12: Microbial Diversity - Role of Microbes in Nutrient Cycling

Microbial Diversity BR19920 - Lecture 12: Role of Microbes in Nutrient Cycling

MCQ Strategy

• Removing obviously wrong answers increases the probability of guessing correctly.
  • Example: Eliminating options can raise the chance of a correct guess to 33% if unsure of the exact answer.

Lyme Disease

• Vector-borne: Transmitted by ticks (Ixodes spp.).
• Zoonosis: Disease spread from animals to humans with limited human-to-human transmission.
• Named after Old Lyme, Connecticut.
• 300,000 cases/year in the USA.
• Pathogen:
  • Borrelia burgdorferi (spirochaete).
  • First isolated by Willy Burgdorfer (1982).
  • Dimensions: 0.3µm width, 5-20µm length.
  • Gram-negative but lacks LPS in the outer membrane.
  • 19 Borrelia species globally; B. garinii and B. afzelii co-occur in Europe with Bb.
• Vector:
  • Ixodes spp. (I. ricinus in Europe) is the tick host.
  • Tick life cycle: egg-larva-nymph-adult.
  • Competent hosts can infect ticks with Bb, while non-competent hosts (e.g., deer) cannot.
• Transmission:
  • Ticks latch onto passing hosts on vegetation.
• Symptoms:
  • Initial symptoms appear after 2 weeks.
  • Erythema migrans (80% of cases).
  • Viral-like symptoms.
  • Later arthritic/neurological problems.
• History:
  • Ancient disease; >5000 years old (Ötzi the Iceman was infected).
• Ecology:
  • Strong climatic patterns influence tick behavior.
  • Nymphs feed on smaller hosts, adults on larger hosts.
  • Humans are dead-end (non-competent) hosts.
  • Class III zoonosis: symptoms only in humans.
  • Ticks require vertebrate blood.
• Coinfection:
  • Possible with Babesia microti (Babesiosis; protozoan) and Anaplasma.

Life Cycles

• European life cycles of Ixodes ricinus tick and Borrelia spp.
• 2-3 year tick life cycle.
• Deer are non-competent hosts for Borrelia spp.
• B. garinii uses bird vectors and can cause neurological symptoms in humans.

Simplified Carbon Cycle

• Terrestrial: Dominated by microbes.
• Aquatic (Littoral Zones): Fungi, bacteria, plankton, ligno-cellulose, and dissolved organic carbon.
• Conversion to $CO_2$ in both terrestrial and aquatic environments.

Photosynthesis vs. Respiration

• Photosynthesis = Respiration (>50% microbial).
• Marine Photo-autotrophs:
  • $CH<em>2O + O</em>2$ (Mainly Microbial)
• Chemo-heterotrophs:
  • $CO<em>2 + H</em>2O$

Carbon Cycle (Major Pools and Fluxes)

• Major Carbon Pools (in GIC):
  • Soils: 1,580
  • Vegetation: 610
  • Marine Biota: 3
  • Atmosphere: 750
  • Fossil Fuels & Cement Production: 92
  • Rivers: 1.6
  • Surface Ocean: 1,020
  • Dissolved Organic Carbon: <700
  • Deep Ocean: 38,100
  • Sediments: 150
• Fluxes (in GIC/yr):
  • 60 (Soils)
  • 60 (Vegetation)
  • 0.5 (Marine Biota)
  • 5.5 (Storage in GIC)
  • 4000 (Sediments)
  • 90 (Rivers)
  • 100 (Surface Ocean)
• Most terrestrial organic carbon is in soils (2.5x more than in plant biomass).

Global Fossil Carbon Emissions

• Data from the Intergovernmental Panel on Climate Change (IPCC).
• Trends in fossil carbon emissions from petroleum, coal, natural gas, and cement production displayed over time.

Major Carbon Reservoirs (Table 10.2)

• Atmosphere before 1850: 560-610 billion metric tons of carbon.
• Atmosphere in 1978: 692 billion metric tons of carbon.
• Oceans and fresh water (inorganic): 35,000 billion metric tons of carbon (Carbonates).
• Dissolved organic: 1,000 billion metric tons of carbon.
• Land biota: 600-900 billion metric tons of carbon.
• Soil organic matter: 1600 billion metric tons of carbon.
• Sediments: 10,000,000 billion metric tons of carbon (Limestone).
• Fossil fuels: 10,000 billion metric tons of carbon (Coal, Oil).

