Slide 1
Recall that most of the bacteria that exist on earth either go
unnoticed by us or are beneficial. In this tutorial, we are discussing
examples of bacteria that are beneficial - not only for us but for all life on
earth.
Slide 2
Nitrogen is an essential element for all living things.
Recall the structure of the macromolecules - nitrogen is found in the amino
acids and nucleic acids and in many vitamins.
While nitrogen is an abundant element on earth - it is found mainly as nitrogen gas which is not a biologically useful form on the element. The only organisms on earth that can use nitrogen gas are the nitrogen fixing bacteria. Nitrogen fixation is the process of converting nitrogen gas into ammonia. This ammonia is then converted by other organisms into other nitrogen containing compounds that can travel through the environment.
Slide 3
This movement of nitrogen-containing compounds through the
environment is known as the nitrogen cycle. You do not need to know the
details of the nitrogen cycle but you do need to recognize that if it were not
for the nitrogen-fixing bacteria - none of these nitrogen containing compounds
would be available for other organisms. Nitrogen-fixing bacteria make
life on earth possible.
Slide 4
The nitrogen-fixing bacteria live in association with the roots of
certain plants. Here, for example, we see the root system of a soybean
plant. The small nodules on the roots house the nitrogen-fixing
bacteria.
Plants in the legume family (which includes plants like peas, alfalfa, clover, and peanuts) are common partners with these nitrogen-fixing bacteria. This relationship is a mutualism - the bacteria get a place to live and a supply of sugars from the plants and the plants get a supply of nitrogen which is essential for plant growth.
Slide 5
Traditional farming methods involved planting legumes to help
maintain soil fertility. Fertile soils have a relatively high nitrogen
content - this nitrogen is consumed by plants and when we harvest the plants
we remove the nitrogen from the soil. Legumes can help return this
nitrogen to the soil. Here we see a planting of coffee shrubs grown
interspersed with a leguminous tree. The tree provides nitrogen to the
soil and this reduces the need for synthetic fertilizers.
Slide 6
Prokaryotes are also responsible for the oxygen we breathe.
Earth's early atmosphere was anaerobic and the first organisms were
chemoautotrophs that used the high energy chemical compounds that were
abundant on earth at the time. As the bacterial population increased,
the competition for these high energy compounds increased and the supply of
these nutrients ran low.
This resulted in the evolution of photosynthesis - using sunlight for energy rather than chemicals. We will talk about photosynthesis in more detail later in the course but for now you need to recognize that oxygen gas is a by-product of photosynthesis. As the population of photosynthetic bacteria grew - the amount of oxygen in the environment increased and the atmosphere became aerobic.
For earth's earliest inhabitants the appearance of oxygen was toxic. Recall that oxygen is highly electronegative - it attracts electrons. This electronegativity can damage cell membranes, proteins, and DNA by pulling electrons away and creating nasty compounds known as free radicals. Free radicals can cause even further damage to cell structures. Organisms that had evolved in an oxygen free environment had little ability to protect themselves from the damaging effects of oxygen. This dramatic change in the environment caused mass extinctions and marked a major change in the evolution of life on earth.
Slide 7
We can classify organisms based on their tolerance or need for
oxygen. Obligate aerobes are organisms that require oxygen for life - we
are obligate aerobes.
Obligate anaerobes are organisms that have no need for oxygen and may be poisoned by it.
Facultative anaerobes have the best of both worlds - they can use oxygen if it is present but they don't need it.