First evidence of the induction of domoic acid production in Pseudo-nitzschia australis by the copepod Temora longicornis from the French coast.

Diatoms of the genus Pseudo-nitzschia are widespread in marine waters. Some of them can produce the toxin domoic acid (DA) which can be responsible for amnesic shellfish poisoning (ASP) when transferred into the food web. These ASP events are of major concern, due to their ecological and socio-economic repercussions, particularly on the shellfish industry. Many studies have focused on the influence of abiotic factors on DA induction, less on the role of biotic interactions. Recently, the presence of predators has been shown to increase DA production in several Pseudo-nitzschia species, in particular in Arctic areas. In order to investigate the relationship between Pseudo-nitzschia species and grazers from the French coast, exposures between one strain of three species (P. australis, P. pungens, P. fraudulenta) and the copepod Temora longicornis were conducted for 5 days. Cellular and dissolved DA content were enhanced by 1,203 % and 1,556 % respectively after the 5-days exposure of P.australis whereas no DA induction was observed in P. pungens and P. fraudulenta. T. longicornis consumed all three Pseudo-nitzschia species. The copepod survival was not related to DA content. This study is an essential first step to better understanding the interactions between planktonic species from the French coast and highlights the potential key role of copepods in the Pseudo-nitzschia bloom events in the temperate ecosystems.

Influence of nutrient enrichment on colonisation and photosynthetic parameters of hard substrate marine microphytobenthos.

This study aimed to assess the influence of nutrient enrichment on the development of microalgal biofilm on concrete and PVC cubes. Three mesocosms were utilized to create a nutrient gradient over a period of 28 days. Various parameters including biomass, photosynthetic activity, microtopography, and extracellular polymeric substances (EPS) were measured. Imaging PAM techniques were employed to obtain surface-wide data. Results revealed that nutrient availability had no significant impact on Chl a biomass and the maximum quantum efficiency of PSII (Fv/Fm). The photosynthetic capacity and efficiency were minimally affected by nutrient availability. Interestingly, the relationship between microphytobenthic (MPB) biomass and photosynthesis and surface rugosity exhibited distinct patterns. Negative reliefs showed a strong correlation with Fv/Fm, while no clear pattern emerged for biomass on rough concrete structures. Overall, our findings demonstrate that under conditions of heightened eutrophication, biofilm photosynthesis thrives in the fissures and crevasses of colonized structures regardless of nutrient levels. This investigation provides valuable insights into the interplay between nutrient availability and surface rugosity.

Colimitation assessment of phytoplankton growth using a resource use efficiency approach in the Bay of Seine (French-English Channel).

Eutrophication and dystrophy are two of the main problems affecting coastal ecosystems. In the Bay of Seine, phosphorus (P) inputs from the Seine estuary have been largely reduced in the last decade, in contrast to nitrogen (N), which leads to high N/P ratio inputs. To study the effect of dystrophy, an enrichment bioassay using water sampled from the Bay of Seine was repeated 19 times over a period of 18 months with six different enrichments. After a few days, chlorophyll a (chl a), alkaline phosphatase activity (APA), transparent exopolymeric particles (TEPs), cytometric size structure, and maximum quantum yield of photosystem II were measured. The data provide strong evidence for an N & P colimitation system in the vast majority of the incubations, as only the N + P and N + P + Si enrichments supported phytoplankton growth, and Si only appeared to play a secondary role in our incubations. A N/P ratio of 16 equal to the Redfield ratio was identified as the optimum for balanced growth, as chl a was the highest and TEP and APA production was the lowest at this ratio. To fit the requirements of the colimited system, a new resource use efficiency (RUENP) calculation was developed to account for N and P colimitation instead of only one nutrient, as is usually the case. This calculation allows better representation of RUE in dystrophic conditions, as found in many highly anthropized ecosystems. The relationships between RUENP and the size structure of the phytoplankton community were explored, and a significant positive correlation between RUENP and larger cells (>2 µm) and a negative correlation with smaller cells (<2 µm) were noted, showing a better use of nutrients by larger cells. This study highlights an increase of RUENP with the phytoplankton cell size in a colimited system.

Interactions between Filter-Feeding Bivalves and Toxic Diatoms: Influence on the Feeding Behavior of Crassostrea gigas and Pecten maximus and on Toxin Production by Pseudo-nitzschia.

Among Pseudo-nitzschia species, some produce the neurotoxin domoic acid (DA), a source of serious health problems for marine organisms. Filter-feeding organisms-e.g., bivalves feeding on toxigenic Pseudo-nitzschia spp.-are the main vector of DA in humans. However, little is known about the interactions between bivalves and Pseudo-nitzschia. In this study, we examined the interactions between two juvenile bivalve species-oyster (Crassostrea gigas) and scallop (Pecten maximus)-and two toxic Pseudo-nitzschia species-P. australis and P. fraudulenta. We characterized the influence of (1) diet composition and the Pseudo-nitzschia DA content on the feeding rates of oysters and scallops, and (2) the presence of bivalves on Pseudo-nitzschia toxin production. Both bivalve species fed on P. australis and P. fraudulenta. However, they preferentially filtered the non-toxic Isochrysis galbana compared to Pseudo-nitzschia. The presence of the most toxic P. australis species resulted in a decreased clearance rate in C. gigas. The two bivalve species accumulated DA in their tissues (up to 0.35 × 10-3 and 5.1 × 10-3 µg g-1 for C. gigas and P. maximus, respectively). Most importantly, the presence of bivalves induced an increase in the cellular DA contents of both Pseudo-nitzschia species (up to 58-fold in P. fraudulenta in the presence of C. gigas). This is the first evidence of DA production by Pseudo-nitzschia species stimulated in the presence of filter-feeding bivalves. The results of this study highlight complex interactions that can influence toxin production by Pseudo-nitzschia and accumulation in bivalves. These results will help to better understand the biotic factors that drive DA production by Pseudo-nitzschia and bivalve contamination during Pseudo-nitzschia blooms.

Coupling high frequency monitoring and bioassay experiments to investigate a harmful algal bloom in the Bay of Seine (French-English Channel).

Coastal ecosystems are increasingly threatened by eutrophication and dystrophy. In this context, the full pattern of a bloom dominated by the dinoflagellate, Lepidodinium chlorophorum, was investigated by a high frequency monitoring buoy equipped with sensors allowing nutrients and photosynthesis measurements. An increase of the N/P ratio affected phytoplankton physiology leading to bloom collapse with a slight oxygen depletion. In parallel, enrichment experiments were performed on the natural bloom population. After 5 days of incubation the community structure, using flow cytometry and several physiological parameters were analysed. The data reveal a potential N and P co-limitation and a decoupling between primary production and productivity in fully enriched conditions. Under unbalanced N/P inputs, high level of alkaline phosphatase activity and transparent exopolymeric particle production, which favour phytoplankton sedimentation, were observed. Nutrient inputs and their stoichiometry control phytoplankton growth, the community structure, physiological regulations, the fate of the bloom and consequences.