What type of phosphorus is readily soluble and available for algal growth?

The correct answer is reactive phosphorus. This form of phosphorus is readily soluble in water, which makes it highly available for biological processes, including algal growth in aquatic ecosystems. Reactive phosphorus can exist in various forms, such as phosphate ions, which are easily absorbed by plants and algae. When phosphorus enters water bodies, it can promote nutrient pollution, leading to eutrophication, which is characterized by excessive growth of algae and depletion of oxygen in the water.

Other forms of phosphorus, such as particulate phosphorus, are bound to soil particles and are not as readily available for immediate biological uptake. Organic phosphorus, while an important nutrient source, requires microbial action to convert it into reactive forms before it becomes available to algae. Fixed phosphorus refers to phosphorus that is chemically bound to soil minerals and is often not accessible for plant uptake, thus reducing its availability for algal growth.