Proliferations of cyanobacteria have detrimental effects on ecosystem functioning, and on the global freshwater food chain. Many studies have focused on the “in situ” dynamics of bloom-forming cyanobacteria, including Cylindrospermopsis raciborskii and Planktothrix agardhii. Few have used experimental assays to explore the fast-growing ability of naturally co-occurring species. Here we investigated the growth of these species when exposed separately (i.e., in monocultures) to a range of light and nutrient conditions, plus their interactive performances in mixed cultures in a short-time experiment (6 days). The use of microplates made it possible to carry out multiple measurements of in-vivo fluorescence (IVF), and to monitor species-dependent biovolumes. No allelopathic effect was significantly observed for any target species, while significantly lower growth rates were obtained in mixed cultures, which may reflect other interference interactions between the species. We showed that Planktothrix grew faster with low light intensity and high nutrient concentrations, and was drastically inhibited by nitrogen deprivation, in contrast to Cylindrospermopsis. However, Cylindrospermopsis outgrew Planktothrix at high NH4 + concentrations, suggesting that this species may be a serious competitor for the native species in many water systems. - Initial fast-growing abilities of bloom-forming species assessed in mono vs mixed cultures.- A low growth rate recorded in mixed cultures, suggesting possible competitive interactions under specific conditions.- P. agardhii outcompete C. raciborski at low light intensity and high NO3 - source; while C. raciborski outgrow at high NH4 + levels.

Brevetoxins (BTXs) are marine biotoxins responsible for neurotoxic shellfish poisoning (NSP) after ingestion of contaminated shellfish. NSP is characterized by neurological, gastrointestinal and/or cardiovascular symptoms. The main known producer of BTXs is the dinoflagellate Karenia brevis, but other microalgae are also suspected to synthesize BTX-like compounds. BTXs are currently not regulated in France and in Europe. In November 2018, they have been detected for the first time in France in mussels from a lagoon in the Corsica Island (Mediterranean Sea), as part of the network for monitoring the emergence of marine biotoxins in shellfish. To prevent health risks associated with the consumption of shellfish contaminated with BTXs in France, a working group was set up by the French Agency for Food, Environmental and Occupational Health & Safety (Anses). One of the aims of this working group was to propose a guidance level for the presence of BTXs in shellfish. Toxicological data were too limited to derive an acute oral reference dose (ARfD). Based on human case reports, we identified two lowest-observed-adverse-effect levels (LOAELs). A guidance level of 180 µg BTX-3 eq./kg shellfish meat is proposed, considering a protective default portion size of 400 g shellfish meat.

Recent reports predict increases in harmful cyanobacteria in water systems worldwide due to climatic and environmental changes, which would compromise water quality and public health. Among abiotic changes, higher salinities are expected to promote the growth of certain harmful species, such as Planktothrix agardhii, which forms blooms in brackish waters. Since P. agardhii is a common producer of cyanotoxin, we investigated the growth and tolerance of this species when exposed in vitro to a range of salinities, while assessing variations in its microcystin diversity and production in batch cultures during a time-frame experiment spanning 18 days. The study revealed salt acclimation of the brackish P. agardhii, which continued to produce microcystins in salty cultures, while maintaining its growth capacity at low to medium NaCl (ranging from 0 to 7.5 g l⁻¹). With higher concentrations (10 to 15 g l⁻¹) significantly less growth occurred, corresponding to the shortening of cyanobacterial filaments, which nevertheless maintained their metabolic functions, as revealed by the high intensity of chlorophyll auto-fluorescence and persistent microcystin production. These findings showed that moderate to high salt levels do not inhibit microcystin production by P. agardhii, at least for several weeks. This raises questions concerning the persistence of harmful cyanobacteria in shallow water systems more exposed to evaporation and consequently to an increase in salinity in the future, as predicted by various climate models.

