Ecosystems are communities of organisms, including their physical
and chemical environments.
Habitat - environment (physical
place) in which an organism lives; microbial activities are
affected by biological (other organisms and their by-products) and
physical (temperature, pH, light, nutrient quality and quantity,
oxygen level, etc.) environmental factors
Niche - abstract concept describing
all environmental conditions needed for an organism to maintain a
viable population, specifying amounts of each resource required to
do so (not a physical place)
Symbiosis
Symbiotic relationships are those in which two or more dissimilar
organisms live in close association; may be endosymbiosis or
ectosymbiosis
Parasitism
one organism (parasite) benefits, while the other (host) is
harmed
example - infections that lead to damage of host tissues by
the parasite, which causes disease (occurs in plants as well as
animals)
Commensalism
association in which one organism (commensal) benefits,
while the other (host) is neither helped nor harmed
example - Clostridium and other microbes live in the
human colon where they benefit from nutrients, warmth and
shelter, but generally do not cause disease (normal
microbiota)
Mutualism
association in which both organisms (mutuals) benefit
examples:
Escherichia coli lives
in the human colon and benefits from nutrients, warmth and
shelter found there, generally not causing disease, but
benefiting the host by producing vitamins and preventing
growth of harmful microbes
lichens
- mutualistic
associations between a mycobiont (ascomycete) and a
phycobiont (green alga or cyanobacterium)
mycobiont provides a stable environment (protects
from excessive light, supplies water and minerals,
provides a firm substrate) within which the phycobiont
can grow somewhat free from environmental stress
phycobiont provides oxygen, organic carbon, other
nutrients for both microbes
mostly ectosymbiotic, although some fungi obtain
nutrients from their phycobionts using haustoria
(projections of hyphae that penetrate the phycobiont cell
wall)
protozoan
symbiosis with bacteria and archaea
Personympha (a protozoan that lives in the gut
of termites) can digest cellulose only because it has
cellulase-producing endosymbiotic bacteria
Metopus contortus (an anaerobic cilliated
protozoan) has a number of symbiotes
two endosymbiotes
hydrogenosomes - degenerate bacteria that use
pyruvate and NADH (both generated via glycolytic
catabolism of the cilliate) to generate
CO2, H2 and acetate
methanogens - use CO2 and
H2 generated in the hydrogenosomes to
generate methane
one ectosymbiote - sulfate-reducing bacteria
oxidize the acetate generated in the hydrogenosomes to
obtain energy via anaerobic respiration and reduce
sulfate in the process
Strombidium purpureum - anaerobic protozoan
that has photosynthetic purple sulfur bacteria living in
its cytoplasm and furnishing it with energy
rumen
ectosymbiosis - cattle, deer, camels, sheep, goats,
giraffes have a four-compartment "stomach" (rumen) in which
cellulolytic bacteria, fungi and protozoa (~1010
cells per mL) live
large amounts of grass, etc. (containing insoluble
polysaccharides and cellulose) are quickly mixed with
saliva by perfunctory chewing, then swallowed into the
rumen where the food is churned until it becomes a
pulpy mass and mixed with the large microbial population
which partially digests and anaerobically ferments the
food, generating fatty acids (absorbed by bloodstream;
used for gluconeogenesis in liver) and vitamins as well
as carbon dioxide and methane (2 liters of methane per
minute are released by eructation)
the food later moves into the reticulum, from
which it is regurgitated as a "cud," thoroughly chewed
(mixing it with more saliva) and swallowed into the
rumen
this process continues until the food becomes liquid
and flows into the omasum (water absorbed), then
into the abomasum (gastric stomach) where the
animal's own enzymes digest the microbes from the rumen
(supplying amino acids, sugars, etc.) and the remaining
food nutrients in the "normal" mammalian way
This is the immediate environment of very small organisms.
Surface Attachment
Microbes are generally attached (via glycocalyx) to:
stones, etc. - especially in moving water
other living things (frequently symbiotic) - plants or
animals
dead organic matter (saprophytic)
Nutrients
Nutrients are frequently adsorbed to surfaces; their
concentrations are frequently 100-1000X lower than in common
laboratory culture media.
Growth Rates
Growth rates are much lower on average in nature than in the
laboratory; characterized by "spurts" of exponential growth
interspersed with long periods of stationary conditions when
nutrients are to dilute for fast growth.
Competition
Competition is due to high population densities and short
generation times.
competition arises when two or more organisms need the same
resource and that resource is present in limited amounts
organism with the shortest generation time frequently "wins"
by outgrowing and thus replacing its competitors; this may be
mediated by inhibiting the growth of other organisms by secreting
antibiotics, toxic substances, etc. into the environment
local diversity is limited by competition
Food Chains
A food chain is a series of organisms through which energy
flows (a food web is a network of interlinked food chains):
producers - organisms that generate organic compounds
using inorganic compounds as sources of carbon, nitrogen,
phosphorus, etc. and light (phototrophs) or inorganic chemicals
(lithotrophs) as sources of energy
consumers - organisms (organotrophs) that obtain carbon
and energy from organic compounds made by other organisms
herbivores "prey" upon producers
carnivores prey upon herbivores
omnivores prey upon producers, herbivores and carnivores
decomposers (generally organotrophic microbes)
recycle some carbon and energy to other organotrophs
(via organic compounds) and inorganic compounds to all
organisms (mineralization of organic compounds) via biogeochemical
cycles
Communication
Microbes communicate with one another via chemical messages.
