17.2 What Are the Functions That Microbes Provide
to Benefit Their Hosts?
Microbial communities provide many different functions to their hosts. In this
section, I outline the relationship between the diversity of microorganisms and
their functional output. In an effort to organize this information, I classify microbial
functions into four major categories based on the contribution they make to the host
fitness. Microorganisms can (1) supply scarce nutrients, (2) prevent disease, (3) facilitate morphogenesis and development, and (4) extend the host’s phenotype. There
are other ways to classify microbial function in the context of host associations (see,
for example, McFall-Ngai et al. 2013, Christian et al. 2015), and this is not meant to
be a review of the extensive literature. Instead, by examining some select studies, I
aim to show the widespread range of species-rich and species-poor microbial
communities that provide hosts with function.
17.2.1 Nutrient Provision
Nutrient provision has been documented for a long time and includes the acquisition
of environmental nutrients by plant roots and the transformation of ingested nutrients
in the animal gut (Hacquard et al. 2015). Procuring more and typically inaccessible
nutrients can increase individual host’s fitness by increasing reproductive outputs,
population persistence (by increasing growth or reducing mortality), and community
diversity for plants and animals (by reducing competition).
Overall, the rhizosphere and root endophytic microbial community, including
bacteria and fungi, are species rich and contribute to plant growth, productivity, and
carbon sequestration through nutrient acquisition (Philippot et al. 2013; Bulgarelli
et al. 2013). Rhizosphere diversity and function are particularly well-studied in
agricultural crops, where it is clear that diverse microbial communities contain the
functional capabilities to transform nutrients, breakdown compounds toxic to the
plants, and respond to environmental fluctuations (Xu et al. 2018; Yurgel et al.
2019).
Two important plant–microbe systems central to rhizosphere studies are the
rhizobia–legume (Sprent et al. 1987) and mycorrhizae associations (Martin et al.
2017). In the rhizobia–legume association, nitrogen-fixing bacteria synthesize
ammonia from atmospheric nitrogen which the plant obtains in exchange for carbohydrates and mineral nutrients (Kiers et al. 2003). Although plants can be infected
with multiple strains of rhizobia, their interaction seems to be mostly antagonistic,
resulting in lower plant productivity (Barrett et al. 2015). On the contrary, diversity
in mycorrhizal infections seems to result in functional complementarity. In exchange
for carbohydrates from the plant host, each species of fungi extends the reach of
plant root systems in different ways, collectively improving access to nitrogen and
phosphorus for the plant (Ferlian et al. 2018). Both of these host–microbe
300
C. Cuellar-Gempeler
to Benefit Their Hosts?
Microbial communities provide many different functions to their hosts. In this
section, I outline the relationship between the diversity of microorganisms and
their functional output. In an effort to organize this information, I classify microbial
functions into four major categories based on the contribution they make to the host
fitness. Microorganisms can (1) supply scarce nutrients, (2) prevent disease, (3) facilitate morphogenesis and development, and (4) extend the host’s phenotype. There
are other ways to classify microbial function in the context of host associations (see,
for example, McFall-Ngai et al. 2013, Christian et al. 2015), and this is not meant to
be a review of the extensive literature. Instead, by examining some select studies, I
aim to show the widespread range of species-rich and species-poor microbial
communities that provide hosts with function.
17.2.1 Nutrient Provision
Nutrient provision has been documented for a long time and includes the acquisition
of environmental nutrients by plant roots and the transformation of ingested nutrients
in the animal gut (Hacquard et al. 2015). Procuring more and typically inaccessible
nutrients can increase individual host’s fitness by increasing reproductive outputs,
population persistence (by increasing growth or reducing mortality), and community
diversity for plants and animals (by reducing competition).
Overall, the rhizosphere and root endophytic microbial community, including
bacteria and fungi, are species rich and contribute to plant growth, productivity, and
carbon sequestration through nutrient acquisition (Philippot et al. 2013; Bulgarelli
et al. 2013). Rhizosphere diversity and function are particularly well-studied in
agricultural crops, where it is clear that diverse microbial communities contain the
functional capabilities to transform nutrients, breakdown compounds toxic to the
plants, and respond to environmental fluctuations (Xu et al. 2018; Yurgel et al.
2019).
Two important plant–microbe systems central to rhizosphere studies are the
rhizobia–legume (Sprent et al. 1987) and mycorrhizae associations (Martin et al.
2017). In the rhizobia–legume association, nitrogen-fixing bacteria synthesize
ammonia from atmospheric nitrogen which the plant obtains in exchange for carbohydrates and mineral nutrients (Kiers et al. 2003). Although plants can be infected
with multiple strains of rhizobia, their interaction seems to be mostly antagonistic,
resulting in lower plant productivity (Barrett et al. 2015). On the contrary, diversity
in mycorrhizal infections seems to result in functional complementarity. In exchange
for carbohydrates from the plant host, each species of fungi extends the reach of
plant root systems in different ways, collectively improving access to nitrogen and
phosphorus for the plant (Ferlian et al. 2018). Both of these host–microbe
300
C. Cuellar-Gempeler