a pictograph of the life cycle of marine ich. From a cartoon clownfish on the top center that is clearly infected with white spots, ich detaches from the fish body, settles in the sand and reproduces, then finds a fish host to complete the cycle

Week 18: The Saltwater Nitrogen Cycle

Updated 2023

In our previous discussion, we covered the basics of cycling your newly established saltwater aquarium. In this week’s lesson, we’ll delve deeper into the intricacies of the saltwater nitrogen cycle. Gaining a comprehensive understanding of this cycle will provide you with valuable insights into the inner workings of your aquarium. Armed with this knowledge, you’ll be better equipped to care for your aquarium and make informed decisions, ultimately preventing unnecessary expenditures on items you may not require.

This Week's Video:

What Is The Nitrogen Cycle?

The nitrogen cycle encompasses the biogeochemical transformations that convert nitrogen into various chemical forms, facilitating its circulation among the atmosphere, terrestrial landscapes, and marine ecosystems. Despite the prevalence of nitrogen in the atmosphere, its availability for biological utilization is limited, resulting in a scarcity of accessible nitrogen across various ecosystems. This scarcity necessitates the involvement of bacteria or other processes to convert nitrogen into biologically usable forms.

Nitrogen plays a pivotal role in the sustenance of all life forms. It serves as a fundamental building block for cells, amino acids, proteins, and DNA. Additionally, nitrogen is a crucial component for the synthesis of chlorophyll, the pigment essential for photosynthesis—the process that sustains organisms like corals, macroalgae, and many marine species. While atmospheric nitrogen is abundant and can enter the Earth’s surface and oceans through a process known as “nitrogen fixation,” human activities such as agriculture and sewage contribute significantly to the influx of nitrogen into marine environments through waste disposal.

How Does it Work?

Both on land and in the ocean, the nitrogen cycle operates, albeit with distinct variations in the finer details.

  • Nitrogen gas (N2) constitutes a significant portion (78%) of Earth’s atmosphere, but it’s inaccessible for direct use by our bodies.
  • In ocean systems, N2 enters through atmospheric diffusion and runoff from land.
  • Nitrogen fixation entails assimilating atmospheric nitrogen (N2) into organic compounds, converting it into ammonium (NH4+), an organic form, through a process called ammonification.
  • Ammonium (NH4+) serves as a foundational building block for nitrogen-based compounds. While crucial for biochemical processes, it’s toxic to fish. Early in the saltwater cycle, an ammonia spike indicates bacteria’s conversion of N2 into NH4+.
  • NH4+ forms the “amino” component of amino acids, essential for protein synthesis involved in cell structure and enzyme reactions.
  • Plants, fungi, and bacteria can incorporate NH4+ into amino acids, structuring their cells. Consequently, organisms consuming these plants absorb amino acids.
  • All organisms produce nitrogen-based waste; for humans, it’s excreted as urine. Fish secrete ammonium (NH4+) waste through their gills.
  • From atmospheric nitrogen, ammonification yields NH4+. Nitrification ensues, converting NH4+ into nitrite (NO2-) and nitrate (NO3-), achieved by nitrosomonas bacteria.
  • Nitrite (NO2-) is also toxic to fish. Nitrification progresses further as nitrobacter bacteria oxidize nitrite into nitrate (NO3-).
  • NO3- removal occurs through water changes or plant uptake. However, neither directly converts NO3- into N2.
  • Denitrification marks the cycle’s final stage. Anaerobic bacteria reduce nitrate (NO3-) to N2, concluding the process. Unlike earlier steps reliant on aerobic bacteria, this stage operates anaerobically.
  • Ideally, anaerobic bacteria would populate your saltwater aquarium, converting nitrate into nitrogen gas (N2) instead of relying on water changes or macroalgae.
  • Anaerobic bacteria thrive in extremely low oxygen environments, often found in deep sand beds or within porous rock formations within your tank.

Glossary of Terms

N2 nitrogen gas (atmospheric nitrogen)

NH4+ Ammonium

NO2- Nitrite

NO3- Nitrate

Amino Acid– structural units that make up proteins

Ammonia & Ammonium

  • Ammonia and Ammonium are compounds that contain Nitrogen and Hydrogen. Ammonia contains one Nitrogen and three Hydrogen whereas Ammonium contains one Nitrogen and Four Hydrogen.
  • Ammonia is a weak base and is un-ionized. On the other hand, Ammonium is ionized.

Denitrification- Denitrification is a microbially facilitated process where nitrate is reduced and ultimately produces molecular nitrogen through a series of intermediate gaseous nitrogen oxide products

Nitrate Reduction– process involving anaerobic bacteria reducing NO3- (nitrate) to N2 (nitrogen gas.

Nitrification- Biological oxidation (break-down) of NH4+ to NO2- then NO3- (nitrite & nitrate)

Nitrogen Fixation– the chemical processes by which atmospheric nitrogen is assimilated into organic  compounds, especially by certain microorganisms as part of the nitrogen cycle.

Nitrobacter- genus of gram-negative bacteria that oxidizes NO2- into NO3-.

Nitrosomonas– genus of Gram-negative bacteria that consume NH4+ (ammonium) and produce NO2- (nitrite)

Organic- carbon based

Oxidation- Oxidation occurs when an atom, molecule, or ion loses one or more electrons in a chemical reaction. The opposite of oxidation is reduction.

Proteins- Proteins are large, complex molecules that play many critical roles in the body. They do most of the work in cells and are required for the structure, function, and regulation of the body’s tissues and organs.

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