The overuse of nitrogen and phosphorus fertilizers has a profound impact on aquatic ecosystems, primarily through a process known as eutrophication. When excess fertilizers run off into water bodies, they lead to an overabundance of nutrients, particularly nitrogen and phosphorus. This nutrient overload stimulates the excessive growth of algae and aquatic plants. While initially, this might seem beneficial, it leads to several problems. As the algae bloom dies, it decomposes, a process that consumes a significant amount of dissolved oxygen in the water. This depletion of oxygen creates hypoxic (low-oxygen) conditions, severely impacting aquatic life and leading to the death of fish and other organisms. Furthermore, some algal blooms can be harmful, producing toxins that affect both aquatic life and humans. Eutrophication often results in a loss of biodiversity, changes in species composition and abundance, and a decline in water quality. This ecological imbalance highlights the need for careful management of fertilizer use to protect aquatic ecosystems.

Nitrogenous waste in animals is a byproduct of the metabolism of proteins and nucleic acids. When proteins are digested, amino acids are released. Amino acids consist of an amino group (NH₂) that contains nitrogen. When the body uses these amino acids, it removes the amino group in a process called deamination, primarily occurring in the liver. This process converts the amino group into ammonia (NH₃), a toxic compound. In aquatic animals, ammonia is excreted directly. However, in terrestrial animals, including humans, ammonia is converted into less toxic forms such as urea or uric acid before being excreted. This conversion is crucial as ammonia is highly soluble and can disrupt cellular pH balance. The production and excretion of nitrogenous waste are critical for maintaining nitrogen balance in the body and preventing the accumulation of toxic levels of ammonia. This process illustrates the direct relationship between the metabolic use of proteins and nucleic acids and the need for efficient waste management systems in animals.

Phosphorus plays a critical role in the structure of DNA and RNA, essential for the preservation and transmission of genetic information. In both DNA and RNA, phosphorus is part of the phosphate group that forms the backbone of the nucleotide chain. Each nucleotide consists of a sugar (deoxyribose in DNA and ribose in RNA), a nitrogenous base, and a phosphate group. The phosphate groups link adjacent nucleotides together through phosphodiester bonds, connecting the 5' carbon of one sugar to the 3' carbon of the next. This structure provides stability and integrity to the nucleic acid chains. In DNA, the phosphate backbone, along with the sugar molecules, forms the double helix's sides, positioning the nitrogenous bases in the center to pair and encode genetic information. In RNA, the single-stranded structure allows it to act as a template for protein synthesis. The presence of phosphorus is thus indispensable for the structural formation of nucleic acids, ensuring the accurate storage, replication, and expression of genetic information in all living organisms. Without phosphorus, the vital processes of life, including heredity and protein synthesis, would be impossible.

The phosphorus cycle is termed a 'sedimentary' cycle because it primarily involves the movement of phosphorus through the earth's crust, water, and living organisms, without a significant gaseous phase like the nitrogen or carbon cycles. Phosphorus originates from rocks and is released into the soil and water through weathering processes. Plants absorb inorganic phosphate from the soil, which then moves through the food chain. When organisms die, phosphorus returns to the soil or sediment through decomposition. This cycle is slow because phosphorus is not readily available in the atmosphere. Consequently, the availability of phosphorus in ecosystems is often limited by the rate of rock weathering and the recycling of organic phosphorus, making it a limiting nutrient in many ecosystems. This limited availability can impact plant growth, which in turn affects the entire food web. Additionally, human activities, such as the use of phosphate fertilizers and detergents, can disrupt this cycle by adding excess phosphorus to ecosystems, leading to problems like eutrophication in aquatic systems.

Nitrogen-fixing bacteria play a crucial role in the nitrogen cycle by converting atmospheric nitrogen (N₂) into a form that plants can use, typically ammonia (NH₃). These bacteria, found in soil or in symbiotic relationships with certain plants like legumes, possess the enzyme nitrogenase, which enables this conversion. This process is vital because atmospheric nitrogen is inert and cannot be directly utilized by plants or animals. Once converted to ammonia, it is further processed into nitrates and nitrites by other soil bacteria. Plants absorb these nitrogen compounds through their roots and use them to synthesize amino acids, the building blocks of proteins. Since proteins are essential for various structural and metabolic functions in plants, nitrogen fixation is fundamental for plant growth and development. Furthermore, plants use these nitrogen-containing compounds to synthesize nucleic acids (DNA and RNA), which are critical for genetic information storage and transmission. Without nitrogen-fixing bacteria, the availability of usable nitrogen would be severely limited, impacting plant growth and, consequently, the entire food chain.