Requirements for Life

George Lebo

23 October 2012

AST 2037

1

2

Life: What is it?

• Things with the ability to reproduce AND the ability to evolve and adapt

• Why both of these?• Flames can spread or “reproduce”, but they aren’t alive• Crystals (i.e. salt) can also spread or grow, but they aren’t alive

either• Only living things evolve – meaning develop adaptations to

their environment that improve their ability to continue

2

Implications• Need: an energy source• Something to power the “doing” of things• Including reproduction

• Need: means of reproduction• Access to material components of life• Way of passing on the information about the structure of

life (“genetic code”)

3

Energy Source• Want something easy to make, easy to store, capable of

making things happen in a “typical” environment• Options:• Nuclear energy? (Requires 10,000,000K and high

pressure)• Solar energy? (Hard to store light)• Thermal energy? (tends to “leak” out; hard to store)• Kinetic energy? (hard to store)• Chemical energy? Works!!

4

Genetic Code• Need lots of ability for variation in the code (especially if

adaptation/evolution are important)• Need ways of “writing” and “reading” code• Likely solution: chemical coding (like DNA)• Need large/complex chemical molecules

• What element is really good at making complex chemical molecules?

5

Medium of Life• Solids? Chemical reactions are very slow in most solids• Gas? Chemicals are often (not always) easily dispersed in

air/gas• Liquid? Chemical reactions can proceed quickly, while

density of reacting materials stays high

6

Solvent• Chemical that can break

apart solids into liquid phase

• Chemical that can separate and mix apart many complex structures into the liquid phase

• What is the best solvent known in the world?

• (Not molecular acid)

7

Summary• Need energy source and reproductive code• Likely energy source: chemical energy• Reproductive code: likely chemical, and requires complex

molecules/chains• A little weaker: May have a preference for liquid phase?• Probably need a powerful solvent

• At the risk of seeming Earth-centric: carbon does a great job of storing chemical energy and forming complex molecules suitable for reproduction; water is a GREAT solvent

8

Extreme Life on Earth

George Lebo

23 October 2012

AST 2037

9

Life on Earth• So far, we have focused on “normal” life on Earth• The sort of standard critters, plants, and bacteria we are used

to• We will use this as a standard “baseline” for evaluating

conditions for life to develop elsewhere• But …

10

The Goldilocks Syndrome• Earth is “just right” for this

sort of life• Conversely, standard life is

“just right” for Earth• Does that mean that life can

ONLY be that way?• Or is it just that, because we

live on Earth, we mostly see “Earth-standard” life?

11

“Extreme” Life on Earth• There are forms of life on Earth which seem “extreme”

compared to standard life• These forms of life show how far life deviates from

“normal” and still survives and reproduces• This gives us some idea of the limitations of life in the

Universe (at least Earth-like life)

12

Extreme Life: Aquifex Aeolicus

• In the 1960’s, biologists were interested in studying “how extreme” life could be

• They knew that microbes lived in water downstream from hot springs in Yellowstone National Park

• The springs themselves reached temperatures of ~85C (185 F) – near the boiling point of water

• The question: How far upstream (close to the hottest water) could microbes survive? 13

High Temps: So What?• What’s the Big Deal about life

at high temperatures?• Experience says that putting

living creatures in boiling hot water kills them

• Mmmmm … lobster!• How?• Denaturing of the proteins• High heat causes proteins

to lose some of their structural/chemical properties

• Breaks down the structure of the living cells

14

Aquifex Aeolicus Surprise• Biologists discovered

bacteria in the hottest parts of the hot springs themselves

• These creatures survive – even thrive and reproduce!! – at ~85C (185 F), near the boiling point of water

• Picture shows microbial mats (as in stromatolites) in Yellowstone hot spring

15

Aquifex Aeolicus Properties• These are very small bacteria• Prokaryotes• Genome structure is only 1/3 as long (complex) as E. coli (a

model “simple” bacteria)• Single DNA molecule in a circular chromosome

16

Aquifex Aeolicus Metabolism• A. aeolicus survives from H,

O, CO2, and mineral salts• Requires oxygen for

respiration (so, not that primitive)

• But … no need for sunlight, nor sunlight-using food !!