Microcystins (MCs) are the most commonly-reported hepatotoxins produced by various cyanobacterial taxa in fresh waters to constitute a potential threat to human and animal health. The biological role of MCs in the producer organisms is not known, and it would be very useful to understand the driving force behind the toxin production. Recent studies have suggested that MCs may have a protective function in cells facing environmental stress. Following this starting premise, we speculate that under adverse conditions the expression of stress-related genes coding for Heat Shock Proteins (Hsp) might be different in an MC-producing strain and its MC-deficient mutant. We therefore used RT-qPCR to compare the expression of 13 hsp genes of an MC-producing strain of Planktothrix agardhii (CYA126/8) and its MC-deficient ΔmcyD mutant over different periods of exposure to high light stress (HL). Three reference genes (RGs) were selected from six candidates to normalize the RT-qPCR data. Of these three RGs (rsh, rpoD, and gltA), gltA is used here for the first time as an RG in prokaryotes. Under HL stress, five genes were found to be strongly up-regulated in both strains (htpG, dnaK, hspA, groES, and groEL). Unexpectedly, we found that the MC-producing wild type strain accumulated higher levels of htpG and dnaK transcripts in response to HL stress than the MC-deficient mutant. In addition, a significant increase in the mcyE transcript was detected in the mutant, suggesting that MCs are required under HL conditions. We discuss several possible roles of MCs in the response to HL stress through their possible involvement in the protective mechanisms of the cells.

We examined the phylogenetic assignment of the Chinese strain PCC 9901, a cyanobacterium previously identified as Lyngbya laxespiralis, using phenotypic and molecular approaches. Phylogenetic analysis of the 16S rRNA gene sequence showed that strain PCC 9901 clustered with Arthrospira rather than Lyngbya. While all the Arthrospira strains were assigned to four genotypes (clusters IA, IB, IIA and IIB) based on the 16S-23S internal transcribed spacer (ITS) rRNA sequences, we reported here an unexpected new genotype for strain PCC 9901 that we named cluster III. A mosaic-like structure was found in two of the three intergenic spacer regions of the ITS (ISR1 and ISR3). In addition, a short repeated sequence was specific for genotype I, and a motif was found in genotype II sequences, but they were absent from the PCC 9901 ITS sequence, suggesting multiple recombination events and/or divergent evolution. Strain PCC 9901 was therefore placed into Arthrospira, and it constituted a new ITS cluster, together with o...

Proliferations of cyanobacteria are becoming increasingly widespread in many artificial reservoirs, and may have detrimental effects on ecosystem functioning, especially when the water sustains commercial fisheries. This is the case of the Bir M'Cherga reservoir in North Tunisia, where sporadic fish die-offs have been recently reported. A two-year survey investigated a dual-community structure including both phytoplankton and the main fecal bacterial indicators facing to environmental factors. Low abundances of fecal indicator bacteria were recorded, indicating that no direct human contamination had occurred. However, the ecological status of this reservoir did show signs of degradation, Bacillariophyceae being superseded by Cyanobacteria, with a “nearlyexclusive” dominance of Planktothrix agardhii lasting several months, in association with Planktolyngbya limnetica and Pseudanabaena limnetica. In contrast, the rapid decline of P. agardhii observed in spring 2007 favored greater phytoplankton diversity, with the summer occurrence of Cylindrospermopsis raciborskii. Even though no significant relationships were identified between fish mortality and biological factors, the very presence of two bloomforming and potentially toxic cyanobacterial species may be viewed as a potentially serious issue with regard to water use and fish farming in the Tunisian reservoir. Des phénomènes de proliférations cyanobactériennes s'observent à la surface des écosystèmes limniques, et ce quel que soit le continent étudié. Or ces fortes biomasses engendrent un profond dysfonctionnement de l'hydrosystème, notamment lorsque ces étendues d'eau servent à la pisciculture. Une étude a ainsi été réalisée sur la structure de la double communauté phytoplanctonique et des bactéries fécales en réponse aux conditions environnementales, dans le réservoir de Bir M'Cherga (Tunisie), à la suite de mortalités inexpliquées de poissons. Une dégradation de l'état écologique des eaux a été confirmée par le remplacement des Bacillariophyceae par les Cyanobactéries, avec une dominance quasi-exclusive de Planktothrix agardhii pendant plusieurs mois, associée à Planktolyngbya limnetica et Pseudanabaena limnetica. Un rapide déclin de P. agardhii au printemps, a favorisé une augmentation de la diversité phytoplanctonique, avec l'apparition estivale de Cylindrospermopsis raciborskii. Bien qu'aucune corrélation n'ait pu être directement établie entre ces mortalités épisodiques et les facteurs biologiques, la seule présence d'espèces de cyanobactéries à fort potentiel invasif et toxique justifie une vigilance accrue de la qualité des eaux et du peuplement piscicole dans les eaux du réservoir Bir'M Cherga.