3.2.3. Alexandrium and PSP Toxins
Two cells of the genus Alexandrium (ca. 20 cells·L−1) were counted only in sample 1A, whereas the microarray detected it throughout the sampling period (Figure 4(b)) with the highest signal at the end of October (sample 6A). Furthermore, PSP toxins were detected with ELISA in late August (sample 3A) and the remaining sampling period, as well as with the Multi SPR in sample 6A (Table 5, see [23] for more discussion on toxin found in these samples). If toxin probes are efficient and therefore PSP toxins are indeed present, there are two different ways to explain the absence of Alexandrium in cell counts: either Alexandrium cells have effectively been missed with the microscope, or there are other PSP-containing microorganisms in the water that are not identified. Neither A. ostenfeldii, A. minutum nor A. tamarense probes were detected by the microarray and their calibration curves for each specific probe have a detection limit of 200 cells [24]. Thus, we are unsure as to which species could be contributing to the PSP toxin profile. It could be Gymnodinium catenatum (see below) or another member of the genus Alexandrium. A. pseudogonyaulax could be a potentially missed Alexandrium species. There are no A. pseudogonyaulax-specific probes on the microarray. There are also many species that are not well investigated for toxin production. However, our data underlines the importance of including additional genus- and species-level probes for Alexandrium, in order to capture the full variability found in this genus. In any case, the detection of Alexandrium and its PSP toxins shows the advantage of the combination of the two methods (species and toxins) to detect harmful species, as well as to detect new invasive species as climate changes and tropical species move into temperate regions.