Lecture 19:Diversity of microbial mats

Objectives• What are microbial mats? Where are they found?– Early earth (Stromatolites, banded-iron formations)– Biological soil crusts– non-sulfur hot springs

• Describe the three green phototrophic bacterial phyla.– Chloroflexi (green nonsulfur bacteria)– Chlorobi (green sulfur bacteria)– Cyanobacteria (blue-green algae)

• How does bacterial photosynthesis turn light into energy? How do bacteria use light to fix Carbon?

• Describe the two thermophilic bacterial phyla.– Aquifex and Thermotoga

PhotosyntheticMicrobial Mat

• Top thin green layer of macro- and micro- green algae that adhere to the surface

• green-brown layer is composed of cyanobacteria and diatom species

• Pink layer are purple sulfur bacteria• orange-black layer is formed predominately by

a single species of purple sulfur bacteria, Thiocapsa pfennigii, and spirochetes.

• The thin, bottom layer is made up of green sulfur bacteria belonging to the Prosthecochloris genus, which photsynthetically consume sulfide

• Black: sulfate reducers make sulfide, “rotten egg” smell, insoluble and strongly resists dissolution

• Below the mat is iron sulfide-rich sediments and remnants of decaying mats

Sippewisset Salt Marsh, Falmouth MA (L)Yellowstone National Park, CA (R)

Microbial mats

Nonsulfur hot spring microbial mats• 14C-labeled acetate diffusion experiment into mat core

in the light and in the dark followed by cryosectioning and autoradiography.

• Silver grains, which appear as tiny black dots, demonstrate where the labeled substrate has diffused during the incubation.

• C flow for photautotrophic and photoheterotrophic microbes

Green phototrophic bacteria

Phylum Chloroflexi(green nonsulfur bacteria)

Fig 9.3

Fig 9.4

Fig 9.5

Phylum Chlorobi(green sulfur bacteria)

Fig. 9.9 Chlorobium tempidum

Fig. 9.10 Chlorobium limicola

Symbiotic consortiaChlorobium (seen) encapsulating a flagellated betaproteobacterium (unseen)

Phylum Cyanobacteria(blue-green algae)

Fig. 9.17

Fig. 9.16

Fig. 9.15

Photoautotrophy

• There are several types of chlorophyll (L), but all share the chlorin magnesium ligand (R)

• Water is a natural attenuator of light, absorbing most of the infrared wavelengths within the first meter

• Longer wavelengths penetrate further into shallow water sediments

Bacterial photosynthesis

• Cyclic photophosphorylation• Obtaining reducing power for carbon fixation– Rhodopsin phototrophy

Cyclic photophosphorylation• Light energy is captured by the RC & excites an electron• The RC transfers the electron (becomes oxidized) to the

ETC• The ETC pumps a proton out of the cell, generating proton

motive force (PMF) which drives ATP production• CytC returns the electron to the RC, reducing it and

completing the cycle

RC, reaction centerBph, bacteriopheophytinQ, quinoneETC, electron transport chainCytC, cytochrome c

Obtaining reducing power for C fixation

• NADH must be generated for organisms wanting to fix C in addition to generating energy from light

• How do organisms make NADH?– Photoheterotrophs: from organic compounds– Purple non-sulfur bacteria & Chloroflexi: from reverse

electron flow– Chlorobi & heliobacteria: from ferridoxin from the

ETC– Cyanobacteria: from oxygenic photosynthesis

Obtaining reducing power for C fixation:reverse electron flow

• Purple non-sulfur bacteria & Chloroflexi get their NADH from this pathway

• Requires a strong chemical reductant: sulfide, thiosulfate, elemental sulfur, a ferrous cation or H2

• Running the ETC in reverse drains the proton gradient & costs ATP Q, quinone

ETC, electron transport chainCytC, cytochrome cS=, sulfideS0, elemental sulfur

Obtaining reducing power for C fixation:sulfur-dependent photosynthesis

• Members of the Chlorobi have strongly reducing RCs

• Electrons passed from Chl a to FeS proteins are able to generate reducing power as Fdred

