3.2.1. Pseudo-nitzschia and ASP Toxins
Pseudo-nitzschia was observed throughout the sampling period and is the only potentially toxic phytoplankton genus that formed a dominant bloom according to the cell counts. The microarray detected three of five Pseudo-nitzschia genus-level probes (PSN + some Frags_25_dT, PSN + FRAGS02-25new_dT and PsnGS02_25_dT) throughout the sampling period (Figure 3(a)). The other two generic-level probes (PsnGS01_25_dT, and PSN no pungens_25_dT) were excluded because the S/N ratio was not always above two. These two are not as strong as the other three probes, which are positioned at the top of the hierarchy file and thus do not cause the hierarchy test to fail. Weaker probes are always placed inside stronger probes to prevent such failure of true positives. Domoic acid (DA) was detected with ELISA [23] (Table 5) in both October samples (4A and 6A) but not in sample 3A (22.08.2011) where 400 cells·L−1 of P. multistriata were counted. This result suggests that the threshold for detecting DA with ELISA is somewhere between 400 and 1,700 cells·L−1 for the species P. multistriata. Furthermore, the Multi SPR gave no signal even though the last October sample had 40,600 cells·L−1. In general, it was found that the ELISA was more sensitive to lower amounts of toxin than the Multi SPR [23]. Because it is quite arduous to identify Pseudo-nitzschia multistriata to the species-level with light microscopy, and because some of our species-specific probes are still being optimized, we focused our comparison on P. multistriata (i.e., the “sigmoid” group) with two genus-level probes and three species-level probes on the array. The October bloom of 40,600 cells·L−1 of Pseudo-nitzschia multistriata matched the microarray with positive hits (S/N ratio above 2) of the two genus-level probes (PSN + some Frags_25_dT and PSN+FRAGS02-25new_dT) and the three species-level probes (PmulausD01_25_dT, PmulacalD02_25_dT, and PmulaD03_25_dT) (Figure 3(b)). The probe PcalfrauD04_25_dT (now interpreted to be a genus-level probe because it cross-reacted with all Pseudo-nitzschia spp. tested) showed consistent high signals for all Pseudo-nitzschia spp. in calibration curves (data not shown) and field samples.
Figure 3  Microarray signals of (a) the Pseudo-nitzschia spp. Genus-level probes (PSN + some Frags_25_dT, PSN + FRAGS02-new_dT and PsnGS02_25_dT) and (b) P. multistriata species-level probes (PmulausD01_25_dT, PmulacalD02_25_dT, PmulaD03_25_dT) normalized against Dunaliella tertiolecta (DunGS02_25_dT) for the field samples taken in Arcachon Bay, France and compared to cell counts. The graphs show only probes that yielded a signal above the detection limit (signal/noise ratio > 2), except for PmulaD03_25_dT, which is only in sample 6A above the S/N ratio. The sampling dates (24.07.2011, 08.08.2011, 22.08.2011, 04.10.201 and 20.10.2011) correspond to the sampling names: 1A, 2A, 3A, 4A and 6A. Cell counts are depicted in log10 on the secondary y-axis and as columns.
microarrays-02-00001-t005_Table 5 Table 5  Toxins measured by Multi SPR and ELISA during the sampling period in Arcachon Bay, France, adapted from [23].

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