Which organism indicates past or possibly intermittent pollution in water?

Clostridium spores are indicative of past or possibly intermittent pollution in water because they have a resilient life cycle that allows them to survive harsh environmental conditions. These spores can remain dormant in the environment for extended periods. When they are detected in water sources, it suggests that there has been fecal contamination or pollution in the past, as Clostridium species are often associated with anaerobic environments found in decaying organic matter or animal waste. Their presence indicates that there may have been periods of pollution that allowed these spores to enter the water system.

In contrast, other organisms such as Escherichia coli are more commonly associated with recent fecal contamination, reflecting ongoing pollution rather than historical. Lactobacillus acidophilus is typically a non-pathogenic bacterium associated with healthy gut flora and not a reliable indicator of water quality. Staphylococcus aureus can be found in various environments and may be present due to current contamination, but it does not have the same implications regarding intermittent or past pollution as Clostridium spores do. Thus, the resilience and history associated with Clostridium make it the correct choice for identifying past or intermittent pollution in water.