Lecture 17: Marine Microbes and their Significance in Biogeochemical Cycles

• 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.

• 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

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

Get something into a form that is useable

• 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

• Phytoplankton • Seaweeds • Few species from angiosperms • Some corals • Many heterotrophic bacteria

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)

• 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

none have been definitively established

1. Marine biology generally lacking in molecular biological approaches compared to terrestrial systems 2. Marine eukaryotes that make DMSP lack genetic manipulation tools

• > 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

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

• 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

• 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

• ddd genes • approx. 30% of DMSP eaten generates Dimethylsulfide (DMS) DMSP -> DMS + Acrylate or 3-hydroxypropionate

DMSP to produce MeSH (using dmd genes) for sulfur, or produce DMS (using dddK genes)for carbon

• (alpha) Roseobacter 2. (gamma) Halomonas

• 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)

• 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

• 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

• 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

• 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

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