Requirements for Life George Lebo 23 October 2012 AST

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

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 ~85  C (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 ~85  C (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, CO 2, 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 70  F 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 H 2 S and bring it into tube, where billions of microbes live Microbes “digest” minerals with sulfur metabolism, releasing CO 2 byproduct Worm uses CO 2 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