Light modulates the effect of antibiotic norfloxacin on photosynthetic processes of Microcystis aeruginosa

Highlights

• The response mechanisms to norfloxacin were investigated in Cyanobacterium Microcystis aeruginosa under two different light regimes.

• Under High light intensity, norfloxacin induce more inhibition of growth and photosynthesis in M. aeruginosa compared to low light intensity.

• Norfloxacin disrupt reaction centers and pigments and induce accumulation of ROS in M. aeruginosa.

• Photosystem II could be one of the main photosynthetic targets of norfloxacin in M. aeruginosa.

Abstract

Norfloxacin is one of the widely used antibiotics, often detected in aquatic ecosystems, and difficultly degraded in the environment. However, how norfloxacin affects the photosynthetic process of freshwater phytoplankton is still largely unknown, especially under varied light conditions. In this study, we investigated photosynthetic mechanisms of Microcystis aeruginosa in responses to antibiotic norfloxacin (0-50 μg/L) for 72 h under low (LL; 50 μmol photons m⁻² s⁻¹) and high (HL; 250 μmol photons m⁻² s⁻¹) growth light regimes. We found that environmentally related concentrations of norfloxacin inhibited the growth rate and operational quantum yield of photosynthesis system II (PSII) of M. aeruginosa more under HL than under LL, suggesting HL increased the toxicity of norfloxacin to M. aeruginosa. Further analyses showed that norfloxacin deactivated PSII reaction centers under both growth light regimes with increased minimal fluorescence yields only under HL, suggesting that norfloxacin not only damaged reaction centers of PSII, but also inhibited energy transfer among phycobilisomes in M. aeruginosa under HL. However, non-photosynthetic quenching decreased in the studied species by norfloxacin exposure under both growth light regimes, suggesting that excess energy might not be efficiently dissipated as heat. Also, we found that reactive oxygen species (ROS) content increased under norfloxacin treatments with a higher ROS content under HL compared to LL. In addition, HL increased the absorption of norfloxacin by M. aeruginosa, which could partly explain the high sensitivity to norfloxacin of M. aeruginosa under HL. This study firstly reports that light can strongly affect the toxicity of norfloxacin to M. aeruginosa, and has vitally important implications for assessing the toxicity of norfloxacin to aquatic microorganisms.

Introduction

Norfloxacin (NOR), as a member of the fluoroquinolone antibiotic family, is one of the most important class of second-generation synthetic antibiotics. Due to its broad-spectrum antibacterial effects on Gram-negative and Gram-positive bacteria, it is widely used in medicine, agriculture and aquaculture (Nie et al., 2009; Lian et al., 2016; Moreau et al., 2018; Yuan et al., 2019). Norfloxacin is unavoidably released into freshwater through urine and feces as it is extensively used and is partially metabolized in the body. Moreover, norfloxacin cannot be completely degraded by conventional biological wastewater treatment plants and is not known to be biodegradable (Ou et al., 2016). Therefore, norfloxacin concentrations found in the freshwater range from nanogram to microgram per liter, which could represent a major threat to the health of freshwater organisms (Kümmerer, 2008; Zhang et al., 2015; Li et al., 2018).

Phytoplankton as primary producers plays a key role for driving energy and material cycles in freshwater ecosystems (Välitalo et al., 2017). It was found that norfloxacin could inhibit the growth of Microcystis aeruginosa and decreased its cell size (Du et al., 2018). Moreover, high concentrations (>10 mg/L) of norfloxacin could induce irreversible damages to M. aeruginosa and finally lead to its death (Du et al., 2018). On the other hand, norfloxacin decreased the formation of colonies in Scenedesmus quadricauda (Pan et al., 2017a), and these varied colony sizes affected interspecies interactions between phytoplankton species. For example, alteration of Scenedesmus obliquus colony size by norfloxacin was found to further affect the stability and persistence of plankton systems (Pan et al., 2017b; Pan et al., 2020). Although several studies investigated the effect of antibiotics (such as chlortetracycline, enrofloxacin and florphenicol) on photosystem II (PSII) photosynthetic activity of various phytoplankton species (such as Tetraselmis suecica, Microcystis flos-aquae and Pseudokichneriella subcapitata) (Seoane et al., 2014; Wan et al., 2014,2015; Carusso et al., 2018; Du et al., 2018), the effect of norfloxacin on photosynthetic processes and underlying mechanisms is not clear. It is therefore urgent to investigate the effect of norfloxacin on photosynthetic processes in phytoplankton (Li et al., 2018).

The toxicity of pollutants, such as trace metals, herbicides and antibiotics, can be modulated by environmental factors, such as light and temperature (Ota et al., 2015; Xu and Juneau, 2016; Guo et al., 2016; Gomes and Juneau, 2017; Xu et al., 2019). Light is one of the most important environmental factors, which drives photosynthesis, the cornerstone of metabolism for phototrophs to convert carbon dioxide into organic matter (Pilon et al., 2011; Wahidin et al., 2013). In freshwater ecosystems, phytoplankton can adjust its photosynthetic states under varied light conditions such as season/weather change (MacIntyre et al., 2002). The physiological status induced by different light regimes in phytoplankton, may further affect its sensitivity to pollutants. It has been found that phytoplankton grown under strong light reduces their sensitivity to atrazine (Deblois et al., 2013). This observed effect is supposed to be a result of diluting atrazine toxicity by increasing the availability of binding sites (quinones) and increasing the ability of the light-regulating process. On the other hand, our previous studies found that high light increased mesotrione toxicity to Chlamydomonas reinhardtii compared to low light conditions (Xu et al., 2019), and higher light could increase zinc toxicity to M. aeruginosa by increasing zinc absorption (Xu and Juneau, 2016). Those studies suggest that light can decrease/increase the toxicity of pollutants to phytoplankton by changing their absorption or photosynthetic status. However, to the best of our knowledge, there is no information about how phytoplankton responds to antibiotics under varied light conditions.

In this research, we aimed to understand the effect of norfloxacin on photosynthetic processes, such as light absorption efficiency, electron transport, and PSII activity under varied growth light regimes in M. aeruginosa, a target aquatic phytoplankton as its cell membrane is similar as Gram-negative bacteria's. It was hypothesized that light could modify the toxicity of norfloxacin to M. aeruginosa by changing its absorption and/or photosynthetic status under different light conditions. We exposed M. aeruginosa to different concentrations of norfloxacin for 72 h under two growth light regimes to address the above questions. M. aeruginosa is one of the most widely distributed phytoplankton species in freshwater ecosystems, which is a common strain for toxicity test of pollutants (Deng et al., 2015; Liu et al., 2015; Guo et al., 2016; Wang et al., 2018). We found that norfloxacin could quickly and sharply decreased the PSII activity with stronger inhibitory effects under high growth light conditions in M. aeruginosa. This study advances our understanding of toxic effects of norfloxacin and possible mechanisms on photosynthetic processes, and has important implications for assessing ecotoxicological impacts of norfloxacin to phytoplankton in freshwater ecosystems.