Sulfur dioxide (SO2) air pollution is often monitored using bioindicator organisms. Which of the following groups of organisms is widely used as an indicator of sulfur dioxide pollution in the atmosphere?

Introduction / Context:
Sulfur dioxide (SO2) is a major gaseous air pollutant released mainly from thermal power plants, industries, and the burning of fossil fuels. Instead of measuring SO2 only with mechanical instruments, environmental scientists also rely on living organisms that respond in a sensitive and predictable way to pollution. Such organisms are called bioindicators. This question tests whether you know which group of organisms is classically used as a biological indicator of sulfur dioxide pollution in the atmosphere.

Given Data / Assumptions:

• We are dealing with air pollution by sulfur dioxide gas in the atmosphere.
• The task is to identify a group of organisms that is widely used as an indicator of such pollution.
• The options include algal blooms, lichens, bryophytes, protozoa and cyanobacteria.
• We assume a standard school level understanding of environmental biology and pollution indicators.

Concept / Approach:
The core concept is the idea of bioindicators, which are organisms that show clear changes in presence, absence, or health in response to particular pollutants. Lichens are well known for being extremely sensitive to atmospheric pollutants, especially sulfur dioxide. In clean air, lichens show healthy growth on tree bark, rocks, and walls, while in polluted air they decline or disappear. Therefore, their distribution pattern can be used to monitor long term air quality. Algal blooms, on the other hand, are mainly associated with nutrient pollution in water bodies rather than with gaseous air pollutants like sulfur dioxide.

Step-by-Step Solution:
Step 1: Recall that sulfur dioxide is a gaseous pollutant that primarily affects air quality and plant surfaces. Step 2: Review which organisms are commonly introduced in textbooks as bioindicators of air pollution, especially SO2, in urban and industrial areas. Step 3: Lichens are famously used as indicators because many lichen species are very sensitive to sulfur dioxide and heavy metals, and their health reflects long term air quality. Step 4: Algal blooms usually indicate nutrient enrichment in water (eutrophication) due to nitrates and phosphates, not specifically sulfur dioxide in air. Step 5: Bryophytes, protozoa, and cyanobacteria can be affected by pollution but are not the standard textbook bioindicators of sulfur dioxide pollution in air. Step 6: Therefore, the best answer is the option that refers to lichens on tree bark and rocks.

Verification / Alternative check:
A quick way to verify is to remember that many environmental science diagrams show maps of lichen distribution around cities to monitor air pollution. In areas with very high sulfur dioxide levels, lichens are absent or greatly reduced, whereas in cleaner rural or hilly regions they are abundant. This cross checks our reasoning. No such standard mapping is routinely done with algal blooms, bryophytes, protozoa or cyanobacteria to monitor sulfur dioxide levels in air.

Why Other Options Are Wrong:
Algal blooms in nutrient rich waters: These indicate eutrophication of lakes and ponds due to excess nutrients, not sulfur dioxide air pollution. Bryophytes such as mosses on rocks and walls: Mosses can be affected by pollution but they are not the classic or most widely used indicator of sulfur dioxide in air. Protozoa in freshwater habitats: Protozoa are mainly used to study water quality or disease, not to monitor gaseous air pollutants. Cyanobacterial mats on the surface of ponds: Cyanobacteria can bloom in nutrient rich water and indicate water pollution rather than sulfur dioxide in air.

Common Pitfalls:
Students sometimes confuse any kind of pollution indicator with specific indicators of sulfur dioxide. Many learners know that algae and cyanobacteria form blooms in polluted water, so they may pick algal blooms without reading that the question is about sulfur dioxide in air, not nutrients in water. Another common error is to think that bryophytes are used as indicators in the same widespread way as lichens. In competitive exam biology, lichens are the standard answer for sulfur dioxide air pollution indicators and that pattern is important to remember.

Final Answer:
Thus, the organisms widely used as bioindicators of sulfur dioxide air pollution are Lichens growing on tree bark and rocks.