OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

What role do bacteria play in the process of eutrophication?

1. They produce oxygen for aquatic life

2. They decompose dead plants, using up oxygen in the water

3. They prevent the growth of algae

4. They enhance photosynthesis in aquatic plants

The correct answer is: They decompose dead plants, using up oxygen in the water

Bacteria play a crucial role in the process of eutrophication, particularly through their involvement in decomposition. In aquatic ecosystems, when excess nutrients, such as nitrogen and phosphorus, enter the water due to agricultural runoff or wastewater, algae often bloom as a response to these nutrients. When these algal blooms die, bacteria rapidly decompose the dead algae to break them down into simpler substances. This decomposition process requires oxygen, which bacteria consume from the water. As the bacterial population increases to break down the organic matter, they deplete the oxygen levels in the water. This reduction in dissolved oxygen can lead to hypoxic conditions, which are detrimental to aquatic life, particularly fish and other organisms that rely on oxygen to survive. Thus, the activity of bacteria in decomposing organic matter contributes significantly to the negative effects of eutrophication on aquatic ecosystems. The other options do not accurately reflect the role of bacteria in eutrophication. They do not produce oxygen or prevent algal growth; in fact, their action can exacerbate the problem by leading to oxygen depletion. Similarly, they do not directly enhance photosynthesis in aquatic plants, as their primary role is in the breakdown of organic material and nutrient cycling.