Light Manipulation Experiment
0
50
100
150
200
250
300
350
400
450
-80-60-40-20020406080100120
time (seconds)
nA H2S
Jacket onJacket offHat onHat off

34. 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…

35. 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:
–S
2O
3
2-
+ H
2O H
2S + SO
4
2-

36. S stable isotopes
•4 stable isotopes of sulfur:
32
S (95.04%),
33
S
(0.749%),
34
S (4.20%),
36
S (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!