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Photo by Sean Davies- Surfer Magazine
prokaryotic
diversity eukaryotic
diversity
One point of view on prokaryotic evolution
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How to count the How to count the number number of prokaryotes on of prokaryotes on earth?earth?Because prokaryotes are everywhere,
it might seem impossible to count all of them.
Fortunately, most of the earth’s prokaryotes are found in a few large habitats.
Whitman, W.B., D.C. Coleman, and W.J. Wiebe (1998) Proc. Natl. Acad. Sci. USA 95: 6578-6583.
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Numbers of prokaryotesNumbers of prokaryotes in various habitats in various habitats
habitathabitat % of total% of total
marine subsurfacemarine subsurface 6666
terrestrial terrestrial subsurfacesubsurfaceaa
2626
soilsoil 4.84.8
seawaterseawater 2.22.2
freshwater and freshwater and saline lakessaline lakes
0.00430.0043
domesticated domesticated animalsanimals
0.0000800.000080
sea icesea ice 0.0000740.000074
termitestermites 0.0000120.000012
humanshumans 0.00000720.0000072
domesticated birdsdomesticated birds 0.000000040.0000000444
aAverage of range of 0.25-2.5 x 1030
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Number and Number and biomass (carbon)biomass (carbon) of prokaryotes in of prokaryotes in the worldthe worldenvironmentenvironment number of number of
prokaryotic cells prokaryotic cells (x 10(x 102828))
Pg of C in Pg of C in prokaryotprokaryot
esesaa
Aquatic habitatsAquatic habitats 1212 2.22.2
Oceanic Oceanic subsurfacesubsurface
355355 303303
SoilSoil 2626 2626
Terrestrial Terrestrial subsurfacesubsurface
25-25025-250 22-21522-215
TOTALTOTAL 415-640415-640 353-546353-546
aPg = 1015 g. For aquatic, subsurface, and soil habitats, prokaryotic cells were estimated to contain 10-20, 86, and 100 fg C, respectively.
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Relationship of plant and Relationship of plant and prokaryotic prokaryotic biomass to primary biomass to primary productivityproductivity
ecosysecosystemtem
net net primary primary productiproductivityvity
total carbon content (Pg C)total carbon content (Pg C)
(Pg (Pg C/yr)C/yr)
plantplant soil and soil and aquatic aquatic
prokaryoteprokaryotess
subsurfasubsurface ce
prokaryoprokaryotestes
terrestrterrestrialial
4848 560560 2626 22-21522-215
marinemarine 5151 1.81.8 2.22.2 303303
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If the abundance of prokaryotes is high today, what has it been like in the past?
Geochemical evidence suggests that the total amount of organic carbon on earth has remained unchanged for billions of years.
If this is true, it is likely that the biomass has also been about the same.
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Stable isotope ratio is evidence of the source of carbon:
δ13C = [(13C/12C)sam/ (13C/12C)std-1] x 1000 [o/oo,PDB]
13C:12C ~ 1:89
Organic carbon is typically depletedin 13C: δ13C = -25 o/oo
Inorganic carbon is the standard: δ13C = 0 o/oo
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Where is the carbon on earth?
Mostly in rocks. Of the carbon in the biosphere, 80 % is inorganic carbonate, 20 % is organic carbon in soils, sediments, and biomass.
Inorganic Cδ13C = 0 o/oo
Organic Cδ13C = -25
Weighed average δ13C = -6 o/oo
Total carbon in biosphere ~7.5 x 1022 g
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If the abundance of prokaryotes is high today, what has it been like in the past?
Geochemical evidence suggests that the total fraction of organic carbon on earth has remained unchanged for billions of years
From Schidlowski et al., 1983
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If the abundance of prokaryotes is high today, what has it been like in the past?
Modern organic carbon is depleted in 13C relative to modern carbonates carbon, and modern carbonates are enriched in 13C. However, the δ13C of the organic carbon is less stable than that of the carbonate.
From Schidlowski et al., 1983
average13 of earth’s carbon
average13 of organic carbon
average13 of carbonates
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If the abundance of prokaryotes is high today, what has it been like in the past?
The enrichment of ancient carbonates in 13C is a measure of the fraction of the total biosphere carbon that is in organic carbon and hence an indirect measure of biomass. From Schidlowski et al., 1983
average13 of earth’s carbon
average13 of carbonates
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If the abundance of prokaryotes is high today, what has it been like in the past?
Ancient carbonates are enriched in 13C, suggesting that the biomass of the earth has remained unchanged over geological time scales and predating the origin of most modern life forms.