• Purely chemical food source (in the presence of thermal energy from the water)

17

The colors of Prismatic Spring in Yellowstone come primarily from the hyperthermophile microbes in it

Archaea• Genetic diversity studies show

that A. aeolicus is one of the most “divergent” bacteria known

• I.e. it has little in common with many of the other bacteria

• This and others led to the re-classification of 3 “Domains” of life on the basis of genetic linkage: • Archea• Bacteria• Eukaryota

18

Archaea• Very small critters (~1 micron in length)• No nucleus (like bacteria)• Different tRNA from bacteria and Eukaryotes (which have

same tRNA as each other)• Cell structure LOOKS like other cells, but made from

different chemicals• All bacteria/eukaryotes use D-glycerol isomers; Archaea only

use L-glycerol

19

Archaea & Extremophiles• Archaea are typically “primitive” organisms• Most single-celled “extremophiles” are members of archaea

20

Chemosynthesis• Energy generation NOT dependent on sunlight• Often (but NOT always) depend on other critters• A. aeolicus survives by pure chemosynthesis (no

photosynthesis; no eating other life forms)• Types of chemosynthetic life:• Methanogens (Methane)• Halophiles (Salt)• Sulfur reducers• Thermoacidophile (i.e. Aquifex aeolicus)

21

Methanogens• Things that use chemosynthesis to survive, and produce

methane (CH4) as a by-product• Well-known examples:• Swamp gas bubbles (methanogen byproduct)• Flatulence (bovine, human) – mmmm … Tijuana Flats!

• Methanogens typically only thrive (and only survive for long) in environments where other “chemically aggressive” elements (like O) are rare

• Methanogens have been found thriving as slime mats on deep rocks below Earth’s surface (endoliths)

• Also found in extreme cold/dry desert environments

22

Halophiles• Microbes that survive by

chemosynthesis in VERY salty water (i.e. 5x to 10x that of ocean water)

• Locations:• Great Salt Lake (Utah)• Dead Sea (Israel/Jordan)• Owens Lake (California)• Evaporation estuaries in

San Francisco Bay

23

Black Smokers – Sulfur Reducers

• Black smoker vents• Found in deepest parts of the

ocean• Volcanic, mineral-enriched

water outflows• Rich in iron, sulfur

compounds• Very little/no oxygen• Discovered in the 1970s• Temps as high as 750 F (!!)• Does not boil, though, due to

extreme pressure at this depth

24

Black Smoker Structure

25

Black Smoker Ecology• Deep sea exploration vehicles investigate black smokers in

the 1980’s• Much to everyone’s surprise, they find LIFE !!

26

Black Smoker Ecology• Not just life – fully-developed ecosystems!• Crabs, shrimp, clams, Pompeii worms

27

Pompeii Worms• Tube worms anchored near black smoker vents• Bottom end has very high temps; top end more like 70F• Hot water flows through tubes; length as much as 10 feet!

28

Pompeii Worms• “Hairy” back is heat-resistant microbe mat (symbiotic with

worm mucus)• Red “feathers” include hemoglobin; separates hydrogen

sulfide from vent flow

29

What feeds the ecosystem?• Sulfur-reducing extremophile archaea!• Metabolism centers on hydrogen sulfide (not oxygen, nor

CO2!)• Pompeii worms (and some clams) seem to have symbiotic

relationship with microbes• Worm “feathers” gather H2S and bring it into tube, where

billions of microbes live• Microbes “digest” minerals with sulfur metabolism, releasing

CO2 byproduct• Worm uses CO2 to digest minerals as well• Other life forms live on microbes, worms, etc.• Worms may live as long as 200+ years (!)

30

Summary• Life is weird• Extremophiles are found everywhere from petroleum

reservoirs to the Dead Sea to hot springs to deep sea vents• Most single-celled extremophiles are Archaea• Genetically distinct from eukaryota and bacteria• tRNA differences and chemical differences too

• Metabolism may be oxygen-independent (even oxygen-phobic!)

• Black smoker ecosystems show tremendous diversity, with basis in (and symbiotic relationships with) sulfur-reducing Archaea

31