27.10
Il ciclo del fosforo
A differenza del carbonio, dell'acqua e dell'azoto, il fosforo non è presente nell'atmosfera come gas. Invece, la maggior parte del fosforo nell'ecosi…
- [Narratore] Nel ciclo del fosforo,
gli atomi di fosforo esistono inizialmente nella roccia,
non in stato gassoso.
Quando la roccia viene erosa, il fosforo viene rilasciato
e si discioglie in fiumi, laghi e acque freatiche.
Piante e alghe utilizzano questi fosfati inorganici liberi
nel suolo o nell'acqua per produrre molecole organiche.
Nutrendosi di piante, gli eterotrofi attingono
alle scorte di fosforo per costruire i propri composti.
Quando gli organismi muoiono, i decompositori rompono
le molecole di fosforo e rilasciano fosfati inorganici
che verranno nuovamente usati da piante e alghe,
in un processo chiamato mineralizzazione del fosforo.
Mentre i fosfati concludono rapidamente
il loro ciclo negli organismi della catena alimentare,
permangono più a lungo nell'acqua.
Gli ioni fosfato disciolti reagiscono
formando composti insolubili
che precipitano negli oceani e sedimentano, stratificano
e come risultato dei sollevamenti tettonici
ad un certo punto ritornano nell'ambiente.
Tuttavia dato che la quantità di fosforo
nell'ambiente è limitata,
questo va fornito alle colture come fertilizzante,
per massimizzare la resa.
L'eccesso di fosforo ruscella nell'ecosistema acquatico,
determinando diversi problemi ambientali,
come la fioritura delle alghe.
View the full transcript and gain access to JoVE Core videos
Q1: Why is phosphorus not found in the atmosphere like carbon or nitrogen?
Phosphorus exists primarily in rock and mineral forms rather than as a gas. Unlike carbon and nitrogen, phosphorus lacks a gaseous phase in the atmosphere. Instead, it cycles through soil, water, sediment, and organisms as inorganic phosphate ions and organic compounds found in biological structures like DNA and cell membranes.
Q2: How does phosphorus move from rocks into soil and water?
Rock and sediment weathering releases inorganic phosphate over time. This phosphate gradually reaches soil and surface water through natural erosion processes. Once dissolved in streams, lakes, and groundwater, phosphate becomes available for plants and algae to absorb and incorporate into organic molecules for growth.
Q3: What role do decomposers play in the phosphorus cycle?
When organisms die or excrete waste, decomposers break down organic phosphate compounds in a process called phosphate mineralization. This converts organic phosphates back into inorganic forms that plants and algae can reuse. This recycling allows phosphorus to cycle quickly through food chains and support continued biological productivity.
Q4: How does phosphorus eventually return to land from the ocean?
Phosphates transported to oceans by natural runoff accumulate in sedimentary layers on the ocean floor. Over thousands of years, geological uplift returns phosphorus-containing rocks from the ocean to land, completing the geochemical cycle. This slow process replenishes terrestrial phosphorus reserves over extended timescales.
Q5: Why do farmers add phosphorus to agricultural fertilizers?
Phosphorus is often a limiting nutrient in natural environments, restricting plant growth. Adding phosphorus to fertilizers increases crop yield by providing plants with adequate phosphate for organic molecule synthesis. However, excess phosphorus from agricultural runoff can cause environmental problems such as algal blooms in aquatic ecosystems.
Q6: What environmental problems result from excess phosphorus in water?
Agricultural runoff containing excess phosphorus stimulates rapid growth of aquatic producers, causing algal blooms and other environmental problems. Unlike the nitrogen cycle, phosphorus has a longer residence time in water, allowing dissolved phosphate ions to accumulate and precipitate as insoluble compounds that disrupt aquatic ecosystems.
Q7: How do heterotrophs obtain phosphorus in food chains?
Heterotrophs acquire phosphorus by consuming plants and other animals that have already incorporated phosphate into their organic structures. This transfers phosphorus through trophic levels, allowing animals to build their own phosphorus-containing compounds for bones, teeth, cell membranes, and other biological molecules.