Lecture 14 - Nitrogen Sulfur Phosphorus and Other Nutrients

Nitrogen

N2 gas is most common form of N (78% of atmosphere) • Not easily used by organisms

• In aquatic ecosystems N can be in inorganic and organic forms

• Most important inorganic forms of N in aquatic ecosystems are:

• Nitrate (NO3 −) & Ammonium (NH4 +)

• Nitrite (NO2 −) and Nitrous Oxide (N2O) can also be present in significant quantities

• Many organic forms • Amino acids, Nucleic Acids, Proteins and Urea • Organic forms can be dissolved or particulate

Nitrogen Fluxes

• Nitrogen is required for life • Component of many essential biological molecules • Amino acids, DNA, etc •

Many consumers can only assimilate organic nitrogen • Fish, insects, mammals • Use enzymes to break down complex organic molecules • Important source of organic and inorganic nitrogen to system → excretion

• Primary producers and bacteria can take dissolved inorganic nitrogen from the water

• Nitrate, Nitrite and Ammonium • Ammonium is preferred (mostly) because requires less energy to use

Nitrogen Fixation

Many bacteria and some archaea can assimilate (“Fix”) N2 gas

• Includes some cyanobacteria (often associated with algal blooms)

• Enzyme nitrogenase → Energetically costly

• Inactivated by oxygen →use specialized cells (heterocysts) to protect enzyme from oxygen

• Fixation also occurs from lightning

Nitrification/Denitrification

Denitrification is process of reducing nitrate to nitrogen gas

• Process is used to convert organic carbon to energy

• Only occurs in anoxic waters

• Results in removal of inorganic N from aquatic systems

Presence of N in the Environment

Inorganic N is most abundant as ammonium or nitrate • Balance depends on presence of oxygen

Phosphorus

• Limiting nutrient in many aquatic ecosystems

• One dominant inorganic form → Phosphate (𝑃𝑂4 3−)

• Concentrations often below detection in pristine waters (< 1 ug P/l)

• Many organic phosphorus forms • Lipids, nucleic acids, etc.

• P is often bound to particles such as sediments • Not bioavailable

Phosphorus Transformations

• Availability of phosphate determined by interactions with iron

• In oxic conditions, phosphate precipitates with 𝐹𝑒3+ and other metals • Leads to deposition of particulate phosphorus in the sediments

• In anoxic conditions (e.g., hypolimnion), phosphates dissociates from metals and is released to water column

Sulfur

Complex cycle driven by numerous redox states in which sulfur can occur

• Many bacteria and archaea involved in cycle

• Important as is tightly linked to inorganic metal cycles (e.g., iron) and thus indirectly to phosphorus cycle

Silicon

Highly abundant element on Earth, but…

• Limited solubility so not a major dissolved ion in water

• Abundance in water dependent on geology/weathering

• Most in volcanic areas, least in limestone areas

•Critically important to diatoms

• Make frustules (specialized cell wall) from silicon

• Can deplete silicon in lakes in summer → replenished by weather and erosion

Iron

• Key element in many biological elements

• But generally low demand compared to C, N, and P

• Present in several dissolved forms – often dependent on O2

• For example, 𝐹𝑒3+ (oxic conditions), 𝐹𝑒2+ (anoxic conditions)

• Recall: Important to P availability

Other important nutrients

• Manganese, Copper, Selenium, Zinc, Molybdenum

• Sodium, Potassium, chloride and boron

• Needed in trace amounts and most are not naturally abundant

• Many are toxic when abundan