This study found that particular marine bacteria have well-defined niches, like many metazoan species. This means for for many bacteria, the presence or absence of them can indicate changes in the environment.
The idea of monitoring species to provide some idea of the health of an ecosystem has been around for a long time. Usually, though, researchers have looked to the opposite edge of the food web, at top predators, for instance, to fill the role of an indicator species.
There are a number of potential advantages to using particular bacterial species for this task. First, there is ubiquity. If you use a top predator, you have a lot of work ahead of you doing surveys and accounting for bias due to the level of effort put into the sampling. This isn’t a problem with bacteria. Second, there are the numbers. Lots and lots of bacteria versus exceedingly few top predators. You can make various quantitative measurements that likely just aren’t possible with rarer metzoans. Third, nobody gives a rip about the bacteria in your water samples. No paperwork, no permits required, and no PETA protesters to worry about.
Despite these considerations, I find it personally difficult to get worked up over microbial ecology. But it looks good for folks who already work in microbiology who want to get into marine science.