Astronomy 294
Life in the Universe
Prof. Scott Gaudi

Lecture 5: Life on the Edge

Key Ideas

Key Ideas

Life appears to thrive in almost every possible environment

Organisms living in extreme conditions are Extremophiles

--Thermophiles and hyperthermophiles (temperatures >100? C)

--Psychrophiles (temperatures -20? C)

--High pressure, radiation, salinity, acidity, alkalinity?

--Endoliths (living in rock)

The limits of life and the driest place on Earth: the Atacama desert

Carbon is unique; silicon-based life is probably not viable

Water is a uniquely good solvent for many reasons

We can imagine very bizarre life

Typical conditions for life on Earth

Temperature

-15-80º C (59 -176º F)

Pressure

-1 atm

Salinity

-Seawater - 3.5% salt.

pH level

-pH~8

Radiation

-6,000 rad/hr

Location

-Land or sea (on the surface or in liquid)

Extremophiles (versus mesophiles)

Either thrive in, or just tolerate, extremes

Different stages do better (e.g., dormant endospores)

All three domains: archaea, bacteria, eukarya

But: no known hyperthermophilic eukaryotes

Temperature Extremes - Thermophiles

Why too hot is bad

--Many problems

--Structure of proteins

--Solubility of oxygen and carbon dioxide drops

--Chlorophyll degrades and stops working

Thermophiles

--Hot springs

--Deep sea vents

--Pyrobolus fumarii (113 ºC)

How they survive.

--Unique proteins that are sturdier

Temperature Extremes - Thermophiles

Prokaryotes tend to be heat tolerant

Earliest life was prokaryotic

First life?

Temperature Extremes - Psychrophiles

Why too cold is bad

--Freezing causes expansion

--Increase in viscosity

Psychrophiles

--<15 ºC

How they survive.

--Lower freezing point of water (salts and other solutes)

--Form spores

Large biomass of psychrophiles

Salt Extremes - Halophiles

Why too much salt is bad

--Strong osmotic pressure desiccates cells (draws water out)

Most organisms tolerate ocean water

Halophiles

--Survive in 10 times salt concentration as ocean

--Great Salt Lake, Dead Sea

How they survive.

--Use ions to reduce osmotic pressure

Implications

--Salt seas on early Mars, Europa today

pH Extremes - Acidophile

Low pH

--Acidic

--Accepts electrons

High pH - alkaline

--Basic

--Gives electrons

pH is a measure of acidity

--Water (by definition) pH=7

--Sea water pH=8

--Blood pH=7.5

pH Extremes - Acidophile

Why too much acidity is bad

--Change structure of the protein

Tolerances of most organisms

--pH > 4 (ocean 8.2)

Acidophiles

--Ferroplasma acidarmanus

--pH = 0 or 1 (battery acid)

How they survive.

--Neutralize the inner cell

--Evolve hardy proteins

Radiation extremes - Radioresistant

Earth is protected from radiation:

--Atmosphere

--Ozone layer

Why too much radiation is bad

--Directly damages the molecules

--DNA particularly suceptible

--Render DNA unable to reproduce, code corrupted

Radiation extremes - Radioresistant

rad = energy input per kg of cells.

Background 0.5 rad/yr

Immediate death: 2,000-5,000 rad/hr

Deinococcus radiodurans

--6,000 rad/hr

--1.5 Mrad burst

--Fully repaired in 24 hours

How they survive.

--Repairs DNA much better

--4-10 copies of the genome

The good and the bad

--Good: hard to kill

--Bad: hard to kill

Rock Dwellers - Endoliths

Where they have been found

--As deep as 3 km

Difficulties:

--High temperature

--High radiation

--Few nutrients

--High pressure (1,000 atm)

autotrophs

The biomass of rock dwellers

--May be more than all life on Earth!

The Driest Place on Earth

Atacama Desert (Chile, South America)

--100 times more arid than Death Valley

"Double rain shadow"

--Rain blocked on both sides by Andes and coastal mountains

How dry is it?

--No glaciers

--Some weather stations have never recorded rain

--May not have had any significant rainfall for 400 years!!

Limit of Life?

Scientists collected samples from the driest region of the Atacama

--No culturable bacteria

--Two samples had no DNA.

But! Only sampled the surface (10 cm)

Samples by another group down to 30 cm found life!

Place that had not harbored plant life for more than a million years.

Life seems to be able to exist even in the driest environments on Earth.

Panspermia

Surveyor 3 - unmanned probe that landed on the moon

--Apollo 12 astronauts brought back a camera

--Contained 50 to 100 viable specimens of Streptococcus mitis,

--The microbes must have been on the probe since it departed the Earth

--Survived 31 months in the absence of air or water

--Subjected to huge monthly temperature variations and bombardment by ultraviolet radiation from the Sun.

Can life survive interstellar space?

--Existence of radioresistant, thermophilic, high-pressure organisms

Ballistic panspermia

--Rocks from Mars

Directed panspermia?

Panspermia is a possible explanation for left-handed preference of proteins.

Alternatives to Carbon Chemistry

All Hail Carbon

Has 6 protons (and 6 neutrons)

Wants 6 electrons.

Two form a complete shell.

Four are available for bonds with other elements.

All Hail Carbon

--Carbon is very versatile.

--Able to form a vast variety of structures.

--Carbon compounds dissolve in liquid

--H+C = hydrocarbons

All Hail Silicon?

Has 14 protons (and 14 neutrons)

Wants 14 electrons.

Two form one complete shell.

Eight form another complete shell

Four are available for bonds with other elements.

--SiH4

Silicon Based Life?

Bonds between two silicon atoms are only 1/2 as strong as C-C bonds

Si-H and Si-O bonds are stronger than Si-Si bonds

Thus Silicon does not like to form long chains (e.g. cyclic molecules)

Problem is Silicon's affinity for Oxygen.

--Forms SO2 rather than SH4

--SO2 is very hardy!

Carbon compounds appear more natural

Life Without Water?

Water: the ideal solvent

Solvent is needed for:

--Carries nutrients in

--Carries waste products out

--Provides a 'medium' for chemical reactions

--Provide thermal balance

--Liquid seems better than gas (e.g., large heat capacity)

Water works so well because

--It is abundant.

--It is liquid over a large range of temperatures.

--Liquid at temperatures where reaction rates are high (but not too high).

--Dissolves chemical compounds.

--Large heat capacity.

--Less dense when it freezes.

--High surface tension (high than any other known liquid).

Life on Neutron Stars?

Collapsing cores of highly evolved stars

--Temperatures of 1 million Kelvin

--Gravitational force 1 trillion times larger

--Nuclei governed by strong force,

--Survive for 10-15 of a second

--Motions of 1000 km per second!

--Move distance equal to its own size in 10-21 seconds

--Density about equal to that of a nucleus

--Packed very close - collide with each other every 10-21 s

Can have million of collisions before they decay.

Evolution has time to operate

--Everything is sped up -

--Life evolves in 10-5 of a second!

Difficult to communicate

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.

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