L6. Microbial Photosynthesis and Decomposition in C cycling

What defines major microbial metabolic groups?

• Energy source: Light → phototrophy; Chemicals → chemotrophy

• Electron source: H₂O vs. non‑H₂O; Organic C vs. inorganic compounds

• Carbon source: CO₂ → autotrophy; Organic C → heterotrophy

• Terminal electron acceptor: O₂ vs. non‑O₂

Which combination correctly describes chemolithoautotrophy?

B. Chemical energy, inorganic e⁻ donors, CO₂ as C source

What are the major carbon pools?

• Atmosphere: 760–837 Pg

• Soils: 1200–2500 Pg

• Oceans: 38,400 Pg (largest active pool)

• Fossil fuels: 4130 Pg

1 Pg = 1012 Kg = 109 tonnes (1 billion tonnes)

What are major fluxes in C cycling?

Major fluxes in carbon cycling include photosynthesis, respiration, decomposition, and combustion, which transfer carbon among various pools, such as the atmosphere, soils, and oceans.

How much C is fluxed through different processes?

• Photosynthesis: ~120 Pg yr⁻¹

• Plant respiration: 60 Pg yr⁻¹

• Soil respiration: 60 Pg yr⁻¹

• Fossil fuel emissions: +3.5 Pg yr⁻¹

Which pool contains the largest amount of carbon?

C. Oceanic pool

What happens in oxygenic photosynthesis?

• Light reaction: Splits water → O₂ + ATP + NADPH

• Dark reaction: Calvin–Benson–Bassham cycle fixes CO₂ → CH₂O

Equation summary of photosynthesis + respiration

• Photosynthesis: CO₂ + H₂O + light → CH₂O + O₂

• Respiration: CH₂O + O₂ → CO₂ + H₂O

Which pigment is universal in oxygenic phototrophs?

C. Chlorophyll a

Why do microbes use different pigments?

• Adaptation to light quality in different environments

• Pigments include chlorophyll a, carotenoids, phycoerythrin, bacteriochlorophylls

Which pigment absorbs strongly in the green region?

B. Carotenoids

What are the different stages in the seasonal phytoplankton cycle?

• Spring bloom: High nutrients + increasing light

• Summer: Nutrient depletion → decline

• Autumn bloom: Mixing restores nutrients

What is meant by NPP vs GPP?

NPP (Net Primary Production) is the amount of organic matter produced by photosynthesis minus the organic matter consumed by respiration, while GPP (Gross Primary Production) is the total amount of OM produced by photosynthesis.

NPP = GPP – Respiration

Which biome has the highest NPP per m²?

C. Wetlands

What are the 5 Major phytoplankton groups?

1. Diatoms: Silicate users; bloom-formers

2. Coccolithophores: CaCO₃ producers

3. Phaeocystis: Dimethylsulfide producers

4. Diazotrophs: N₂-fixers (Anabaena, Trichodesmium)

5. Picophytoplankton: Synechococcus, Prochlorococcus

2 main Cyanobacteria types?

• Filamentous: Anabaena, Trichodesmium (N₂ fixation)

• Coccoid: Synechococcus, Prochlorococcus (dominant in oligotrophic oceans)

Which cyanobacterium is a major N₂ fixer in marine systems?

B. Trichodesmium

Key features of microbes capable of anoxygenic photosynthesis?

• Uses H₂S, S⁰, S₂O₃²⁻, H₂ as electron donors

• Produces no O₂

• Pigments: bacteriochlorophylls (a, b, c, d, e, g)

• Occurs in anoxic, sulfur-rich environments

3 main Carbon fixation pathways:

1. RPP (Calvin cycle) – purple bacteria

2. RCA (reverse TCA) – green sulfur bacteria

3. 3‑HP cycle – green non‑sulfur bacteria

Which group uses the reverse TCA cycle?

B. Green sulfur bacteria

Who performs most respiration, plants or microbes?

• Microbes dominate respiration in both aquatic and terrestrial systems, account for 50% or more of global respiration

• Small size fractions (<2 µm) contribute disproportionately

Composition of detritus in terrestrial vs aquatic systems:

• Terrestrial: High lignin, high C:N (e.g., pine wood C:N = 640)

• Aquatic: High protein, low C:N (e.g., diatoms C:N = 6.7)

Which component of detritus decomposes slowest?

D. Lignin

4 key hydrolases involved in decomposition:

• Proteins: Aminopeptidase, endopeptidase, carboxypeptidase

• Chitin: Chitinase

• Peptidoglycan: Lysozyme

• Cellulose: Cellulase

The fluorogenic substrate Leu-MCA is digested by →

Leu‑MCA → aminopeptidase

The fluorogenic substrate MUF‑phosphate is digested by →

MUF‑phosphate → phosphatase

The fluorogenic substrate MUF‑cellobioside is digested by →

MUF‑cellobioside → cellulase

What is the priming effect?

• Plant roots release C‑rich exudates & stimulate microbial degradation of SOM

• Leads to increased NH₄⁺ mineralisation

• Grazers release additional NH₄⁺

What is the immediate effect of root exudates?

B. Increased SOM mineralisation

Major decomposers

• Bacteria: dominate simple soils, aquatic systems

• Fungi: dominate lignin‑rich, forest soils

Which group dominates decomposition of lignin?

B. Fungi