Environmental Microbiology: Hydrologic Cycle, Biochemical Oxygen Demand, Eutrophication, Dead Zones, and Wastewater Treatment

Environmental Microbiology

Learning Objectives

• Define biochemical oxygen demand (BOD).

• Explain how hypoxic zones occur using eutrophication as an example.

• Describe the stages of wastewater treatment.

• Describe the factors that promote effective wastewater treatment.

• Describe the three sides of the nitrogen triangle.

The Hydrologic Cycle

Overview of the Hydrologic Cycle

The hydrologic (water) cycle is the continuous movement of water between the atmosphere and the biosphere. This cycle is essential for sustaining life and influences the distribution of nutrients and contaminants in the environment.

• Precipitation: Water falls to earth as rain, snow, or other forms.

• Evaporation: Water returns to the atmosphere from land and water surfaces.

• Transpiration: Water is released from plants into the atmosphere.

• Runoff: Water passing through the ground picks up nutrients and organic matter, which can affect water quality.

Example: Rivers carry nutrients into lakes and oceans, increasing the organic carbon flux and potentially raising BOD.

Biochemical Oxygen Demand (BOD)

Definition and Importance

Biochemical oxygen demand (BOD) is the amount of oxygen removed from the environment by microbial respiration of organic matter. It is a key indicator of water quality and pollution.

• Formula for aerobic respiration:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$

• High BOD can threaten the viability of fish and other aquatic animals, especially when oxygen levels fall below 5 mg/L.

• Saturated water: 8 mg/L O2

• Sewage: BOD often exceeds 600 mg/L.

• BOD is measured using a dissolved oxygen probe over time; the rate of decrease in oxygen is proportional to the concentration of organic matter.

Example: Water samples with the fastest decrease in dissolved oxygen have the highest BOD.

Dead Zones and Hypoxia

Formation and Impact

Dead zones are regions of the ocean or lakes that become hypoxic (low in oxygen), making them uninhabitable for most fish and invertebrates. These zones are often caused by excessive nutrient input and subsequent microbial activity.

• Example: The Gulf of Mexico off the Louisiana Coast is a well-known dead zone, which expanded after the 2010 oil spill.

• Heterotrophic microbes increase in response to organic nutrient influx, and their aerobic respiration depletes oxygen.

• Dead zones are mapped by measuring oxygen concentrations; areas with <2 mg/L O2 are considered hypoxic.

Eutrophication

Process and Consequences

Eutrophication is the enrichment of water bodies with excess nutrients (mainly nitrogen and phosphorus), leading to increased growth of algae and bacteria. This process can result in hypoxic zones and dead zones.

• Nutrient influx: Run-off containing NH4+, NO3-, and PO43- enters water bodies.

• Algal bloom: Rapid growth of algae and cyanobacteria occurs due to nutrient availability.

• Once nutrients are depleted, the algal mass dies and sinks to the bottom.

• Heterotrophic bacteria feed on dead algae, increasing oxygen consumption through aerobic respiration.

• Oxygen depletion leads to hypoxia, threatening aquatic life.

Example: Ponds and streams receiving agricultural runoff often experience eutrophication and subsequent fish kills.

Wastewater Treatment

Stages of Wastewater Treatment

Wastewater treatment is designed to reduce BOD and remove pathogens before water is released back into the environment. The process involves several stages:

• Preliminary treatment: Removal of large debris and solid waste.

• Primary treatment: Fine screens and sedimentation tanks remove smaller particles and some soluble substances.

• Secondary treatment: Microbial decomposition of organic content, often using activated sludge processes.

• Tertiary (advanced) treatment: Chemical disinfection (e.g., chlorination) to eliminate pathogens and further reduce contaminants.

Activated Sludge Process

The activated sludge process uses a biological floc composed of bacteria and protozoa to decompose organic matter. Floc formation is critical for settling and removal of microbes from the effluent.

• Healthy sludge contains saprotrophic bacteria and protozoa (amoebae, ciliated protozoans, stalked ciliates).

• Filamentous bacteria are important for floc structure; the ratio of filamentous to planktonic types affects effluent quality.

The Nitrogen Cycle

Overview and Biological Importance

The nitrogen cycle describes the transformations of nitrogen between its various oxidation states in the environment. It is unique because nitrogen has multiple oxidation states and relies heavily on prokaryotic metabolism.

• Major accessible nitrogen source: Atmospheric N2 (79% of air).

• Industrial fixation: The Haber process converts N2 + 3H2 → 2NH3 for fertilizer production.

The Nitrogen Triangle

The nitrogen cycle can be visualized as a triangle with three major processes:

• Fixation: Conversion of N2 to NH3 by nitrogenase (anaerobic, energy-intensive).

• Nitrification: Oxidation of NH3 to NO2- and then to NO3- by lithotrophic bacteria (e.g., Nitrosomonas, Nitrobacter).

• Denitrification: Reduction of NO3- to N2 (anaerobic respiration), leading to nitrogen loss from the environment and production of N2O (a greenhouse gas).

Key Equations in the Nitrogen Cycle

• Nitrogen fixation:

$N_2 + 8H^+ + 8e^- \rightarrow 2NH_3 + H_2$

• Nitrification:

$NH_3 \xrightarrow{Nitrosomonas} NO_2^- \xrightarrow{Nitrobacter} NO_3^-$

• Denitrification:

$NO_3^- \rightarrow NO_2^- \rightarrow NO \rightarrow N_2O \rightarrow N_2$

Symbiotic and Free-Living Nitrogen Fixation

• Symbiotic: Bacterial associations with plant roots (e.g., legumes, Rhizobium), cyanobacteria in aquatic environments (heterocysts).

• Free-living: Soil bacteria and archaea capable of fixing nitrogen independently.

Summary Table: Key Processes in Environmental Microbiology

Process	Main Function	Key Microbes	Environmental Impact
Biochemical Oxygen Demand (BOD)	Measures oxygen used by microbes to decompose organic matter	Heterotrophic bacteria	High BOD leads to hypoxia, fish kills
Eutrophication	Excess nutrients cause algal blooms	Algae, cyanobacteria, heterotrophic bacteria	Oxygen depletion, dead zones
Wastewater Treatment	Removes organic matter and pathogens	Bacteria, protozoa	Improves water quality, reduces disease risk
Nitrogen Fixation	Converts N2 to NH3	Rhizobium, cyanobacteria	Provides usable nitrogen for plants
Nitrification	Oxidizes NH3 to NO3-	Nitrosomonas, Nitrobacter	Can lead to nitrate runoff, eutrophication
Denitrification	Reduces NO3- to N2	Denitrifying bacteria	Removes nitrogen from ecosystems, produces N2O

Additional info: Academic context and definitions have been expanded for clarity and completeness. Equations and table entries have been inferred and formatted for study purposes.