Study Guide - Biogeochemistry & Carbon/Redox Cycling

Redfield Ratio

in aquatic ecosystems, marine and freshwater plankton construct cellular material in a highly conserved atomic ratio of Carbon, Nitrogen, and Phosphorus (C:N:P = 106:16:1)

Schindler Whole-Lake Experiments

pioneering whole-lake manipulation experiments conducted by David Schindler in the Experimental Lakes Area (ELA) demonstrated nutrient limitation in freshwater systems

Experimental Design of Schindler Whole-Lake Experiments

lakes were divided with vinyl curtains with one basin receiving C and N, while the other received C, N, and P

Key Findings of Schindler Whole-Lake Experiments

basin receiving P produced massive algal blooms dominated by nitrogen-fixing cyanobacteria

• confirms that P is the primary limiting nutrient controlling cultural eutrophication in freshwater lakes

• C and N drawn from the atmosphere can compensate for internal deficits, but P availability strictly dictates maximum primary production

Bicarbonate-Carbonate Equilibrium System

dissolved CO2, carbonic acid, bicarbonate, and carbonate ions form an aqueous chemical equilibrium that buffers aquatic ecosystems against rapid pH fluctuation

Dissociation and pH Dependence under acidic conditions

• pH < 6.4

• dissolved CO2 and carbonic acid H2CO3 dominate

Dissociation and pH Dependence under circumneutral conditions

• pH 6.4-10.3

• bicarbonate ions HCO3 dominate, acting as an effective buffer by absorbing incoming H+ ions or releasing them as base is added

Dissociation and pH Dependence under alkaline conditions

• pH > 10.3

• carbonate ions CO3 dominate

Photosynthetic pH Shifts - Daytime

photosynthetic uptake of CO2 by aquatic plants and algae consumes dissolved carbonic acid, shifting the chemical equilibrium left

• this reduction in free H+ ions causes ambient stream/lake pH to increase

Photosynthetic pH Shifts - Nightime

photosynthesis stops while plant, animal, and microbial respiration continues producing CO2 that dissolves into the water

• forms carbonic acid and releases free H+ ions, causing ambient pH to decrease

Anaerobic Respiration causes a _______ of dissolved oxygen

depletion

When aerobic respiration depletes DO, microorganisms shift to…

alternative terminal electron acceptors (TEAs) for anaerobic respiration

Microbes use electron acceptors in order of…

the thermodynamic energy yield, from highest energy yield to lowest

High → to Low Energy Yield Cascade

1. Molecular Oxygen (O2)

2. Nitrate reduction / Denitrification (NO3 → N2 gas)

3. Manganese Reduction (Mn4+ → Mn2+)

4. Iron Reduction (Fe3+ → Fe2+)

5. Sulfate Reduction (SO4²- → H2S)

6. Methanogenesis / Carbon Dioxide Reduction (CO2 → CH4)

Aerobic Respiration terminal electron acceptor

O2

Aerobic Respiration Reduced End Product

H2O

Aerobic Respiration Biogeochemical Implication

highest energy yield; standard aerobic cellular respiration

Denitrification terminal acceptor

NO3^-

Denitrification reduced end product

Nitrogen gas, N2

Denitrification Biogeochemical implication

converts bioavailable fixed nitrogen into inert N2 gas, removing excess nitrogen from aquatic systems

Manganese Reduction terminal electron acceptor

Mn^4+

Manganese Reduction Reduced end product

soluble Mn²+

Manganese Reduction Biogeochemical Implication

dissolves insoluble manganese oxides into water column

Iron Reduction terminal electron acceptor

Fe³+

Iron Reduction reduced end product

soluble Fe²+

Iron Reduction Biogeochemical Implications

dissolves ferric iron complexes, releasing bound phosphate, PO4³-, from sediments into water

Sulfate Reduction terminal electron acceptor

sulfate, SO4²-

Sulfate Reduction reduced end product

Hydrogen Sulfide, H2S

Sulfate Reduction Biogeochemical Implication

generates toxic, rotten-egg-odor hydrogen sulfide gas; precipitates toxic heavy metals as insoluble metal sulfides

Methanogenesis terminal electron acceptor

CO2 / organic C

Methanogenesis reduced end product

Methan CH4

Methanogenesis Biogeochemical Implication

lowest thermodynamic yield, produces methane gas in strictly anoxic conditions