Geographic distribution of Karlodinium veneficum in the US east coast as detected by ITS–ferredoxin real-time PCR assay

Karlodinium veneficum is an ichthyotoxic dinoflagellate suspected to be responsible for massive fish kills. Because it is morphologically similar to Pfiesteria and related dinoflagellates, accurate abundance data of K. veneficum in the natural environment are difficult to obtain using light microscopy. In this study, species-specific real-time PCR assays were developed using the ferredoxin gene (KvFERR) and rDNA locus (partial internal transcribed spacer regions and 5.8S rDNA, KvITS). These assays were used to investigate the geographic and temporal distribution of K. veneficum from various estuaries and coastal regions of the Pacific and West Atlantic Oceans. In general, both genes gave similar quantitative results, detecting down to 0.1–1 K. veneficum cells mL−1. The higher occurrence frequency and abundance of K. veneficum were observed in Neuse River, North Carolina, Narragansett Bay, Rhode Island and Chesapeake Bay, MD, USA. Most of the KvITS sequences from North Carolina to Maine were identical to that of K. veneficum strain CCMP1975 (initially isolated from Chesapeake Bay), with 1–2 nucleotide substitutions occurring in sequences from Texas and Narragansett Bay samples. This result suggests the same origin and recent separation of K. veneficum populations living in this wide geographic range. In this study, K. veneficum was more abundant at 5–15°C and 4–20 psu.