Lecture 14 - Nitrogen Sulfur Phosphorus and Other Nutrients

Nitrogen, Sulfur, Phosphorus, and Other Nutrients

Overview of Nutrient Cycles

• Availability of nutrients plays a crucial role in ecosystems.

• Photosynthesis is less limited by carbon, but nitrogen and phosphorus are often the limiting factors in freshwater systems.

• Other nutrients such as sulfur, silicon, and iron can also be significant depending on the ecological context.

• Nutrient cycles are deeply interrelated, impacting overall ecosystem health.

Nitrogen

Forms of Nitrogen

• N2 gas constitutes 78% of the atmosphere but is not easily accessible to most organisms.

• In aquatic ecosystems, nitrogen exists in both inorganic and organic forms:

  • Inorganic Forms:

    • Nitrate (NO3−)

    • Ammonium (NH4+)

    • Nitrite (NO2−)

    • Nitrous Oxide (N2O)

  • Organic Forms:

    • Include amino acids, nucleic acids, proteins, and urea which can be dissolved or particulate.

Nitrogen Fluxes

• Nitrogen is essential for life; it constitutes key biological molecules (e.g., amino acids, DNA).

• Consumers like fish, insects, and mammals primarily assimilate organic nitrogen.

• Enzymatic breakdown of complex organic molecules releases both organic and inorganic nitrogen.

• Primary producers and bacteria assimilate dissolved inorganic nitrogen (preferably ammonium due to lower energy requirements).

Nitrogen Fixation

• Certain bacteria and archaea can fix atmospheric N2 into usable forms.

• Cyanobacteria are notable for their role in nitrogen fixation, especially linked to algal blooms.

• The process requires nitrogenase enzyme, which is energy-intensive and sensitive to oxygen, hence bacteria often utilize specialized cells (heterocysts) for protection.

• Natural fixation also occurs via atmospheric lightning.

Nitrification and Denitrification

Nitrification

• Nitrification involves two steps: oxidizing ammonium (NH4+) to nitrite (NO2−) then to nitrate (NO3−).

• Conducted by chemoautotrophic bacteria under oxic conditions while rapidly depleting ammonium and nitrite in these waters.

Denitrification

• Denitrification reduces nitrate back to nitrogen gas (N2), facilitating energy conversion from organic carbon.

• This process occurs exclusively in anoxic environments, leading to removal of inorganic nitrogen from systems.

Presence of Nitrogen in the Environment

• In aquatic systems, inorganic nitrogen exists mainly as ammonium or nitrate; the balance is influenced by the presence of oxygen.

• The nitrogen cycle illustrates the transformations and states of nitrogen amidst various biological processes.

Phosphorus

Characteristics of Phosphorus

• Phosphorus serves as a limiting nutrient in many aquatic ecosystems, primarily in the form of phosphate (PO4^3−).

• Often present in very low concentrations in pristine waters (< 1 ug P/l).

• Organic phosphorus forms are found in lipids and nucleic acids but are frequently bound to sediments, rendering them bio-unavailable.

Phosphorus Transformations

• Availability of phosphates is tightly linked to interactions with iron in the environment.

• In oxic conditions, phosphates precipitate with Fe3+ and other metals, leading to particulate deposition in sediments.

• Under anoxic conditions, phosphates dissociate from metals, becoming available in the water column.
  

Sulfur and Other Trace Nutrients

The Sulfur Cycle

• The sulfur cycle consists of multiple redox states and involves diverse bacteria and archaea.

• Sulfur is critical as it connects to inorganic metal cycles, influencing phosphorus availability indirectly.

Silicon, Iron, and Trace Nutrients

• Silicon:

  • Abundant on Earth but often has limited solubility in water.

  • Crucial for diatoms, which form siliceous frustules.

  • Summer depletion of silicon in lakes requires replenishment through weathering.

• Iron:

  • Key for various biological processes; however, its demand is lower relative to carbon, nitrogen, and phosphorus.

  • Exists in different dissolved forms based on the oxygen state: Fe3+ (oxic) and Fe2+ (anoxic).

• Other Nutrients:

  • Include manganese, copper, selenium, zinc, molybdenum, sodium, potassium, chloride, and boron—often needed in trace amounts and can be toxic at higher concentrations.

Summary of Key Points

1. Photosynthesis is frequently limited by nitrogen and/or phosphorus.

2. Nitrogen cycles between inorganic and organic forms; fixation is facilitated by nitrogen-fixers and lightning.

3. Phosphorus exists in both organic and inorganic forms, with inorganic often bound to sediments and thus bio-unavailable.

4. Sulfur, silica, and iron can also influence ecosystem dynamics, particularly phosphorus availability.