Assessing the abundance of the ichthyotoxic Karenia mikimotoi by quantitative PCR and microscopy

The dinoflagellate Karenia mikimotoi may form blooms causing fish kills in coastal waters of the North East Atlantic region, including Norway and Sweden. A K. mikimotoi specific quantitative PCR (qPCR) assay was developed, based on the use of SYBR Green 1. The abundance of Karenia was assessed in samples from Outer Oslofjorden (Station OF2) collected monthly from August 2011 to June 2012, and 10 stations in Skagerrak collected during a cruise in April/May 2012. According to the qPCR assay, Karenia was present in all samples at station OF2, with a peak in August and September and with very low numbers in February, March, April and May. Karenia was also identified by light microscopy of net hauls (20 μm mesh) in both August and September. Cell counts were performed on the same samples and Karenia was only recorded in September (180 000 cells L-1). At the stations of the 2012 April/May Skagerrak cruise the abundance of Karenia was either very low or below the detection limit of our qPCR assay and Karenia was not observed by microscopy in any of the corresponding cell counts or net hauls. These results demonstrate that the qPCR assay was more sensitive than traditional counting methods using light microscopy in the detection of Karenia. In addition to traditional microscopy based methods, qPCR assays can be a powerful tool in the identification and quantification of Karenia and other harmful algae and may prove especially useful if applied in early warning programs.