From Schidlowski et al., 1983
first land plants
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If the abundance of prokaryotes is high today, what has it been like in the past?
Ancient carbonates are enriched in 13C, suggesting that the biomass of the earth has remained unchanged over geological time scales and predating the origin of most modern life forms.
From Schidlowski et al., 1983
first metazoa
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If the abundance of prokaryotes is high today, what has it been like in the past?
Ancient carbonates are enriched in 13C, suggesting that the biomass of the earth has remained unchanged over geological time scales and predating the origin of most modern life forms.
From Schidlowski et al., 1983
first multicellular organisms
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If the abundance of prokaryotes is high today, what has it been like in the past?
Ancient carbonates are enriched in 13C, suggesting that the biomass of the earth has remained unchanged over geological time scales and predating the origin of most modern life forms.
From Schidlowski et al., 1983
abundant microfossils
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If the abundance of prokaryotes is high today, what has it been like in the past?
Ancient carbonates are enriched in 13C, suggesting that the biomass of the earth has remained unchanged over geological time scales and predating the origin of most modern life forms.
From Schidlowski et al., 1983
first microfossils
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Conclusion
The biomass and numbers of prokaryotic cells are large today and have been large for billions of years.
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Understanding Understanding prokaryotic prokaryotic diversitydiversity
The cellular abundance makes it possible to estimate of cellular production, the number of mutations, and other rare genetic events
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Annual cellular production of prokaryotesAnnual cellular production of prokaryoteshabitathabitat PopulatiPopulati
on sizeon sizeTurnover Turnover
time time (da)(da)
Cells yrCells yr-1-1
(x 10(x 102929))
marine marine heterotrophs,heterotrophs,
above 200 above 200 m m
3.6 x 3.6 x 10102828
1616 8.28.2
below 200 below 200 mm
8.2 x 8.2 x 10102828
300300 1.11.1
marine marine autotrophsautotrophs
2.9 x 2.9 x 10102727
1.51.5 7.17.1
soilsoil 2.6 x 2.6 x 10102929
900900 1.01.0
subsurfacesubsurface 4.9 x 4.9 x 10103030
550,000550,000 0.030.03
domestic domestic mammalsmammals
4.3 x 4.3 x 10102424
11 0.020.02
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Frequency of rare Frequency of rare genetic eventsgenetic eventsGiven high rates of production of prokaryotic cells, rare
genetic events would be common. For instance, if the mutation frequency is 4 x 10-7 cell-1 generation-1, the frequency of four simultaneous mutations in any gene shared by the population becomes high.
habitathabitat cells cells produced produced
yryr-1-1 (x (x 10102929))
frequency of four frequency of four simultaneous simultaneous
mutations (hmutations (h-1-1))
marine marine heterotrophs,heterotrophs,
above 200 above 200 mm
8.28.2 2.52.5
marine marine autotrophsautotrophs
7.17.1 2.02.0
soilsoil 1.01.0 0.30.3
subsurfacesubsurface 0.030.03 0.0090.009
domestic domestic mammalsmammals
0.020.02 0.0060.006
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yearsyears Total Total number of number of
cells cells producedproduced
Rare Rare genetic genetic eventseventsaa
OneOne 2 x 102 x 103030 44
ThousThousandand
2 x 102 x 103333 55
MillionMillion 2 x 102 x 103636 55
BillionBillion 2 x 102 x 103939 66
aLargest number of mutations likely to have occurred within the same gene and within the same generation
Rare genetic events are common over geological periods of time
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Is prokaryotic evolution Is prokaryotic evolution fundamentally different fundamentally different from eukaryotic from eukaryotic evolution?evolution?Most studies of evolution are
performed with eukaryotes, which possess very small populations and long generation times.
For prokaryotes, the timescale and the numbers of individuals are much larger. Could these factors lead to a fundamentally different process?
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Does the large Does the large number of number of prokaryotes imply a prokaryotes imply a high diversity?high diversity?Many microbiologists have longed believed
that most of the biodiversity on earth was prokaryotic. Qualitative evidence for this opinion comes from the tremendous diversity observed in energy metabolism, motility, and other prokaryotic processes.
The abundance of prokaryotes and high cellular production rates supports the hypothesis that rapid evolution and diversification is possible in the prokaryotes.
How can we compare the biodiversity between prokaryotes and eukaryotes?
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Congruence between DNA pairing and phenetic similarity (Jones and Sneath [1970] Bact. Rev. 34:40-81).
>70 % pairingwithin a species
Definition of species in
prokaryotes
Strains with >70 % hybridization (pairing) of their genomic DNAs or a Tm of hybrids of
their genomic DNAs of <5 oC constitute a species. A large amount of phenetic diversity is included within a species.
