Effect of environmental factors on survival and growth of quahog parasite unknown (QPX) in vitro

Quahog parasite unknown (QPX) is a protistan microorganism associated with mass mortalities of hard clams (Mercenaria mercenaria) along the northeastern coasts of the United States and maritime Canada. Because several studies indicate modulatory effects of prevailing environmental parameters on disease outbreaks, this study tested the effect of major environmental parameters (temperature, salinity and oxygen concentration; individually or combined) on QPX survival in artificial seawater and parasite growth in culture media in vitro. Three QPX isolates from two different geographic locations were compared. Results indicated that in vitro growth of QPX was optimal in standard culture medium at 34. ppt between 20°C and 23°C. Additionally, significant differences in temperature optima were observed for geographically distinct QPX isolates (p<0.001) confirming previous studies suggesting the existence of different QPX strains (or ecotypes). When tested in seawater, QPX exhibited opposite trends with higher survival at 15°C and 15. ppt. Results also demonstrated limited survival and growth of QPX under anoxic conditions. Additionally, results showed that the parasite is able to survive extreme temperatures (-12°C to 32°C) suggesting that QPX could overcome short periods of extreme conditions in the field. These results contribute to a better understanding of interactions between QPX and its environment, but potential impacts of environmental conditions on QPX disease development need further work as it also involves clam response to these factors.