Week 2

Today's Topics:
○ Origin of life on Earth & microbial evolution

Lecture Topic:
During the lecture, take notes here.
○ Life cannot exist without liquid water
○ The atmosphere was anoxic when life first evolved, so the first life must have been anaerobic.
○ Temperatures were higher, so early life must have been thermophilic, living in environments such as hydrothermal systems.
○ Photosynthetic bacteria were probably not the first organisms, but are thought to have evolved early. First Pb were probably not oxygen producing.
○ Archea are not photosynthetic.
○ PS thought to have evolved in purple-green bacteria.
○ Cyanobacteria developed oxygenic photosynthesis, about 2.5-3.0 billion years ago. By 2.5 to 1.5 billion years ago free oxygen was present in atmosphere, as a result.
○ This had a profound effect - aerobic respiration provides massively more energy than anaerobic, used by almost all life today.
○ Also lead to development of ozone layer, reducing UV at surface and its mutagenic effects.
○ This permitted evolution of more complex forms of life.
○ Anaerobic prokarya: 4.0 billion years ago.
○ Anoxygenic photosynthetic bacteria: 3.0-3.5 billion years ago.
○ Eukarya appeared about 2 billion years ago
○ Microbial Nutrition
○ Energy: Light, inorganic, or organic compounds?
○ Carbon: CO2 or organic compounds
○ Photoautotrophic: obtain energy from light, and carbon from CO2.
○ Photoheterotrophic: obtain energy from light, and carbon from organic compounds. (Cannot fix from CO2.)
○ Chemoautotrophic: Energy from oxygenation of inorganic compounds, egg ammonia or H2S, and carbon from CO2.
○ Chemoheterotrophic: obtain both energy and carbon from organic compounds.
○ Possible early metabolic types
○ Oparin argues that initial metabolic type was simple heterotrophic bacterium. - only needs a few enzymes.
○ Autotrophic organisms are more complex, as they need multiple pathways, and so would have evolved later.
○ Others argue that early life forms may have been hydrogen bacteria, obtaining energy from oxidation of hydrogen. Both bacteria and archea types are known, using CO2 for carbon.
○ Many of these are anaerobic, and so could have fit in with early conditions.
○ Temperature and growth
○ Cardinal temperatures:
§ Optimum: fastest replication: range 0-75C
§ Minimum: no growth below: enzymes inactive, membranes solidify; al low as -20C
§ Maximum: no growth above: cell may die - enzymes denatured, membranes break down; as high as 120C.
§ Pressure can permit fluid water at unusual temps.
○ Upper limit for protozoa: 50C
○ Upper limit for algae & fungi: 55-60C
○ Classification by temp:
§ Psychrophile: Optimum temperature <15C
§ Psychrotroph (facultative psychrophiles): 20-30C eg: fungi, etc.
§ Mesophiles: 20-45C eg most human pathogens
§ Thermophiles: 55-65C
§ Hyperthermophiles: 80-113C
○ Oxygen and growth
○ Obligate aerobes - completely dependent on oxygen - includes almost all multicellular organisms
○ Facultative aerobes - do not need oxygen, but grow better with it.
○ Aerotolerant anaerobes - grow equally well with or without oxygen
○ Obligate anaerobe - killed by oxygen.
○ Microaerophiles - damaged by normal concentration of oxygen (20%)
○ pH and growth
○ Habitats range fro 0.2-2.0 at the acid end to 9-10 at the alkaline.
○ Acidophiles optimum 0-5.5
○ Neutrophiles optimum 5.5-8.0
○ Alkaliphiles optimum 8.0-11.5
○ Eg fungi prefer 4-6; malt extract agar is used to culture.
○ Most bacteria and protozoa are neutrophiles.
○ Osmotic concentration
○ Nonhalophile:adversely affected by high concentration.
○ Halotolerant: Able to grow over a range of concentrations.
○ Moderate Halophile: prefers high levels of NaCl - above 0.2M to grow. (eg Staphyllococcus)
○ Extreme Halophile: requires high levels - 2M to saturation (about 6.2M)
○ Microorganisms and Ecosystems
○ Primary producers: eg Algae - make organic matter from CO2 and water.
○ Primary consumers: eg Protozoa - feed on bacteria and fungi.
○ Decomposers: eg Fungi - break down organic matter into inorganic compounds.
○ Prefered conditions:
○ Protozoa prefer wet, fungi prefer dry.
○ Some prefer rhizosphere (area around roots) - eg nitrogen fixing bacteria (no eukarya can fix nitrogen)
○ Symbiosis: eg e. coli in gut -make vitamins that we cannot.
○ Impact of Bacteria and Archea on biogeochemical cycles
○ Carry out significant reactions that are crucial to operation of biosphere
○ Nitrogen, iron, sulphur, carbon cycles.
○ Carry out steps that no eukarya can.
○ Carbon cycle: carbon fixation (photosynthesis), methanogenesis, CO oxidation.
○ Nitrogen cycle: nitrogen fixation, nitrification, nitrate reduction, mineralisation.
○ Microbial Interactions
○ Mutualism: eg aphid & buchnera aphidicola - cannot reproduce independantly - 60-80 bacterisides, containing buchnera, which produce nitrogen compounds, vitamins, and amino acids not provided by diet.
○ Tube worm - trophosomes contain microorganisms that oxidise H2S and fix CO2.
○ Rumen ecosystem - read up for coursework.
○ Commensalism:eg e.coli in the gut - one gets advantage, other is neutral (some list as mutualistic - we get vitamin b from e coli)
○ Parasitism one is at expense of other. (pathogens)
○ No archea are parasitic
○ Predation: one eats another - bdellovibrio, vampirococcus, daptobacter.