Global Carbon Cycle Subsystems

• Plant subsystem:
  • $CO_2$: only 1-25%
• Herbivore subsystem:
  • Herbivores >> Carnivores
• Decomposition recycling subsystem:
  • Detritus and Decomposers = Flow of Energy
  • Recycling <99%
• Inorganic Nutrients Input from lithosphere

Carbon Pools and Human Activity

• Effect of human activity on carbon pools (deep ocean, biosphere, fossil fuels, atmosphere) over the past 200 years. Deep ocean Biosphere Fossil fuels Atmosphere

MOR Soil

• Distinct layers due to absence of earthworms.
• Progressive decomposition of plant litter downwards (with fresh material accumulating at the top).

Decomposition

• Agents: Fungi, Bacteria, Soil Fauna.
• Process:
  • Primary resource (R1): Plant Litter -> Secondary resource (R2): Faeces -> R3 -> R4.
• Mineralization:
  • Decomposers -> Detritus -> Dead Organic Matter -> Inorganic nutrients + $CO<em>2 + H</em>2O$

Rate of Decomposition

• Rainforest: 6 months - 100 years to reach 95% decomposition.
• Tundra
• Factors influencing decomposition rate:
  • Resource Quality (Carbon:Nitrogen ratio).
  • Environmental factors (Temperature, Water).
  • Energy and Nutrients

Measuring Decomposition

• Litter bags: Measure dry weight loss over time using mesh bags (7 mm mesh allows access to earthworms).
• Time to 95% decomposition.
• Half-life.

Cotton Strips and $CO_2$ Measurement

• Cotton strips: Assess loss of tensile strength as a measure of decomposition.
• Soil respiration: Measure $CO_2$ emission using Infrared Gas Analysis (IRGA).

Organisms Involved in Decomposition

• Biomass estimates for a wood soil:
  • Bacteria: 36.9 kg ha$-1$
  • Actinomycetes: 0.2 kg ha$-1$
  • Fungi: 454.0 kg ha$-1$
  • Protozoa: 1.0 kg ha$-1$
  • Earthworms: 12.0 kg ha$-1$
  • Other biota: 23.0 kg ha$-1$
  • Annual litter production: 7640.0 kg ha$-1$
• Up to 6km fungal hyphae per gram soil

Effect of Biocides on Litter Respiration

• Control vs. treatments with benomyl (kills fungi), streptomycin (kills bacteria), and DDT (kills soil animals).
• Demonstrates the individual contributions of different groups to decomposition.

Interactions with Soil Animals - Detritivores

• Detritivores (earthworms, nematodes, springtails, woodlice).
• Relatively low biomass (10g/m$^2$), high numbers (120 million nematodes/m$^2$).
• Contribute <10% to soil respiration.
• Main effect is physical - COMMINUTION (chewing up) of litter.
• Increases surface area for microbial attack.
• Example: Earthworm breaking down an oak leaf into 1 billion fragments.

Soil Animals

• Collembolans (Springtail): 50,000/m$^2$
• Mites: 100,000/m$^2$
• Nematodes: 120x10$^6$/m$^2$

Resource Quality & Nutrients

• Nutrients: Nitrogen (N), Phosphorus (P), Potassium (K), Sulfur (S), Magnesium (Mg), Iron (Fe), and Micronutrients
• Nitrogen (N) is major required nutrient [Protein] and is often limiting and of low availability
• Resource Quality (C:N):
  • Wood / Litter: 200:1
  • Fungal mycelium: 30:1
  • Animal tissues: 7:1
• Resource Quality increases during decomposition - Carbon lost as $CO_2$ but nutrients conserved.
  • C:P: Wood / Litter 1:1500, Fungal mycelium 1:100, Animal tissues 1:50

Fungal Nutrition

• Turgor pressure and tissue softening by enzymes allow penetration of substrate by hyphae.
• Oxygen translocation in hyphae
• Exoenzymes:
  • Cellulases (degrade Cellulose) - Carbon
  • Ligninases (degrade Lignin) - Carbon
  • Proteases (degrade Proteins) - Nitrogen
  • Lipases (degrade Lipids) - Carbon
  • Phosphatases (release Phosphate) - Phosphorus
  • Nucleases (degrade Nucleic acids) - Nitrogen, Phosphorus
• Bacteria Surface attack only

Composition of Plant Litter

• Cellulose: 20-45% (Sugar polymers for energy)
• Hemicellulose: 10-30%
• Lignins: 5-30% (Aromatic polymers that are recalcitrant)
• > Lignocellulose: Mainly degraded by fungi

Ligno-cellulose

• Breakdown of cell wall components including hemicellulose, cellulose, and lignin.
• Cell wall structure and C:N ratio (2-300:1).