HC14 Symbiosis, Human
microbiome (BOOK)
Chapter 20.14, 23.3, 23.9, 24.1-24.11
CH20 Microbial Ecosystems
20.14 Hydrothermal vents
Volcanic hydrothermal vents are warm (5 to >50˚C) or very hot (270 to >400˚C) that emit
hydrothermal fluids. The diffuse fluids are emitted from cracks in the sea floor and exterior walls of
hydrothermal chimneys.
Black smokers form upright sulfide edifices called chimneys which form when acidic hydrothermal
fluids rich in metals and magmatic gases are mixed with cold oxygenated seawater.
Bacteria displaying chemolithotrophic metabolisms dominate hydrothermal vent microbial
ecosystems. Sulfidic vents support sulfur bacteria.
In contrast to bacteria, the diversity of volcanic hydrothermal vent archaea is limited.
CH23 Microbial Symbioses with Microbes, Plants, and
Animals
23.3 The Legume-Root Nodule Symbiosis
The partners in a symbiosis are called symbionts and most nitrogen-fixing bacterial symbionts of
plants are called rhizobia. Infection of legume roots by rhizobia leads to the formation of root
nodules in which the bacteria fix gaseous nitrogen. Modulated legumes can therefore grow well on
unfertilised bare soils that are nitrogen deficient.
Legumes can not fix nitrogen themselves, but rhizobia can. However, nitrogen fixation by
nitrogenase is inactivated by high levels of Ow. In the nodule, O2 levels are precisely controlled by
the O2-binding protein leghemoglobin.
There are major steps in the formation of a root nodule in a legume infected by rhizobium.
The roots of leguminous plants secrete organic compounds that stimulate the growth of a diverse
rhizosphere microbial community.
After attaching with help of an adhesion protein called rhicadhesin, a rhizobial cell penetrates into
the root hair which curls in response to substances secreted by the bacterium. The bacterium then
induces formation by the plant of a cellulosic tube, called the infection thread.
Bacteroids
, Rhizobia multiply rapidly within plant cells and become swollen, misshapen, branched cells called
bacteroids. A microcolony of bacteroids surrounded by the plants cytoplasmic membrane is called a
symbiosome, and only after this N2 fixation begins.
Nodule formation
Nod genes are rhizobial genes that direct the steps in nodulation of a legume. NodABC genes code
for Nod factors induce root hair curl in and trigger cell division in the pea plant. NodD encodes the
regulatory protein NodD which controls transcription of other nod genes.
Biochemistry of Root Nodules
Most leguminous plants for nodules on their roots but some form nodules on their stems. These are
widespread in tropical regions where soils are often nitrogen deficient. Some stem-nodulating
rhizobia produce bacteriochlorophyll a and are therefore able to carry out anoxygenic
photosynthesis.
23.9 Marine Invertebrates at Hydrothermal Vents and Cold Seeps
Macroinvertebrates including tube worms and large clams and mussels are present near
hydrothermal vents. They are able to exist in darkness because they receive nourishment through a
symbiotic association with autotrophic bacteria. The mutualistic chemolithotrophs are either tightly
attached to the animals surface (epibionts) or actually live within the animal tissues.
Most symbionts of marine invertebrates have small genomes indicating the reduced function and an
obligate association with their host.
CH24 Microbial Symbioses with Humans
The massive assemblage of microbes on the human body is called the human microbiome. A
microbiome can be defined as a functional collection of different microbes in a particular
environmental system. The term microbiota is used to reference to the types of organisms present in
an environmental habitat.
24.1 Overview of the Human Microbiome
The sites of the human body inhabited by microbes include the mouth, nasal cavities, throat,
stomach, intestines, urogenital tracts and the skin. The human body as the host and its associated
microbes are host-microbiome superorganism.
microbiome (BOOK)
Chapter 20.14, 23.3, 23.9, 24.1-24.11
CH20 Microbial Ecosystems
20.14 Hydrothermal vents
Volcanic hydrothermal vents are warm (5 to >50˚C) or very hot (270 to >400˚C) that emit
hydrothermal fluids. The diffuse fluids are emitted from cracks in the sea floor and exterior walls of
hydrothermal chimneys.
Black smokers form upright sulfide edifices called chimneys which form when acidic hydrothermal
fluids rich in metals and magmatic gases are mixed with cold oxygenated seawater.
Bacteria displaying chemolithotrophic metabolisms dominate hydrothermal vent microbial
ecosystems. Sulfidic vents support sulfur bacteria.
In contrast to bacteria, the diversity of volcanic hydrothermal vent archaea is limited.
CH23 Microbial Symbioses with Microbes, Plants, and
Animals
23.3 The Legume-Root Nodule Symbiosis
The partners in a symbiosis are called symbionts and most nitrogen-fixing bacterial symbionts of
plants are called rhizobia. Infection of legume roots by rhizobia leads to the formation of root
nodules in which the bacteria fix gaseous nitrogen. Modulated legumes can therefore grow well on
unfertilised bare soils that are nitrogen deficient.
Legumes can not fix nitrogen themselves, but rhizobia can. However, nitrogen fixation by
nitrogenase is inactivated by high levels of Ow. In the nodule, O2 levels are precisely controlled by
the O2-binding protein leghemoglobin.
There are major steps in the formation of a root nodule in a legume infected by rhizobium.
The roots of leguminous plants secrete organic compounds that stimulate the growth of a diverse
rhizosphere microbial community.
After attaching with help of an adhesion protein called rhicadhesin, a rhizobial cell penetrates into
the root hair which curls in response to substances secreted by the bacterium. The bacterium then
induces formation by the plant of a cellulosic tube, called the infection thread.
Bacteroids
, Rhizobia multiply rapidly within plant cells and become swollen, misshapen, branched cells called
bacteroids. A microcolony of bacteroids surrounded by the plants cytoplasmic membrane is called a
symbiosome, and only after this N2 fixation begins.
Nodule formation
Nod genes are rhizobial genes that direct the steps in nodulation of a legume. NodABC genes code
for Nod factors induce root hair curl in and trigger cell division in the pea plant. NodD encodes the
regulatory protein NodD which controls transcription of other nod genes.
Biochemistry of Root Nodules
Most leguminous plants for nodules on their roots but some form nodules on their stems. These are
widespread in tropical regions where soils are often nitrogen deficient. Some stem-nodulating
rhizobia produce bacteriochlorophyll a and are therefore able to carry out anoxygenic
photosynthesis.
23.9 Marine Invertebrates at Hydrothermal Vents and Cold Seeps
Macroinvertebrates including tube worms and large clams and mussels are present near
hydrothermal vents. They are able to exist in darkness because they receive nourishment through a
symbiotic association with autotrophic bacteria. The mutualistic chemolithotrophs are either tightly
attached to the animals surface (epibionts) or actually live within the animal tissues.
Most symbionts of marine invertebrates have small genomes indicating the reduced function and an
obligate association with their host.
CH24 Microbial Symbioses with Humans
The massive assemblage of microbes on the human body is called the human microbiome. A
microbiome can be defined as a functional collection of different microbes in a particular
environmental system. The term microbiota is used to reference to the types of organisms present in
an environmental habitat.
24.1 Overview of the Human Microbiome
The sites of the human body inhabited by microbes include the mouth, nasal cavities, throat,
stomach, intestines, urogenital tracts and the skin. The human body as the host and its associated
microbes are host-microbiome superorganism.