• External sources of reductant (sulfide etc) replace the electrons removed from the ETC to generate ATP via PMF Chl a, chlorophyll a

FeS, iron-sulfur proteinsFd, ferredoxinS=, sulfideSO, elemental sulfur

Obtaining reducing power for C fixation:oxygenic photosynthesis

• Cyanobacteria (including chloroplasts in plant cells) carry out oxygenic photosynthesis

• Light activates the RC and reducing power is generated with ferredoxin reduction

• A second reaction center (PS II) transfers electrons to the ETC to generate ATP via PMF

Fdox, oxidized ferredoxinFdred, reduced ferredoxinPSII, Photosystem II

Rhodopsin phototrophy

• A way of capturing light energy for ATP production

• A simple light-driven proton pump composed of a single protein (rhodopsin) and photopigment (retinal)

• See Lec 18…

Carbon fixation

• Calvin cycle (Cyanobacteria)• Reverse TCA cycle (Chlorobi)• Hydroxypropionate pathway (Chloroflexi)– All three of these cycles are cyclical and require

reductant and energy• Reductive acetyl-CoA pathway– Aka the Wood pathway– Non-cyclical, used by non-phototrophic,

acetogenic bacteria and some methanogens

Calvin cycle• Used by Cyanobacteria• Rubisco is the key

enzyme in this pathway (ribulose bis-phosphate carboxylase/oxidase)

• Rubisco carboxylates ribulose 1,5-bisphosphate to make two molecules of 3-phosphoglycerate

• Rubisco is commonly known as the most abundant enzyme on Earth

Reverse (reductive) TCA cycle

• Used by Chlorobi (green sulfur bacteria)

• It consumes CO2, ATP, NADH/NADPH, and reduced ferredoxin to produce pyruvate

Hydroxypropionate pathway• Used by Chloroflexi• Some of the same

reactions as the TCA cycle, but generates glyoxylate

Thermophilic bacteria

Phylum Aquifex

Fig. 8.3 Aquifex pyrophilus

Fig. 8.4 Thermocrinus ruber

Phylum Thermotoga

Fig 8.7 Thermotoga maritimaFig 8.8 Thermosipho melanesienseFig 8.9 Fervidobacterium islandicum

Fig 8.7

Fig 8.8 Fig 8.9

Obsidian pool is an acidic, boiling spring

Stromatolites

Lithification in Stromatolites

• Extracellular polysaccharides (EPS) have carboxyl groups that can bind divalent cations, including calcium (Ca++)

• Biofilms have cation binding capacity

Banded Iron Formations

• Earliest and most pervasive evidence of life on earth

• Have shaped the atmosphere and climate of earth over geologic time

BIF formed during Phase 2 of the Great Oxygenation Event

BIF formed during Phase 2 of the Great Oxygenation Event

• Pre-3.0 Ga biosphere: photosynthetic bacteria almost certainly employed photosystem-I

• (PS-I) and used H2, H2S and/or Fe2+ to reduce CO2 to organic matter

• At a suspected tipping point where the oceans became permanently oxygenated, small variations in oxygen production produced periods of free oxygen in the surface waters, alternating with periods of iron oxide deposition.

Biological soil crusts• Highly specialized community of

cyanobacteria, mosses, and lichens• Crusts can support plant life• Crusts generally cover all soil spaces

not occupied by vascular plants, which may be 70% or more of the living cover.

• Crusts hold soils in place, preventing erosion

• Blowing sand can bury and kill crusts• Crusts grow very slowly, so walking

on them can kill them and take years/decades to recover

Objectives• What are microbial mats? Where are they found?– Early earth (Stromatolites, banded-iron formations)– Biological soil crusts– non-sulfur hot springs

• Describe the three green phototrophic bacterial phyla.– Chloroflexi (green nonsulfur bacteria)– Chlorobi (green sulfur bacteria)– Cyanobacteria (blue-green algae)

• How does bacterial photosynthesis turn light into energy? How do bacteria use light to fix Carbon?

• Describe the two thermophilic bacterial phyla.– Aquifex and Thermotoga