Does prey availability influence the detection of Dinophysis spp. by the imaging FlowCytobot?

The Imaging FlowCytobot (IFCB) is a field-deployable imaging-in-flow cytometer that is increasingly being used to monitor harmful algae. The IFCB acquires images of suspended particles based on their chlorophyll-a fluorescence and/or the amount of light they scatter (side scattering). The present study hypothesized that fluorescence-based image acquisition would undercount Dinophysis spp., a genus of non-constitutive mixotrophs, when prey is limited. This is because Dinophysis spp. acquire plastids via ingestion of their ciliate prey Mesodinium spp., and lose photosynthetic capacity and autofluorescence in the absence of prey. Even small blooms of Dinophysis spp. can be highly toxic and result in diarrhetic shellfish poisoning (DSP), highlighting the importance of accurately detecting low abundances. To explore this, laboratory experiments were conducted to determine optimal IFCB settings for a fed culture of Dinophysis acuminata, and an existing time series of IFCB observations collected in Puget Sound (Washington, U.S.A) was used to compare Dinophysis spp. abundance estimates from samples triggered via side scattering versus fluorescence in relation to Mesodinium spp. abundance. This study introduces a quantitative approach for optimizing the detection of target harmful algae which can be repeated across multiple IFCBs and demonstrates the effects of IFCB calibration on Dinophysis spp. detection. The laboratory experiments showed that IFCB settings for fluorescence-based image acquisition need to be fairly sensitive to accurately detect D. acuminata cells. A poorly calibrated IFCB can miss a significant proportion of D. acuminata abundance whatever the method used to trigger the image acquisition. Field results demonstrated that the physiological status of Dinophysis spp. can influence their detection by the IFCB when triggering on fluorescence. This was observed during a 7-day period when the IFCB failed to detect Dinophysis spp. cells when triggering on fluorescence while cells were still detected using the side scattering triggering method as well as observed by microscopy. During this period, Mesodinium spp. was not detected, IFCB-derived autofluorescence level of individual cells of Dinophysis spp. was low, and less than 50 % of Dinophysis spp. cells exhibited autofluorescence under the microscope. Together, this indicates that the unique feeding ecology of Dinophysis spp. may affect their detection by the IFCB when cells are starved.

On the conditions promoting Pseudo-nitzschia spp. blooms in the eastern English Channel and southern North Sea.

This study investigated the drivers of the blooms of Pseudo-nitzschia seriata and Pseudo-nitzschia delicatissima complexes in the eastern English Channel and southern North Sea. Phytoplankton data series acquired from 1992 to 2020 were analyzed with a multivariate statistical approach based on Hutchinson's niche concept. P. seriata and P. delicatissima complexes were found to be typically present year round, but they bloomed at different periods because they occupied different realized ecological niches. P. delicatissima complex occupied a more marginal niche and was less tolerant than P. seriata complex. P. delicatissima complex typically bloomed in April-May at the same time as Phaeocystis globosa while P. seriata complex blooms were more frequently observed in June during the decline of low intensity P. globosa blooms. P. delicatissima and P. seriata complexes were both favored by low-silicate environments and relatively low turbulence but they responded differently to water temperature, light, ammonium, phosphate and nitrite + nitrate conditions. Niche shifts and biotic interactions played important roles in the control of the blooms of P. delicatissima and P. seriata complexes. The two complexes occupied different sub-niches during their respective low abundance and bloom periods. The phytoplankton community structure and the number of other taxa presenting a niche overlapping the niches of P. delicatissima and P. seriata complexes also differed between these periods. P. globosa was the taxa contributing the most to the dissimilarity in community structure. P. globosa interacted positively with P. delicatissima complex and negatively with P. seriata complex.

Physiological changes induced by sodium chloride stress in Aphanizomenon gracile, Cylindrospermopsis raciborskii and Dolichospermum sp.

Due to anthropogenic activities, associated with climate change, many freshwater ecosystems are expected to experience an increase in salinity. This phenomenon is predicted to favor the development and expansion of freshwater cyanobacteria towards brackish waters due to their transfer along the estuarine freshwater-marine continuum. Since freshwater cyanobacteria are known to produce toxins, this represents a serious threat for animal and human health. Saxitoxins (STXs) are classified among the most powerful cyanotoxins. It becomes thus critical to evaluate the capacity of cyanobacteria producing STXs to face variations in salinity and to better understand the physiological consequences of sodium chloride (NaCl) exposure, in particular on their toxicity. Laboratory experiments were conducted on three filamentous cyanobacteria species isolated from brackish (Dolichospermum sp.) and fresh waters (Aphanizomenon gracile and Cylindrospermopsis raciborskii) to determine how salinity variations affect their growth, photosynthetic activity, pigment composition, production of reactive oxygen species (ROS), synthesis of compatible solutes and STXs intracellular quotas. Salinity tolerance was found to be species-specific. Dolichospermum sp. was more resistant to salinity variations than A. gracile and C. raciborskii. NaCl variations reduced growth in all species. In A. gracile, carotenoids content was dose-dependently reduced by NaCl. By contrast, in C. raciborskii and Dolichospermum sp., variations in carotenoids content did not show obvious relationships with NaCl concentration. While in Dolichospermum sp. phycocyanin and phycoerythrin increased within the first 24 h exposure to NaCl, in both A. gracile and C. raciborskii, these pigments decreased proportionally to NaCl concentration. Low changes in salinity did not impact STXs production in A. gracile and C. raciborskii while higher increase in salinity could modify the toxin profile and content of C. raciborskii (intracellular STX decreased while dc-GTX2 increased). In estuaries, A. gracile and C. raciborskii would not be able to survive beyond the oligohaline area (i.e. salinity > 5). Conversely, in part due to its ability to accumulate compatible solutes, Dolichospermum sp. has the potential to face consequent salinity variations and to survive in the polyhaline area (at least up to salinity = 24).