29. Cyanobacteria out competes the green algae in eutrophic lakes due to organic pollutions because they can
    (1) tolerate low oxygen level
    (2) tolerate high phosphorus level
    (3) fix nitrogen, so it is not limitation for them
    (4) low light is required for photosynthesis

Eutrophic lakes—those rich in nutrients, especially from organic pollution—are often dominated by cyanobacteria (also known as blue-green algae) rather than green algae. This dominance is a result of cyanobacteria’s unique adaptations, particularly their ability to fix atmospheric nitrogen. This article explores why cyanobacteria outcompete green algae in nutrient-rich, organically polluted lakes, the ecological implications of this shift, and the specific advantages that allow cyanobacteria to thrive under these conditions.

Understanding Eutrophication and Algal Blooms

Eutrophication is the process by which water bodies become overly enriched with nutrients, primarily nitrogen and phosphorus, leading to excessive growth of algae and aquatic plants. Organic pollution, such as sewage or agricultural runoff, accelerates this process. While both cyanobacteria and green algae can grow rapidly in nutrient-rich environments, cyanobacteria often become dominant, forming dense blooms that can harm aquatic ecosystems and human health.

Why Cyanobacteria Outcompete Green Algae

Cyanobacteria possess several traits that give them a competitive edge over green algae in eutrophic conditions:

1. Nitrogen Fixation: Overcoming Nutrient Limitation

The most significant advantage of cyanobacteria is their ability to fix atmospheric nitrogen. Nitrogen is an essential nutrient for all algae, but in many freshwater systems, it can become a limiting factor for growth. Green algae rely entirely on dissolved nitrogen (such as nitrates and ammonia) in the water. When these forms of nitrogen are depleted, green algae growth is restricted.

Cyanobacteria, however, have specialized cells called heterocysts that allow them to convert atmospheric nitrogen (N₂) into ammonia (NH₃), a form that can be used for growth. This ability enables cyanobacteria to grow even when dissolved nitrogen is scarce, giving them a critical advantage over green algae in nutrient-rich but nitrogen-limited environments.

2. Tolerance to High Phosphorus Levels

While both cyanobacteria and green algae can tolerate high phosphorus levels, phosphorus is rarely the limiting factor in eutrophic lakes polluted with organic matter. High phosphorus availability is a hallmark of eutrophication, but it does not explain why cyanobacteria outcompete green algae. Both groups can exploit abundant phosphorus, so this factor does not confer a unique advantage to cyanobacteria.

3. Tolerance to Low Oxygen Levels

Cyanobacteria are generally more tolerant of low oxygen (hypoxic) conditions than many other algae, but this is not the primary reason they dominate in eutrophic lakes. Low oxygen conditions often result from algal blooms and decomposition of organic matter, but green algae can also survive in these environments. Oxygen tolerance is a secondary advantage that helps cyanobacteria persist, but it is not the main driver of their dominance.

4. Low Light Requirements for Photosynthesis

Some cyanobacteria can photosynthesize under low light conditions, but this is not a universal trait and does not explain their dominance in most eutrophic lakes. Eutrophic lakes are often turbid due to high algal biomass, but both cyanobacteria and green algae can adapt to reduced light. Light limitation is more relevant in deep or highly turbid waters, but it is not the primary reason cyanobacteria outcompete green algae in typical eutrophic conditions.

The Ecological Impact of Cyanobacterial Dominance

When cyanobacteria dominate, they can form dense surface blooms that block sunlight, reduce oxygen levels, and produce toxins harmful to aquatic life and humans. These blooms can:

• Reduce biodiversity: By shading out other algae and aquatic plants.

• Cause fish kills: Through oxygen depletion and toxin production.

• Harm water quality: Making water unsafe for drinking, recreation, and irrigation.

• Disrupt food webs: Affecting zooplankton, fish, and other aquatic organisms.

Comparing the Options

Let’s review the options in the context of cyanobacterial dominance in eutrophic lakes:

• Tolerate low oxygen level:
  Partially correct. Cyanobacteria are more tolerant of low oxygen, but this is not the main reason they outcompete green algae.

• Tolerate high phosphorus level:
  Incorrect. Both cyanobacteria and green algae can tolerate high phosphorus; this does not explain cyanobacterial dominance.

• Fix nitrogen, so it is not limitation for them:
  Correct. This is the primary reason cyanobacteria outcompete green algae in eutrophic lakes. Nitrogen fixation allows them to grow when dissolved nitrogen is limited.

• Low light is required for photosynthesis:
  Partially correct. Some cyanobacteria can photosynthesize in low light, but this is not the main reason for their dominance in eutrophic lakes.

The Role of Nitrogen Fixation in Cyanobacterial Success

Nitrogen fixation is a unique metabolic capability that sets cyanobacteria apart from most other algae. By converting atmospheric nitrogen into a usable form, cyanobacteria can bypass the limitation imposed by low dissolved nitrogen. This is especially important in eutrophic lakes where phosphorus is abundant but nitrogen may be scarce after initial algal blooms deplete it.

How Nitrogen Fixation Works

• Heterocysts: Specialized cells in some cyanobacteria that create an anaerobic environment necessary for nitrogenase, the enzyme that fixes nitrogen.

• Energy Cost: Nitrogen fixation is energetically expensive, but the payoff is access to a virtually unlimited nitrogen source.

• Ecological Advantage: In nitrogen-limited conditions, cyanobacteria can continue to grow and reproduce while green algae are restricted by nitrogen scarcity.

The Broader Context: Eutrophication and Water Management

Understanding why cyanobacteria dominate in eutrophic lakes is crucial for managing water quality and preventing harmful algal blooms. Strategies to reduce cyanobacterial blooms include:

• Reducing nutrient inputs: Limiting the discharge of phosphorus and nitrogen into water bodies.

• Promoting diverse algal communities: Encouraging the growth of non-toxic algae and aquatic plants.

• Monitoring and early warning systems: Detecting blooms early to minimize their impact.

Key Takeaways

• Cyanobacteria outcompete green algae in eutrophic lakes primarily because they can fix atmospheric nitrogen, overcoming nitrogen limitation.

• High phosphorus tolerance and low oxygen tolerance are secondary advantages, but not the main drivers of cyanobacterial dominance.

• Low light requirements for photosynthesis are not the primary reason cyanobacteria dominate in most eutrophic lakes.

• Cyanobacterial blooms can harm aquatic ecosystems, reduce biodiversity, and threaten human health.

Summary Table

Conclusion

Cyanobacteria outcompete green algae in eutrophic lakes polluted with organic matter because they can fix atmospheric nitrogen, making nitrogen limitation a non-issue for their growth. This unique ability allows them to thrive when dissolved nitrogen is scarce, giving them a decisive advantage over green algae. Understanding this dynamic is essential for managing eutrophication and protecting aquatic ecosystems from harmful cyanobacterial blooms.

In summary, the correct answer is:
(3) fix nitrogen, so it is not limitation for them.
This nitrogen-fixing capability is the key reason cyanobacteria dominate in eutrophic lakes, highlighting the importance of nutrient management in water quality conservation.