Studies on the digestive gland microbiome of freshwater mussels

Abstract

Freshwater mussels are among the most imperiled aquatic invertebrates in North America. In the United States, Alabama has the highest diversity of freshwater mussels with 178 species. To address the conservation needs of Alabama’s native freshwater mussels, ADCNR opened the Alabama Aquatic Biodiversity Center in 2005. The purpose of this facility is to lead captive propagation and reintroduction efforts of several unionid and other non-game species. However, understanding the role of bacterial communities (‘microbiome’) in mussel health has not yet been explored. The microbiome is defined as a community of symbiotic bacteria and their genes that provides the host with many benefits. These bacteria establish complex and dynamic interactions with their host. When the microbiome becomes imbalanced (e.g. the host experiences an environmental insult) those interactions are disrupted and could result in a pathological state for the host. The term dysbiosis is used to refer to an unbalanced microbiome. In these studies, I characterized the bacterial community associated with the digestive gland (‘gut’) of freshwater mussels using 16S rRNA gene sequencing. My results showed that the microbiomes from wild and cultured/captive mussels are significantly different from each other and that Tenericutes is the most abundant bacterial phylum in the microbiome of the mussels species analyzed to date. Tenericutes, specifically the bacterial class Mollicutes, are known intracellular parasites of humans, and their role in the microbiome of freshwater mussels is unknown. I also characterized the bacterial communities of water and sediment surrounding the mussels and found them diverse but very distinct from the digestive gland microbiome of mussels. When mussels were challenged with environmental changes or chemical insults (e.g. rearing environment or antibiotics), freshwater mussels become dysbiotic a stage that was characterized by a loss of Tenericutes and an increase of Proteobacteria and Firmicutes. Overall, there was a significant change in bacterial diversity and a shift to a pathogen-dominated microbiome. I conclude that certain rearing environments modify the natural gut microbiome of freshwater mussels. This was particularly evident when wild mussels were relocated to laboratory facilities.