Specificity of the PNA probe
The specificity of the PNA probe (PP0443A) should be considered as a critical point for FISH detection. To achieve this, the probes were firstly designed based on the multiple sequence alignment involving the LSU D1–D2 sequences of P. donghaiense and all other Procentrumavailable in Genbank. Next, BLAST searches were performed on the designed probes, confirming that the sequences of probes could exclusively match with P. donghaiense. Finally, cross-reactivity of the screened probe against other microalgae was tested. The positive (DU0512A) and negative (DU0499S18) control treatments were included to define a range of labeling intensities possible for any given sample and thereby provided a reference from which to assess the reactivity of specific probe.
The FISH trials served as an intermediate step to determine whether a candidate probe could access its target sequence. Therefore, no attempt was made to optimize the whole cell hybridization conditions and the list of species used in the trials was also limited. The results of hybridization with all test species using the PNA probe and control probes are shown in Table 2. The positive probe could react with all test species, repeatedly giving bright and uniform label intensity for all species examined. Contrarily, the negative probe could not label any species, and the cells treated by negative probe appeared uniformly dark. In contrast, PP0443A reacted exclusively with P. donghaiense. Based on these, the specific PNA probe may be speculated to be useful for molecular identification of the target species in natural samples containing many different microalgae.