Les herbicides inhibateurs du photosystème II, effets sur les communautés algales et leur dynamique The impact of photo system II (PS II) inhibitors on algal communities and dynamics

Résumé

Nous présentons une revue bibliographique à propos des effets des herbicides inhibiteurs du Photosystème II (PS II) sur les communautés algales. Ces herbicides sont abondamment utilisés dans les pratiques phytosanitaires. Ils sont susceptibles de contaminer les milieux aquatiques et, étant donné leur mode d'action inhibitrice de la photosynthèse, ils peuvent agir directement sur les algues. De nombreuses études ont été réalisées afin d'évaluer l'impact des contaminations par ces herbicides sur les microphytes, en particulier leur effet sur la croissance et la physiologie de certaines algues (monocultures en laboratoire). D'autres études expérimentales et quelques rares in situ, ont porté sur l'impact des herbicides inhibiteurs de la photosynthèse sur la structure des peuplements algaux. Certaines tendances ont pu être ainsi dégagées quant à la sensibilité et la résistance aux herbicides des différentes espèces étudiées soit isolément, soit au sein des peuplements. Les herbicides inhibiteurs du PS II perturbent effectivement la structure des peuplements phytoplanctoniques de façon plus ou moins marquée. L'impact des herbicides sur les algues est variable selon la structure des peuplements (liée aux successions) et les paramètres environnementaux, notamment liés à la saison. Nous devons donc développer nos connaissances à propos des interactions entre toxiques et facteurs environnementaux sur des pas de temps correspondant non seulement aux rythmes des contaminations mais aussi aux rythmes des succesions alagles, car ces interactions sont susceptibles de réduire ou d'amplifier les conséquences d'une pollution par ces toxiques dans les milieux aquatiques.

Mots-clés:

• Herbicides,
• inhibiteurs du PS II,
• communautés algales,
• structure,
• sélection,
• successions,
• variabilité saisonnière

Abstract

The aim of this paper is to present a review about the impact of Photo system II (PS II) inhibitors on algae communities. A brief discussion of the use in agriculture, the different chemical families, the photosynthetic inhibition effect and the occurrence of these compounds in aquatics systems is followed by the presentation of the impacts these herbicides have on algae.

Many studies investigate the effects of PS II inhibitors on algae growth and physiology. The response to pollutants were studied by monitoring changes in terms of different parameters : chlorophyll fluorescence induction usually increases with PS II inhibitors. Concentration of pigments decreases with PS II inhibitors, but increases sometimes with low contaminations of these toxicants (it is probably an homeostasis effect). Pigment ratio can change with herbicide exposure. Primary production (measured by ¹⁴C incorporation or dissolved O₂) usually decreases with PS II inhibitors. But, the " excretion " of dissolved organic compounds may increase with PS II inhibitors. These herbicides may alter or change cell morphology of algae. In consequence algae growth is inhibited by PS II inhibitors exposure. But growth inhibition varies, depending on each species' (and strain) sensibility or resistance to each herbicide.

By this way, PS II inhibitors can affect the algae community structure. In consequence, herbicides exert a selection pressure when the exposure reaches a certain level, and this for a sufficient period of time. Since organisms vary in their resistance to toxicants, the selection pressure will exclude the sensitive ones which will be replaced by resistant ones. Sometimes, responses to pollutants which are measured by global changes in biomass, pigments, dissolved O₂... can recover after a lagtime. This apparent ability to recover from effects of herbicides can be explained by the following selection effect : resistant species are indirectly stimulated and develop in the contaminated environment. The result is an algae community which has an increased resistance to these toxicants as compared to a community which has not been affected by the toxicants. This difference in resistance, between the unselected and the selected communities, may be detected by comparison of results from short term physiological tests performed with the respective community and by comparison of each community structure (taxonomy). This methodological approach called Pollution-Induced Community Tolerance (PICT) is of a great interest as a biological marker of specific pollution in aquatic systems.

Indirect effects of algae response to PS II inhibitors occur in the polluted ecosystem according to changes of the physicochemical conditions (decrease of dissolved O₂ concentrations and pH, increase of dissolved organic and inorganic matters ...). Furthermore, the impact of herbicides on algae communities varies, depending on the community's species composition (depending on successions) and on environmental factors such as interspecific interactions and physicochemical parameters (depending on seasonal changes). Interspecific interactions implies competition for the limiting nutrients among algae and allelopathic interactions among algae, as well as interactions between algae and other trophic levels (microbial loop, grazing pressure ...). These interactions between herbicides and environmental factors may reduce or emphasize the consequences of such a pollution in aquatic systems.

Seasonal change of algae communities species composition (algae successions) occurs as a response to changing environmental factors by the way of interspecific interactions and physicochemical parameters. Therefore, algae succession is affected by the herbicide destructuration of the algae communities. At the opposite, interactions and successions may affect the response of algae communities to the toxic. In this sense, herbicides act as a supplementary factor of disturbance in algae successions. Structural changes, induced by these herbicides, are usually accompanied by the attributes which are typical of an early successional stage. But, according to the " Intermediate Disturbance Hypothesis " the species richness should be maximal at intermediate intensities of herbicide contamination and at intermeadite frequencies of contaminations. The question is to compare the algae successions rythms and the frequencies of herbicides contaminations.

Then, the time factor (or the persisting quality, which is difficult to assess in experimental studies) has to be taken in account in monitoring aquatic polluted systems, because of the seasonal variability in the response of algae to PS II inhibitors as well as the seasonal variability of water bodies' contamination by these herbicides through watersheds. Moreover, usually, low values of herbicides' concentrations occur in aquatic environments but are persistant. It results that aquatic organisms are exposed during long periods of time, meaning that indirect effects, via interactions between herbicides and environmental factors, may be emphazised.

To further investigate these interactions and the herbicide persistance in aquatic systems, we have to develop experimental studies. This approach, however, must be complemented with in situ studies monitored with a timing of investigation relative to the natural population fluctuations and " pulses " of herbicides in these systems. Investigation must take place on various aquatic ecosystems. If a greater effort is given to monitor natural systems, for both herbicides and herbicides-induced effects, this will provide greater confidence in future predictions regarding the safety of PS II inhibitors in aquatic environments.

Keywords:

• Herbicides,
• PS II inhibitors,
• algae communities,
• structure,
• selection,
• successions,
• seasonal variability