Probes screening
The results of FISH using all the DNA probes are summarized in Table 2 and Fig. 1. P. donghaiense could not be labeled by the probes targeting both SSU rRNA (DP1704A23) and ITS rDNA (DP0159A25, DP0498A21). The complex second structure of rRNA may preclude its hybridization with DP1704A23, since rRNA expression in cells is often thought to be at a high level. Except for certain species [8], rDNA is generally thought to be unsuitable for probe targeting, because the cells labeled with rDNA targeting probe tend to display weak fluorescence [32], which disturb their differentiation from other species in natural samples [39]. Things seem to get worse for P. donghaiense, since the cells marked by both DP0159A25 and DP0498A21 did not display any visible fluorescence under epifluorescence microscopy. The possible reason for this is that the copies of ITS rDNA within genomic DNA of P. donghaiense are at least less than A. catenella [32], A. tarmarense [32] and H. akashiwo [8]. Therefore, P. donghaiense cells could not provide enough biding molecules for the rDNA targeting probe, and the hybridized cells with less fluorescein labeled probe naturally give out weak and even invisible fluorescence, as shown in this study.The effect of the secondary structure of the LSU rRNA on the accessibility of probes to the target sites has been shown in previous studies [40]–[42]. Again, our findings reconfirm this. The four rRNA-targeted probes with even slight alternation in the 5′ portion of the sequence displayed different performance (Table 2 and Fig. 1). Among them, only DP0443A labeled P. donghaiense cells with fluorescent intensity equivalent to the positive control probe (DU0512A18), while P. donghaiense cells marked by DP0587A did not show any fluorescence. The cells labeled with DP0602A and DP0512A displayed more or less intensive fluorescence compared with the positive control probe labeling cells, respectively. The further quantification analyses of fluorescent intensity of cells labeled with different probes were shown in Fig. 2. Apparently, the fluorescent intensity of DP0443A labeling cells were significantly more intensive (p