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Factors structuring the dynamic of phytoplankton and HAB
HAB and more largely plankton communities, are shaped by biotic and abiotic factors. Within the abiotic factors, physical (e.g. temperature, light, stratification, water motion, turbulences, transport) and chemical (e.g. salinity, nutrient, metals) processes appear to be particularly important in the control of algal blooms (Figueiras et al., 2006; Guallar et al., 2017). For instance, retention (i.e. low hydrodynamic) of water is often considered to support blooms of HAB. Another important abiotic factor controlling HAB is the nutrient composition of seawater (Anderson et al., 2002). Nutrient (relative abundances and type of nutrient) will affect the species composition through phytoplankton species preferences. While there seems to be evidence of HAB stimulation by nutrient enrichment, not all organisms respond to this stimuli (Anderson et al., 2002). For instance, some genus like ALEXANDRIUM seem quite unaffected by nutrient enrichment (Anderson et al., 2012). While the role of abiotic factors in structuring communities has been extensively studied, they only partially explain the protist community composition. In some cases, physico-chemical parameters explain only 30 % of the community variation (Berdjeb et al., 2018) indicating that other factors influence community structure. The authors of this study (Lima-Mendez et al., 2015) have pointed out that the plankton interactome (interactions between organisms) is a thriving force structuring communities. The interactome is shaped with various relationships (e.g. parasitism, symbiosis, competition, predation), and some of interactions are mediated through chemical signaling (i.e.
ALLELOCHEMICAL INTERACTIONS IN MARINE ENVIRONMENT
Allelochemical potency occurs in various microalgal groups including cyanobacteria, dinoflagellates, prymnesiophytes, rapidophytes, or diatoms (Legrand et al., 2003). Concordantly with species diversity, allelochemical diversity is broad (e.g. fatty acids, extracellular peptides, proteinaceous compounds, glycolipids, oxylipins, alkaloids; (Granéli and Turner, 2007; Legrand et al., 2003)). Most of marine allelochemicals are chemically uncharacterized due to the difficulties of extracting significant amounts of these compounds, and purifying them in complex matrices. Mixture of chemicals released by a mono-culture are usually really complex (Pohnert, 2010) and thus difficult to purify/characterize. One way of characterizing allelochemicals is the description of their basic features: target spectra (organisms affected), activity spectra (physiological functions inhibited), polarity of the compounds, molecular size range, lability, etc.
Allelochemical are usually released in the surrounding environment and become toxic when they come into contact with the target cells. There is evidences of another mode of action, some allelochemical species are toxic through a cell-cell contact (Ternon et al., 2018; Uchida, 2001; Zheng et al., 2016) and may not require the release of allelochemicals. Allelochemicals affect target cells by inducing many different effects such as photosynthesis inhibition (Lelong et al., 2011; Poulson-Ellestad et al., 2014), decrease in swimming velocity (Lim et al., 2014) or immobilization (Tillmann et al., 2007), encystment (Tillmann et al., 2007), grazing inhibition (Kamiyama, 1997), growth inhibition (Arzul et al., 1999; Lim et al., 2014), decreased nutrient uptake (Lyczkowski and Karp-Boss, 2014), compromised membrane integrity (Poulin et al., 2018; Prince et al., 2008), inducing cell lysis (Ma et al., 2011a; Tillmann and John, 2002) and induction of programmed cell death (Vardi et al., 2002). However the precise mode of action is usually unknown. There is a clear lack of knowledge and a need for better characterization of allelochemicals nature and mode of action. Many allelochemicals may have a broader activity spectra and may exhibit other biological activities than strict allelochemical interactions.
Ecological significance
Allelochemical activity has been defined as an adaptation by which some phytoplankton species could achieve a competitive advantage over other species (Legrand et al., 2003). While bloom succession is modulated by abiotic factors they can also be mediated by biotic factors including allelochemical interactions. Killing other algal cells or inhibiting other algal growth will decrease competition for resources (e.g. nutrients, light, vitamins, trace metals). The bloom dynamic model of species competing for nutrient (Grover and Wang, 2013) suggested that under rich nutrient condition, the species producing allelochemicals will outcompete other species. Under low nutrients conditions, the situation is different and the species with the best nutrient uptakes are more likely to persist. Moreover, allelochemicals with other biological activities (e.g. antibacterial, anti-grazing, antifungal) will also promote the development and persistence of a bloom.
Allelochemical release is also an advantage for mixotrophic species, the immobilization and death of target cells may enhance phagotrophy (Blossom et al., 2012). Phagotrophy may indeed have coevolved with allelochemical activity (Granéli and Turner, 2007). Allelochemical interactions could be one of the key factors favoring the dominance of HAB species over other plankton species during HAB events.
