Phosphorus is a key component for life on Earth; it performs essential roles in respiration, photosynthesis, and the decomposition of organic material. Phosphorus is primarily acquired by plants in the inorganic, ionic forms, which are found in soil solutions at concentrations of only a few parts per million. Plants use methods of diffusion and active transport to absorb phosphorus at the surface of their roots.
Phosphorus is abundant in soils; however, it is often unavailable to plants because it forms insoluble complexes with positively charged cations. This occurs when negatively charged phosphorus ions bind to positive cations in the soil (i.e. opposites attract). Enzymes such as acid phosphatases play a critical role in the acquisition and manipulation of phosphorus in plants. It has been found that when soils possess low levels of free phosphorus, plants are stimulated to produce acid phosphatase enzymes, which release inorganic phosphorus in the soil.
Plants need nutrients such as phosphorus to grow and proliferate; therefore, understanding how this species uses phosphorus could lead to conservation practices to limits this invasive species’ impact on the environment. A group of scientists wanted to investigate the relationship between plant enzyme activity and phosphorus levels in aquatic biomes. This investigation was targeted at studying the invasive Eurasian milfoil, Myriophyllum spicatum. In doing so, scientists believed that they could control the spread of the plant into neighboring waters if they were able to limit the nutrients in its environment. A study was performed in order to explore how phosphorus concentration in freshwater ecosystems affects phosphorus cycling and plant enzyme production. In order to determine if there was a relationship between the phosphatase activity and concentration of phosphorus in the sediment and water column of specific sites, researchers measured the respective phosphorus concentrations and enzyme activities.
In this study, three standing ponds were sampled at six different time periods in the same year from July to December. The phosphorus content of each sample was determined through an ascorbic acid assay. Sediment collections were divided into samples weighing one sixteenth of a gram using coning and quartering techniques. These samples underwent a persulfate digestion and were vacuum filtered to remove excess sediment. Last, the samples were diluted and analyzed for phosphorus content using the ascorbic acid procedure. The solutions’ ability to absorb specific wavelengths of light was measured using a spectrophotometer set at an absorbance of 880 nanometers. After the sediment phosphorus content of each site was determined, scientists decided to determine the concentration of phosphorus releasing enzymes through an alkaline phosphatase assay. One to two milliliters of collected sediment was centrifuged until the sediment formed a pellet. The scientists chemically induced and observed a reaction between the sediment enzymes and insoluble phosphorus compounds present in an artificial substrate. Enzymes speed up chemical reactions by binding to substrates and releasing their constituent parts: in this case phosphorus and an unknown cation. After this reaction was halted, the samples were centrifuged and their absorbance was measured with a spectrophotometer set at 420 nanometers. This identified the concentration of phosphatase enzymes present in each sample. Now, scientists were able to compare the correlation between phosphorus levels and plant enzymes in the soil.
An exponential regression analysis indicated that there was a significant relationship between phosphatase activity and sediment phosphorus concentration (see Figure 1). The trend in the exponential regression analysis showed evidence of an inducible expression between phosphorus substrate and phosphatase enzymes. In other words, phosphorus is often abundant in soils, but is unavailable due to its formation into insoluble complexes with aluminum and iron. As a result, acid phosphatase enzymes break down these insoluble complexes and release phosphorus for plant acquisition and usage; therefore, phosphorus rich environments should possess greater phosphatase activity.
This study supported the expectation that there would be a relationship between phosphatase activities and sediment phosphorus concentration (i.e. higher phosphorus concentrations increase yields of plants, while phosphorus limitation decreases the productivity of invasive species). The scientists hoped to use the findings in this study for the development of bioremediation techniques aimed at controlling invasive species through green management practices.
Figure 1 represents a correlation between phosphorus concentration and phosphatase activity in all the sites across all time periods.
