(21) Environmental Effects on Growth, Other Environmental Factors

pH scale used to express

the acidity or alkalinity of a solution

pH = 7

neutral

pH < 7

acidic

pH > 7

alkaline or basic

pH scale is logarithmic so

change units indicates a ten-fold change in H+ concentration

most organisms have a range of

2-3 pH units of which growth is possible

most environments have a pH between

4 and 9 and most organisms prefer pH values of 6-8

neutrophiles:

organisms that grow optimally at pH values of 5.5 to 7.9

acidophiles:

organisms that grow best below pH 5.5

most are moderately

acidic but some can grow optimally below pH 3 or even pH 2

most acidophilus cannot grow at

pH 7 or 2 pH units greater than their optimum

membrane stability is critical; some organisms require

acidic conditions and lyse at neutral pH values

alkaliphiles =

organisms with growth optima at pH 8 or higher

alkaliphiles - typically found in

soda lakes and high-carbonate soils

alkaliphiles - often generate

a sodium motive force instead of a proton motive force to drive transport reaction and rotate flagella

alkaliphiles - interesting to study because of bioenergetic problems associated with trying to establish

a proton motive force when the external membrane surface is strongly alkaline (protons will spontaneously combine with hydroxyl ions to form water)

optimal pH for growth refers to

the pH of the extracellular environment

the intracellular pH must stay relatively close to the

neutral (pH 5-9) even if the external pH is highly acidic or basic in order to maintain macromolecules within the cell (DNA is acid-labile and RNA is alkaline-labile)

microbial culture media typically contain

buffers to maintain constant pH

water availability depends upon the

absolute water content of an environment and the concentration of solutes dissolved in the water; dissolved solutes bind water, making the water less available or organisms

water activity =

expression of water availability in physical terms; water activity varies between 0 (no free water) and 1 (pure water)

water diffuses from regions of

higher water concentration (low solute concentration) to regions lower water concentration (higher solute concentration) during osmosis

If the cytoplasm has a higher solute concentration than the environment, water will diffuse

into the cell → called positive water balance

in environments where the solute concentration exceeds that of the cytoplasm, water diffuses

out of the cell; can cause dehydration problems

seawater contains

~3% NaCl and many marine organisms require NaCl (cannot substitute other salts) and grow at the water activity of seawater

halophiles =

require some NaCl for growth

halotolerant organisms can tolerate a reduction in

water activity of their environment but grow best without the solute

extreme halophiles =

requires 15-30% NaCl for optimal growth

osmophiles =

organism that live in environments high in sugar as a solute

xerophiles =

organisms able to grow in very dry environments

if living in an environment with low water activity, cell can only get water by

increasing their internal solute concentration and this moving water in via osmosis

two mechanisms to increase internal solute concentration:

• pumping solutes into the cell from the environment

• synthesizing a solute

compatible solutes

solutes inside the cell most be non-inhibitory to macromolecules in the cell

compatible solutes typically consist of

water-soluble molecules such as sugars, alcohols, or amino acid derivatives

oxygen is poorly

soluble in water and can often become exhausted in bodies of water

oxygen classes of microorganisms:

microbes are grouped according to their need for or tolerance of O2

aerobes =

require oxygen to live

anaerobes =

do not require oxygen and may even be killed by exposure to O2

facultative organisms =

can live with or without oxygen

aerotolerant anaerobes =

can tolerate oxygen and grow in its presence even though they cannot use it

obligate anaerobes =

inhibited or killed by O2

microaerophiles =

can use oxygen only when it is present at levels reduced from that in air

aerobes need

extensive aeration (shaking, bubbling_

anaerobes need

oxygen excluded

reducing agents:

chemicals that may be added to culture media to reduce oxygen

• flush or consume oxygen

molecular oxygen (O2) is not

toxic but can be converted to toxic oxygen by-products that can harm or kill cells

• superoxide anions (O2-), hydrogen peroxide (H2O2), and hydroxyl radicals (OH*)

toxic oxygen by-products from oxygen all form via the reduction

of O2 to H2O in respiration so all organisms exposed to O2 will experience some of these toxic by-producys

catalase and peroxidase convert

H2O2 (hydrogen peroxide) to O2 and H2O

superoxide dismutase converts

2 O2- (superoxide anions) to H2O2 and O2

superoxide reductase (strict anaerobes) coverts

O2- (superoxide anions) to H2O2 without producing O2