L38: Suflur Cycle Flashcards
Three ways to produce H2S
desulfuration
**decomposition of organic matter: yields H2S
– recycling of organosulfur compounds
Sulfur respiration (anaerobic)
**reduction of S0 as terminal electron acceptor via anaerobic
respiration (many species): yields H2S
Dissimilatory sulfate reduction
**via anaerobic respiration by SRB’s also yields H2S
Explain Aerobic oxidation of sulfide and sulfur
-oxidation of H2 S and S0 , with O2 as TEA, in energy generation
– sulfide and sulfur compounds are not only the energy source
but also electron source of for cell growth
– Beggiatoa, Thiothrix etc. (sulfur bacteria) can oxidize H2 S
aerobically and accumulate S0 intracellularly
– some species oxidize S0 to SO42-
– Sulfolobus (archaea) oxidizes S0 , yielding sulfuric acid, prefers
low pH and heat
– chemolithotrophs i.e., Acidithiobacillus thiooxidans
explain the two ways of Anaerobic oxidation (1) wit example
Chemotrophic anaerobic oxidation of sulfide and sulfur
– oxidation of sulfides or sulfur with electron acceptors other than
O2 (anaerobic respiration) : yields S0 or SO42-
– Example: Thiomargarita and Beggiatoa can use H2 S as energy
source with NO3- as TEA and electron source for growth
explain the two ways of Anaerobic oxidation (2)
Phototrophic anaerobic oxidation of sulfide and sulfur
– use of sulfides or S0 as e- donor: yields S0 or SO42-
– Chromatium (purple sulfur), Chlorobium (green sulfur)
– similar to oxidation of H2 O => H2 + O2 in oxygenic P/S
– sulfides and/or S0 used as electron source
Explore gradient organisms in the sulfur cycle
– Example: Beggiatoa grow at the interface of O2 and H2S gradients to oxidize H2 S, forming S0 and/or sulfate
– aerobic chemotrophic sulfide oxidation: TEA is O2
– anaerobic chemotrophic sulfide oxidation: using internally-
stored NO3- as TEA when O2 is unavailable
explain Biological deposition of elemental sulfur (S0) with examples
– S0 deposits form in lakes and geological formations
– Desulfovibrio, Thiobacillus, Chlorobium
– initiated by dissimilatory sulfate reduction to H2 S by SRB’s such as Desulfovibrio
– oxidation of H2 S to S0 in aquatic and geological environments (aerobically or anaerobically) contributes to S0 deposits
What are the two ways of sulfur deposition with an example
Sulfur deposition via aerobic chemotrophic sulfide oxidation:
– H2 S can be oxidized to S0 by Thiobacillus
Sulfur deposition via anaerobic phototrophic sulfide oxidation:
– H2 S can be oxidized to S0 by phototrophs such as Chlorobium,
providing there is light
What are two ways of sulfur reduction
Assimilatory sulfate reduction
Dissimilatory sulfate reduction
Explain the first way of sulfur reduction in depth
– many species are able to assimilate sulfate to obtain S
– similar to assimilatory nitrate reduction
– pathway is tightly regulated as cells only need a small amount of S for synthesis of amino acids (Cys, Met), glutathione, etc.,
– assimilatory pathway is subject to end-product inhibition
Explain the second way of sulfur reduction in depth
Dissimilatory sulfate reduction
– like dissimilatory nitrate reduction, it is not subject to end-
product inhibition because it for is anaerobic respiration, not for
assimilation
– yields H2S as a waste product
– sulfate-reducing bacteria (SRBs) i.e., Desulfovibrio
– use SO42- as terminal electron acceptor in anaerobic
respiration
– use H2 and/or organic carbon as energy & electron source
– H2S produced is an important energy & electron source for
other microorganisms
– SRB’s were previously believed to all belong to the Deltaproteobacteria, but that group has been split and the SRB’s placed in their own phylum Thermodesulfobacteriota
– there are now known to be Gram +ve bacterial and Archaeal organisms that can also do dissimilatory sulfate reduction
What id Risattis paper do
This paper demonstrated how a bacterial habitat containing complex communities
can be studied. The bacteria were detected, located, identified, and the
abundance of different groups was quantified
Break down the methods of risatti
– a core sample was taken and sectioned with respect to depth
– total RNA was isolated and serial dilutions were slot blotted to
membranes
– membranes were hybridized separately with each 32 P-labelled probe, washed and exposed to x-ray films
– the films were developed, and the film exposure (density of silver grains) was quantified (densitometry)
– blots were stripped of the probe then re-hybridized with
each of the other probes, in turn
– densitometry of spots on film quantifies the amount of probe
bound to the RNA on blot, which is relative to the number of
cells in the sample from the SRB group that the probe targets
– relative densities of SRB groups vs. sample depth were plotted
Probes used by Risatti et al., 1994
– specific for parts of the 16S rRNA
– used “signature sequences” of each group
Five different SRB probes
– one for each SRB genus or family of closely-related genera
– to differentiate SRB groups
Universal probe
– to detect all organisms & calculation of relative population sizes
Archaeal probe
– to detect Archaea such as methanogens hypothesized to be deep in the mat (below the SRBs) and extreme halophiles (i.e., Halobacterium sp.) that may be present near the surface
What were the probes from hybridisation
Probes/lanes
A. Desulfococcus
B. Desulfovibrio
C. Desulfobacterium
D. dilution series of C
What were the conclusions from risatti’s paper
– the spatial separation of genera of related microorganisms in a
microhabitat had rarely, if ever, been demonstrated before
– the distribution of the SRB groups is likely significant and
was also observed in later studies (NB also to ~40 mm)
– indicated the phylogenetic groups have different metabolic
capabilities that can influence their distribution in the habitat
– each group appears to have its own food web niche i.e., :
– Desulfovibrio were shallow (9 mm) and numerous
– Desulfobacter were deep (42 mm) and relatively very
numerous
– as SRB’s were detected in the deepest sections Risatti et al.,
would have had to sample deeper to detect methanogens
