Algae and the Phosphorus Cycle

The phosphorus cycle is the simplest of the biogeochemical cycles. Phosphorus is the eleventh most abundant mineral in the Earth’s crust and does not exist in a gaseous state. Natural inorganic phosphorus deposits occur primarily as phosphates, that is, a phosphorous atom linked to four oxygen atoms, in the mineral apatite. The heavy molecule of phosphate never makes its way into the atmosphere; it is always a part of an organism, dissolved in water, or in the form of rock. Cycling processes of phosphorus are the same in both terrestrial and aquatic systems. When a rock with phosphate is exposed to water (especially water with a little acid in it), the rock is weathered out and goes into solution. Autotrophs (algae and plants) assimilate this dissolved phosphorus up and alter it to organic phosphorus using it in a variety of ways. It is an important constituent of lipid portion of cell membranes, many coenzymes, DNA, RNA, and, of course ATP. Heterotrophs obtain their phosphorus from the autotrophs they eat. When heterotrophs and autotrophs die (or when heterotrophs defecate), the phosphate may be returned to the soil or water by the decomposers.

There, it can be taken up by other autotrophs and used again. This cycle will occur over and over until at last the phosphorus is lost at the bottom of the deepest parts of the ocean, where it becomes part of the sedimentary rocks forming there. If the rock is brought to the surface and weathered, this phosphorus will be released. During the natural process of weathering, the rocks gradually release the phosphorus as phosphate ions, which are soluble in water and the mineralized phosphate compounds breakdown. Phosphates PO₄³² are formed from this element. Phosphates exist in three forms: orthophosphate, metaphosphate (or polyphosphate), and organically bound phosphate, each compound containing phosphorus in a different chemical arrangement. These forms of phosphate occur in living and decaying plant and animal remains, as free ions or weakly chemically bounded in aqueous systems, chemically bounded to sediments and soils, or as mineralized compounds in soil, rocks, and sediments.

Orthophosphate forms are produced by natural processes, but major man-influenced sources include: partially treated and untreated sewage, runoff from agricultural sites, and application of some lawn fertilizers. Orthophosphate is readily available to the biological community and typically found in very low concentrations in unpolluted waters. Polyforms are used for treating boiler waters and in detergents. In water, they are transformed into orthophosphate and become available for autotrophs uptake. The organic phosphate is the phosphate that is bound or tied up in autotrophs, waste solids, or other organic materials. After decomposition, this phosphate can be converted to orthophosphate.

Algae and plants are the key elements to passing on phosphates to other living organisms, but their importance in phosphorus cycle is connected mainly to the impact of this element on their growth. As already remarked, both phosphorus and nitrogen are among the nutrients that can become limiting, hence an overloading of these two elements leads to dramatic changes in the structure and functioning of an ecosystem.

Phosphorus, in the form of orthophosphate, is generally considered the main limiting nutrient in freshwater aquatic systems; that is, if all the phosphorus is used, autotroph growth will cease, no matter how much nitrogen is available. In phosphorus limited systems, excess phosphorus will trigger eutrophic condition. In these situations the natural cycle of the nutrient becomes overwhelmed by excessive inputs, which appear to cause an imbalance in the “production versus consumption” of living material (biomass) in an ecosystem. The system then reacts by producing more phytoplankton/vegetation than can be consumed by the ecosystem. This overproduction triggers the series of events determining the aging process of the water body.

Under aerobic conditions, as water plants and algae begin to grow more rapidly than normal, there is also an excess die off of the plants and algae as sunlight is blocked at lower levels. Bacteria try to decompose the organic waste, consuming the oxygen and releasing more phosphate, which is known as “recycling or internal cycling.” Some of the phosphates may be precipitated as iron phosphate and stored in the sediment where it can then be released if anoxic conditions develop. In deeper environments, the phosphate may be stored in the sediments and then recycled through the natural process of lithotrophication, uplift, and erosion of rock formations. In anaerobic conditions, as conditions worsen as more phosphates and nitrates may be added to the water, all of the oxygen may be used up by bacteria in trying to decompose all of the waste. Different bacteria continue to carry on decomposition reactions; however, the products are drastically different. The carbon is converted to methane gas instead of CO₂; sulfur is converted to hydrogen sulfide gas. Some of the sulfide may be precipitated as iron sulfide. Under anaerobic conditions the iron phosphate precipitates in the sediments may be released from the sediments making the phosphate bioavailable. This is a key component of the growth and decay cycle. The water body may gradually fill with decaying and partially decomposed plant materials to make a swamp, which is the natural aging process. The problem is that this process can been significantly accelerated by man’s activities.

Phosphates were once commonly used in laundry detergents, which contributed to excessive concentrations in rivers, lakes, and streams. Most detergents no longer contain phosphorus. Currently, the predominant outside sources of phosphorus are agricultural and lawn fertilizers and improperly disposed animal wastes.