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Congruence between DNA pairing and phenetic similarity (Jones and Sneath [1970] Bact. Rev. 34:40-81).
>70 % pairing implies large genetic variation
Definition of species in
prokaryotes
From analyses of genomic sequences, this definition corresponds to >95 % ANI (average nucleotide identity) for >69 % of the DNA or >85 % of the protein encoding genes (Goris et al. 2007. IJSEM 57:81)
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What if we apply this What if we apply this criterion to criterion to eukaryotes?eukaryotes?Most primates would belong to the same
species.
From Sibley and Ahlquist (1984)
one species?
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Rate of 16S rRNA divergence
Based upon the divergence of 16S rRNA genes of bacterial endosymbionts of insects, 0.05 divergence is about 175 Ma (Ochman et al. 1999. PNAS USA 96:12638)
At this rate, most bacterial genera would be about 350 Ma
175
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A group of related alpha proteobacteria
crown gall
N2-fixing nodules
Agrobacterium, Sinorhizobium, and Rhizobium represent related organisms with similar life styles with an age of about 350 Ma.
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Sagittula stellata: a marine organism that transforms lignin as well as binds lignocellulose particles. Gonzalez et al. (1997) IJSB 47: 773-780.
Cells bound to lignocellulose particles
Radiolabel released from synthetic ligninafter 30 da.
A group of related alpha proteobacteria
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Silicibacter pomeroyi: a marine organism that transforms dimethylsulfoniopropionate to dimethylsulfide and methane thiol. Gonzalez et al. (2003) IJSB 53: 1261-1269.
Phase contrast and electron micrographs of DSS-3. Arrow indicates surface blebs formed from outer membrane.
A group of related alpha proteobacteria
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In fact they are all: heterotrophs, aerobes, auxotrophic for some amino acids and vitamins, and stain Gram negative
horsemouse
What if we applied these same criteria to eukaryotes?
Mammals and marsupials would be in the same genus
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In fact they are all: heterotrophs, aerobes, auxotrophic for some amino acids and vitamins, and stain Gram negative
horsemouse
What if we applied these same criteria to eukaryotes?
Mammals and marsupials would be in the same genus
Time since last common ancestor (Ma)
80 175
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How many species and higher taxa in nature?
Use rRNA gene sequence comparisons to get at this.
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In prokaryotes, the relationship of 16S rRNA sequence similarity (S) and DNA hybridization (D) can be described by a complementary log log plot (r=0.79; Keswani and Whitman, 2001).
These results suggest that D>0.70 when S = 0.998 P = 0.50S = 0.992 P = 0.95S = 0.986 P = 0.99
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Consensus of usage of taxonomic ranks for prokaryotes in Bergey’s
phylum
class
order
family
genus
95%
50%
Cumulative fraction of comparisons at each taxonomic rank with the specified 16S rRNA sequence similarity.
Dyszynski and Whitman, submitted
0.90
0.95
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Sequence data summary for an Sequence data summary for an rRNA gene library of soil bacteriarRNA gene library of soil bacteria
Libraries prepared : 42
Clones prepared : 4032Sequences obtained : 3719Chimeric sequences : 12Non-16S rRNA sequences : 1Sequences used for analyses :
3706Mean read length : 842 bpLess than 0.2 % of the clones possessed
>99.8 % similarity to a described species.
Less than 2.5 % of clones possessed >99 % sequence similarity to a described species.
From Jangid et al., unpublished
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0
500
1000
1500
2000
2500
0 1000 2000 3000 4000
Number of Sequences
Nu
mb
er
of
OT
Us
D = 0.01
0.03
0.05
0.10
Rarefaction curves fail to plateau even for deep phylogenetic groups.
Greater than 2000 OTUs more diverse than species. (D=0.01).
Even this fairly large effort on common soil failed to fully sample the bacterial taxa
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Prokaryotic diversity in natural habitats
Rarefaction analyses of rRNA gene libraries from soil in Georgia detect >400 OTUs with 0.90 divergence.
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Prokaryotic diversity in natural habitats
For 3706 sequences, 400 OTUs detected.Chao1 estimator: 669 (95 % COI 583-797).
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Prokaryotic diversity in natural habitatsImplies that soil contains large numbers of prokaryotic groups each with a diversity comparable to the placental mammals.
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ConclusionsConclusions
The high number of prokaryotes predicts a high capacity for genetic variation.
Although the biodiversity of the prokaryotes is not known, it is likely to be very high, possibly exceeding that of eukaryotes.