Combined effect of temperature and bleaching herbicides on photosynthesis, pigment and fatty acid composition ofChlamydomonas reinhardtii

Abstract

Norflurazon (Nf) and fluridone (Fd) are phytoene desaturase inhibitor herbicides that are widely used for the control of grasses and invasive aquatic weeds, respectively. These herbicides enter aquatic environments where they can negatively affect non-target plant species (e.g. algae). Their toxicity towards algae may be modified by abiotic factors such as light intensity, temperature, pH and nutrients. Investigating the effect of low temperature on the toxicity of Nf and Fd is particularly important because both temperature and herbicides affect some of the same physiological process (e.g. carotenoid biosynthesis). Here we demonstrate that Nf reduced photosynthesis in the green alga Chlamydomonas reinhardtii more strongly at 15 than at 25ºC, while Fd showed stronger effects at 25 than at 15ºC. Neither herbicide significantly inhibited photosynthesis at 8ºC. Although the overall pigment content decreased with lower temperature, there was an increase in photo-protective carotenoids relative to chlorophylls at both 15 and 8ºC in the absence of herbicides. Moreover, most of the measured pigments decreased markedly in the presence of Nf and Fd at 15 and 25ºC, including β-carotene which fell to below detection limits. The fatty acid composition was modified by temperature and the level of unsaturation noticeably increased at 15 compared with 25ºC. At 8ºC, however, despite a 2.4 times decrease in fatty acid content, the unsaturation level was similar to 25ºC acclimated cells. Monounsaturated fatty acids increased concomitant with a decrease in polyunsaturated fatty acid in the 2.5 µM Nf treatment at 25ºC. Differences in the effect of Nf and Fd on photosynthesis at 15 and 25ºC can be attributed to the marked decrease in carotenoids, which play an important role in photoprotection. At 8ºC, the apparent lack of inhibitory effects compared with control cultures could be due to enhanced photoprotection and/or decreased uptake of herbicides by the alga.

Temperature and Light Modulation of Herbicide Toxicity on Algal and Cyanobacterial Physiology

“…Several studies have examined the effects of climatic factors (such as temperature and irradiance) on photosynthesis of aquatic organisms (Huner et al, 2003;Necchi Jr, 2004). Temperature, for example, has been observed to modulate algal and cyanobacterial photosynthesis by modifying pigment and lipid compositions (and thus membrane fluidity) and enzyme activities (Huner et al, 2003;Necchi Jr, 2004;Chalifour et al, 2014). Although photoacclimation processes allow aquatic organisms to adjust their photosynthetic apparatus in response to changing light environments, prolonged exposure to high light intensities can induce photoinhibition and photodamage (Huner et al, 2003).…”

Phytoene Accumulation in the Novel Microalga Chlorococcum sp. Using the Pigment Synthesis Inhibitor Fluridone

“…A study into the inhibitory effects of fluridone on E. coli expressed PDS from the cyanobacterium Synechococcus (PCC 7492), as well as purified Synechococcus PDS, revealed a concentration of 0.3 μM and 3.5 μM to cause 50% inhibition of carotenoid production, respectively [38]. Chalifour, et al discovered that a range of temperatures influences the inhibitory effects of the herbicides norflurazon and fluridone in the model alga C. reinhardtii [35]. It was found that 1.25 μM fluridone had the greatest impact on secondary carotenoid formation at a temperature of 25 °C; whereas, secondary carotenoid formation was affected to a lesser extent at lower temperatures [35].…”

“…These bleaching herbicides target the enzyme phytoene desaturase (PDS), responsible for the downstream production of carotenoids past the metabolic step of phytoene production [34]. The inability to synthesize carotenoids that are essential for structure and function of photosynthetic complexes results in chlorophyll degradation, and ultimately, plant cell death [10,35,36,37]. At non-lethal doses, effective inhibition of PDS leads to the over-accumulation of phytoene [23,29,31,32,35,38].…”

“…The inability to synthesize carotenoids that are essential for structure and function of photosynthetic complexes results in chlorophyll degradation, and ultimately, plant cell death [10,35,36,37]. At non-lethal doses, effective inhibition of PDS leads to the over-accumulation of phytoene [23,29,31,32,35,38]. This has been demonstrated in the microalgae D. bardawil and H. pluvialis , in which phytoene accumulation increased sharply as a result of exposure to bleaching herbicides [29,31,32,33].…”

Impact of diuron and S-metolachlor on the freshwater diatom Gomphonema gracile: Complementarity between fatty acid profiles and different kinds of ecotoxicological impact-endpoints

“….06.347 et al 2014. For instance, an increase of MUFA and a decrease of PUFA and HUFA content were found on the green algae Scenedesmus and Chlamydomonas reinhardtii after respectively chloroacetamide treatment; norflurazon and fluridone exposure (Böger et al 2000, Chalifour et al 2014.…”