ENVIRONMENTAL CONTROL OF ALLELOCHEMICAL INTERACTIONS
Some defense and competing mechanisms can be modified by environmental factors to face changing conditions (Lindström et al., 2017; Selander et al., 2006), and are so called induced-responses. These mechanisms can be induced to face an increasing stress but mechanisms can also be inhibited if the resources are limited. Such responses can be modelled (Chakraborty et al., 2018). Modulation of such mechanisms, that can be energetically costly, allow a better allocation of resources by the cells (Selander, 2007). Like the production of other phycotoxins, the production of allelochemicals can be modulated by environmental parameters (Figure 4) (Granéli and Turner, 2007; Legrand et al., 2003). Different abiotic factors like pH, light, nutrients, temperature were shown to modulate allelochemical potency (Granéli and Johansson, 2003; Granéli and Salomon, 2010; Schmidt and Hansen, 2001). Variability of allelochemical potency in response to changing environmental factors show that allelochemicals can be induced-responses, however the knowledge on the environmental control is still scarce. As an example of environmental abiotic control, the prymnesiophyte PRYMNESIUM PARVUM requires light to produce hemolysin (toxin concentration decreases when culture is in the dark), even if the hemolysin is degraded in presence of light (Granéli and Turner, 2007). Production of allelochemicals is, in some situations, indirectly modulated as some allelochemicals may have dual purposes. For instance, the concentration of metals is known to modulate allelochemical activity in terrestrial environments, as some allelochemicals also have chelating properties (Inderjit et al., 2011; Tharayil et al., 2009). To our knowledge, the modulation of allelochemical potency by metals has not been assessed in marine species. Biotic factors such as cell concentration (Granéli and Salomon, 2010; Schmidt and Hansen, 2001; Tillmann et al., 2008), growth phase or physiological state (Arzul et al., 1999; Schmidt and Hansen, 2001), target cells (Kubanek et al., 2005; Tillmann et al., 2008), or allelochemical strain variability (Alpermann et al., 2010; Brandenburg et al., 2018) are also known to influence allelochemical activity. More recently, the production of allelochemicals was shown to be activated by the presence of competitors in co-culture (Ternon et al., 2018).
Table of contents :
Chapter 1 – State of the art
1 Harmful algal blooms
1.1 Phytoplankton
1.2 Harmful algal blooms
1.3 Increase of HABs events
1.4 Factors structuring the dynamic of phytoplankton and HABs
2 Allelochemical interactions
2.1 Chemical ecology
2.2 Definition of allelopathy
2.3 Allelochemical interactions in marine environment
2.4 Ecological significance
2.5 Environmental control of allelochemical interactions
2.6 Microalgae producing allelochemicals
3 The genus Alexandrium
3.1 Classification
3.2 Life cycle
3.3 Biology
3.4 Toxin production
3.5 Alexandrium spp. worldwide geographical distribution
3.6 Alexandrium spp. outbreaks in Australia
3.7 Alexandrium spp. outbreaks in France
3.8 Environmental control of Alexandrium spp. blooms
4 Alexandrium spp. allelochemical potency
4.1 Allelochemical potency within the genus Alexandrium
4.2 Effects of allelochemicals on protists and microbial communities
4.3 Allelochemicals mode of action
4.4 Effects of allelochemicals on biological membranes
4.5 Environmental regulation of Alexandrium spp. allelochemical potency
4.6 Chemical characterization of Alexandrium spp. allelochemicals
4.7 Alexandrium spp. exudates
5 Biological activities of Alexandrium spp. exudates
5.1 Hemolytic and cytotoxic activities
5.2 Toxicity to bivalves
5.3 Toxicity to fish
6 Chemical nature of Alexandrium spp. exudates
6.1 Characterization of Alexandrium spp. exudates
6.2 Characterization of hemolytic and cytotoxic compounds
6.3 Characterization of ichtyotoxic compounds
Objectives and organization of the thesis
Chapter 2 : Unraveling the mechanisms of allelochemical interactions
Article 1 – Allelochemicals from Alexandrium minutum induce rapid inhibition of metabolism and modify the membranes from Chaetoceros muelleri
Article 2 – Allelochemical interactions between Alexandrium minutum and Chaetoceros muelleri: insights into photosynthesis and membrane integrity
Chapter 3 : Development of a bioassay to quantify allelochemical interactions from Alexandrium minutum
Article 3 – A rapid quantitative fluorescence-based bioassay to study allelochemical
interactions from Alexandrium minutum
Chapter 4 : Effects of abiotic factors on allelochemical potency
Article 4 – A multi-trait approach reveals the effects of Cu on the physiology of an allelochemical-producing strain of Alexandrium minutum
Chapter 5 : What is the chemical nature of Alexandrium minutum allelochemicals ?
Chapter 6 – General discussion
1 Chemical nature of allelochemicals
2 Allelochemicals mode of action
3 Interactions between allelochemicals and cytoplasmic membranes
4 Interactions between allelochemicals and photosynthetic membranes
5 Understanding the mode of action to explain the variability in the sensitivity to allelochemicals
6 Variability of allelochemical potency from Alexandrium strains
7 The relevance of laboratory studies in an environmental context
8 Beyond allelochemical interactions
Conclusions and perspectives
PhD Outcomes
References (Introduction and General discussion)
