Lecture 17: Marine Microbes and their Significance in Biogeochemical Cycles Flashcards
1
Q
What is Sulfur?
A
- Atomic no. 16
- ~1% of dry weight of organisms
- Two mineral forms:
a. Sulfide (S^2-)
b. Sulfate (S04^2-) - Used as:
a. fertiliser (plants = high demand)
b. Manufacturing insecticides, fungicides,
etc.
2
Q
Why is sulfur important?
A
- Metabolism - fuel and respiratory
electron donor - Proteins - AAs; cysteine and methionine
- Disulphide bonds - protein stability +
structure - Cofactors in enzymes - e.g., FeS in
Aconitase (TCA cycle) - Antioxidant molecules glutathione and
thioredoxin
3
Q
What are the different sulfur processes in microbes?
A
1. Assimilation - using sulfate to form
organic compounds (E dependent)
(animals use preformed S-compounds)
2. Dissimilatory sulfur metabolism - bacteria
and archaea use sulfur in E yielding
reactions (Ox or An)
3. Electron acceptors or donors (e.g.,
Sulfate reduction & oxidation)
4. Generating signalling molecules
5. Generating anti-stress molecules
4
Q
What does assimilate mean?
A
Get something into a form that is useable
5
Q
Explain assimilatory uptake of sulfate
A
- Sulfate abundant in marine environments
(~10^6 x more abundant than other S) - Some incorporated into polysaccharides
- Sulfate reduced to Sulfide in many
microbes (requires E) - Sulfide used to make AAs cysteine and
methionine
6
Q
What are different types of organisms transforming sulfur
A
- Phytoplankton
- Seaweeds
- Few species from angiosperms
- Some corals
- Many heterotrophic bacteria
7
Q
What is marine DMSP synthesis?
A
1. Reduce sulfate to Sulfide to make
methionine (all req. E)
2. L-methionine is then turned into
"Dimethylsulfoniopropionate" (DMSP)
a. stable and soluble zwitterion ( can be +
& - charged
b. Prod. at v high [mM] e.g., 1/2M
c. > 8 billion tonnes made per year
3. E.g.,
a. Some marine eukaryotes
b. Marine bacteria (Alphaproteobac teria)
8
Q
Why is DMSP produced?
A
- Suggested roles:
a. Osmoprotectant - OSMOLITES
(balancing the osmotic difference
between cell’s surroundings and
cytosol)
b. Cryoprotectant - CRYOLITES
(maintaining enzyme activities in
high and low temperatures)
c. Grazing deterrent - deters predators
due to their bad taste/toxicity
d. Oxidative stress protectant -
ANTIOXIDANTs DMSP and its
catabolites scavenge oxygen free
radicals generated by oxidative
stress
e. Storage molecule for carbon and
sulfur - lots of carbon and sulfur in
its structure so can be broken down
at a later date
9
Q
Why only suggested roles?
A
none have been definitively established
10
Q
Why is it that we do not know the single gene or enzyme involved in these DMSP pathways, yet we know the biochemical synthesis pathways?
A
1. Marine biology generally lacking in
molecular biological approaches
compared to terrestrial systems
2. Marine eukaryotes that make DMSP lack
genetic manipulation tools
11
Q
What is known about DMSP pathways?
A
- > 8 billion tonnes is made annually in
surface oceans - 3-10% of global marine primary carbon
production (GPP) - DMSP catabolism supplies 3-10% of
carbon requirements of heterotrophic
bacteria - DMSP supplies 50-100% of sulfur
demands for heterotrophic bacteria
(despite there being 1-10 million x more
sulfate) VIA CATABOLISM
12
Q
What is DMSP catabolism
A
1. organisms eat DMSP and gain benefits such as E, carbon, sulfur 2. DMSP - major nutrient for heterotrophic bacteria (N & S) (& E) a. 3-10% of carbon supply b. 30-100% sulfur supply
13
Q
What are the two pathways for DMSP catabolism
A
- Sulfur - demethylation pathway
a. dmd genes
b. 70% of DMSP in marine environment
eaten forms MeSH (methanethiol)
c. MeSH used as a source of sulfate
14
Q
What is SAR11 bacteria?
A
- most abundant and ubiquitous clade of
heterotrophic marine bacteria (1/3 of
cells in photic zone) - Examples
a. Candidatus Pelagibacter ubique - 1st
cultured
b. P. ubique genomes small (1.3Mbp)
c. smallest no. genes in a living organism
d. pathways for all 20 AAs and most
cofactors - these bacteria have lost the genes allowing assimilation of sulfate into Sulfide
a. therefore need reduced sulfur sources
15
Q
What is DMSP lysis?
A
- ddd genes
- approx. 30% of DMSP eaten generates
Dimethylsulfide (DMS)
DMSP -> DMS + Acrylate or 3-hydroxypropionate
16
Q
What can SAR11 do?
A
DMSP to produce MeSH (using dmd genes) for sulfur, or produce DMS (using dddK genes)for carbon
17
Q
What are other examples of bacteria heterotrophic bacteria that lyse DMSP?
A
- (alpha) Roseobacter
2. (gamma) Halomonas
18
Q
What are Roseobacters?
A
- marine alpha-proteobacteria
- 10-25% bacteria in marine surface
waters, sediments and sea ice
(abundant) - Easily cultured (ADVANTAGE OVER
SAR11 FOR STUDIES) - Many genomes available
- Just as abundant SAR11 but more diverse
(more genera)
19
Q
What is the problem with SAR11?
A
- VERY DIFFICULT TO CULTURE
a. cant grow on plates
b. cant isolate single colonies
c. can only grow in liquid culture
d. difficult to monitor growth and its rate
20
Q
Why is roseobacter a good model organism?
A
- First studied: Ruegeria pomeroyi DSS-3
- Isolated from DMSP enrichments of coastal Georgia Seawater
- Genome sequenced in 2004
- Genetically tractable
- Much like SAR11:
a. can produce DMS (using dddP, dddQ,
dddW genes) lyse
b. MeSH (dmd genes) demethylate
21
Q
What happens to DMS?
A
- 90% re-catabolised by marine bacteria as
carbon source (Methylophaga spp.) - Other 10% transferred from marine
system to atmosphere (~35 million
tonnes)
a. major biogenetic source of sulfur in
atmosphere
22
Q
What happens to DMS in the atmosphere?
What is CNN?
A
- DMS oxidation produces products
- these products act as “Cloud
Condensation Nuclei” (CNN) - Climate cooling effect by reflecting
sunlight - AS WELL AS THIS, clouds then move, from marine to terrestrial systems, COMPLETEING GLOBAL SULFUR SYSTEMS
CNN is the ‘nucleus’ for which water droplets form around, leading to a cloud
23
Q
Explain the CLAW hypothesis
A
1. Proposition that DMSP dependent DMS
production as a negative mechanism by
which phytoplankton regulate their
environment in CLAW
a. Phytoplankton prod. DMSP...
b. If it gets too cold they make more
DMSP = more DMS
c. = more DMS in atmosphere
d. increases cloud cover
e. BACTERIA ESSENTIALLY REGULATING
THE ENVIRONEMT FOR THEMSELF
