Detection and quantification of a keystone pathogen in a coastal marine ecosystem

The pathogenic amoeba Paramoeba invadens causes recurrent mass mortalities of green sea urchins Strongylocentrotus droebachiensis in coastal Nova Scotia, Canada, driving regime shifts from urchin barrens to kelp beds. Outbreaks of the disease (paramoebiasis) are sporadic, and the source population(s) and epizootiology of the amoeba are poorly understood. We developed PCR-based detection of P. invadens in urchin tissue, sediment, and seawater. Primers specific to the P. invadens nuclear SSU rRNA gene were designed and used in PCR and qPCR analyses to better detect and quantify P. invadens during, following, and in the absence of a natural disease outbreak. A comparison of pathogen load in asymptomatic and symptomatic sea urchins indicated a lower threshold of ∼1 cell mg^-1 tissue for observing overt signs of paramoebi-asis in urchins. P. invadens was detected for the first time in sediment during and following an outbreak of disease in 2014. It also was detected in low abundance (<10 cells l^-1) in seawater in fall 2015 in the absence of sea urchin mass mortality or a strong storm event, but not under similar conditions in summer/fall 2016 and 2017. The ability to detect and quantify this pathogen in sea urchins and environmental samples sheds new light on mechanisms of introduction, spread, and persistence of P. invadens along the Nova Scotian coast and the role of large-scale meteorological events and ocean warming in these processes.