MICR 2123 Lecture Notes - Lecture 16: Nitric Oxide, Aphotic Zone, Soil Microbiology

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23 Mar 2017
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Microbes in Global Elemental Cycles
Microbes in Ecosystems
Ecosystem: consists of populations of species plus their habitat or environment
The interaction between microbes and their ecosystems include
o Assimilation: process by which organisms acquire an element to build into cells from an
inorganic source
CO2 fixation, N2 fixation
Organisms that do this are primary producers
Their role is absorbing energy from outside the ecosystem (phototrophs) and
assimilating minerals into biomass (autotrophs)
o Dissimilation: process of breaking down organic nutrients to inorganic minerals (CO2)
Examples are respiration and fermentation
Organisms that do this are consumers or decomposers
o Operates as a cycle
To obtain energy and materials for biomass (building bodies of living organisms) all organisms
participate in the food web
o Levels of consumption in a food web are called trophic levels
o Besides the primary producers, there are
Consumers: feed on other organisms or organic matter
Decomposers: break down dead or decaying organisms
Microbial symbiosis
Symbiosis: an intimate association between organisms in different species
o Mutualism: when 2 organisms benefit and may fail to grow independently
Lichens are formed by a mutualistic relationship between an alga (or cyanobacteria)
and a fungus
Algae/cyanobacteria provide fixed carbon
Fungus provides protection
Cyanobacteria also fix nitrogen
Syntrophy
o Termites have endosymbiotic bacteria and archaea in their gut that helps then digest
cellulose
o What happens?
Bacteria of termite gut break down cellulose and ferment the sugars to make acetic
acid, lactic acid, CO2, H2
Bacteria make ATP, acetate feeds the termite
If the CO2 and H2 accumulate they inhibit bacterial fermentation
Methanogens use CO2 and H2 to produce CH4
In the absence of methanogens, CO2 and H2 fermentation cannot grow
In absence of fermentative bacteria, methanogens can not grow because will not get
their substrate for growth (CO2 and H2)
o This kind of mutualism where there is metabolic association requiring two or more
partners to complete the metabolism is called syntrophy
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Marine and aquatic microbiology
Ocean zones
o In the open ocean, the water column (pelagic zone) is divided into distinct regions
Neuston
Air water interface
Contains the highest microbe populations
Euphotic zone
Receives light so phototrophs grow
Aphotic zone
Below the reach of light
Heterotrophs and lithotrophs grow
Benthos
Ocean floor plus sediment below the surface
Thermal vent communities
Trophic levels in the ocean
o Phototrophic primary producers are called phytoplankton (cyanobacteria and algae)
o Consumers are heterotrophic bacteria and archaea
o Decomposers are viruses
Soil microbiology
In well drained soil, top layers are full of oxygen so support aerobic respiration
Lower layers are water saturated prolonged water saturation generates anoxic conditions, so
fermenters, anaerobic lithotrophs, and anaerobic respiring organisms grow
Trophic levels in soil
o Primary producers: green plants
o Consumers: protists and fungi
o Decompsers: fungi and bacteria
Mutualistic relationship
o Mcyorrhizae
Fungal mycelia associating with plant roots
Provide minerals to plants sometimes form meters away
Receive energy rich products of photosynthesis
2 different kinds
Ectomycorrhizae
o Fungal mycelia never penetrate the root. Form mantle on the root
surface and extend mycelia away from the root to absorb nutrients
Endomycorrhizae
o Fungi penetrate plant cells forming arbuscle within root cell
o Arbuscular mycorrhizae
Exist entirely underground
Glomus
Microbial communities within plants Rhizobia fix nitrogen for legumes
o A form of plant-bacteria mutualism
o Rhizobia enter plant cells and differentiate into bacteroids with no cell wall
Plant supplies nutrients and minerals
Bacterium supplies fixed nitrogen
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