In the Great Lakes, invasive dreissenid mussels are threatening the health of coastal ecosystems. These mussels, which were introduced from other regions, change how food webs work and have been linked to harmful algae blooms and outbreaks of bird botulism. Past research has shown that they affect lakes partly by changing bacterial communities in the water.
A recent Michigan Sea Grant graduate research project studied interactions between animals and microbes in the Great Lakes. Nikesh Dahal, Ph.D., from the University of Michigan, examined how zebra and quagga mussels change bacterial communities in Lake Erie and Lake Huron and in some smaller lakes in the region. He looked at how bacteria respond to mussel feeding, how the microbial component of the food webs change, and how environmental conditions affect broader microbial diversity patterns.
Based on predictions of classical ecological theory of resource allocation tradeoffs, scientists might expect that bacteria resistant to being eaten by mussels would grow more slowly, while bacteria that were easier for mussels to eat would grow faster. However, this project found that resistant bacteria often grew just as fast – or even faster – than susceptible bacteria. These resistant bacteria also had a higher diversity of mobile genetic elements that may help them adapt to diverse conditions in lakes. These genetic elements are those features that can move between populations through viruses or other transfer mechanisms.
At the community level, resistant bacteria also tended to stick around over time rather than become more abundant. This situation suggests that microbial resistance to being eaten by invasive mussels may be associated with bacterial survival across a range of environments, rather than providing short-term competitive edge. Overall, taken together, the findings suggest that microbial resistance is a complex trait, and changing environments may lead to evolution of diverse ecological strategies, as opposed to fixed and binary strategies predicted under classical ecological theory.
The western basin of Lake Erie was one of the first places in North America where these mussels became established. In addition, the spread of harmful algal blooms, dominated by cyanobacteria, in this region has been previously linked to the selective feeding behavior of these mussels. As such, if simple trade-offs between growth and defense were the main factor shaping bacterial diversity, then we would expect to see that occurring in this system. Instead, these findings point to a more complex picture of how invasive mussels influence microbial communities.
This research advances our understanding of how species introductions reshape microbial diversity patterns, with implications for management of harmful algal blooms and water quality in Lake Erie.
