Phytoplankton response to polystyrene microplastics: Perspective from an entire growth period

Highlights

• Effects of PS microplastics on algae were studied over its entire growth period.

• PS microplastics caused depression but then stimulation of algal growth.

• Physical damage and oxidative stress are involved in depression of algal growth.

Abstract

Microplastics are widely identified in aquatic environments, but their impacts on phytoplankton have not been extensively studied. Here, the responses of Chlorella pyrenoidosa under polystyrene (PS) microplastics exposure were studied across its whole growth period, with microplastic sizes of 0.1 and 1.0 μm and 3 concentration gradients each, which covered (10 and 50 mg/L) and exceeded (100 mg/L) its environmental concentrations, respectively. PS microplastics caused dose-dependent adverse effects on Chlorella pyrenoidosa growth from the lag to the earlier logarithmic phases, but exhibited slight difference in the maximal inhibition ratio (approximately 38%) with respect to the two microplastic sizes. In addition to the reduced photosynthetic activity of Chlorella pyrenoidosa, unclear pyrenoids, distorted thylakoids and damaged cell membrane were observed, attributing to the physical damage and oxidative stress caused by microplastics. However, from the end of the logarithmic to the stationary phase, Chlorella pyrenoidosa could reduce the adverse effects of microplastics jointly through cell wall thickening, algae homo-aggregation and algae-microplastics hetero-aggregation, hence triggering an increase of algal photosynthetic activity and its growth, and cell structures turned to normal. Our study confirmed that PS microplastics can impair but then enhance algae growth, which will be helpful in understanding the ecological risks of microplastics.

Introduction

Microplastics, which are defined as less than 5 mm in size, have gained much attention due to their potential hazards on aquatic food webs (Horton et al., 2017). Because of poor waste management (Alimi et al., 2018), continuous accumulation from the breakdown of larger plastics debris (Eerkes-Medrano et al., 2015) and minimal biological degradation (Yoshida et al., 2016), high abundance of microplastics has been identified (Fig. S1) in freshwater systems (Lechner et al., 2014; Yonkos et al., 2014; Su et al., 2016), marine environments (Browne et al., 2011; Cincinelli et al., 2017; Guven et al., 2017) and sediments (Klein et al., 2015; Anderson et al., 2016; Wang et al., 2017). Polyethylene (PE), polystyrene (PS), and polypropylene (PP) were the most common types of microplastics found in these environments (Klein et al., 2015; Zhang et al., 2015; Cincinelli et al., 2017).

An increasing number of studies have paid attention to the ecological impact of microplastics in the aquatic environment. It was found that microplastics can be directly or indirectly ingested by organisms of various trophic levels, such as mammals (Besseling et al., 2015), fishes (Alomar and Deudero, 2017), crustaceans (Watts et al., 2016) and zooplankton (Cole et al., 2016), resulting in reduced growth, inhibited fecundity, alteration of oxygen consumption, decreased lifespan, limited feeding capacity and increased antioxidant-related enzymes activity.

Phytoplankton are the vital primary producer at the base of the trophic chain and are essential in oxygen production, as well as nitrogen and phosphorus biogeochemical cycle (Cherchi et al., 2011), however, phytoplankton are sensitive to environmental hazards (Wang et al., 2015). Therefore, understanding the interactions between phytoplankton and microplastics is crucial for evaluating the emerging impact of microplastics on ecological property in aquatic environment. Recently, the effects of microplastics on algae have been explored, with studies mostly focused on the growth dynamic of algae under microplastics exposure. It had been demonstrated that there were adverse effect of microplastics on algae growth, but the effects seemed impaired with increasing microplastics sizes (Besseling et al., 2014; Bergami et al., 2017). When the size reached millimeter-scale, microplastics had no significant effects on growth of marine microalgae (Zhang et al., 2017). Great endeavors have also been made to interpret the physiological response of phytoplankton to microplastics. Results suggested that hetero-aggregation between microplastics beads and phytoplankton was a potential way for phytoplankton communities to escape the impacts of these hazardous materials (Long et al., 2017). Lagarde et al. (2016) characterized the hetero-aggregates and found the compound comprised of approximately equal amounts of polypropylene and algae, which were attributed to the over-expression of algal genes involved in the biosynthesis of galactose and xylose. The available literature substantially increased our knowledge on the role of microplastics in algal growth, however, the ecological risks from microplastics are not adequately understood (Hale, 2018). One issue that remains to be addressed has been the question of microplastics dosage. In natural aquatic systems, for example, microplastics concentration were found to range from 0–3.8 × 10⁻³ mg/L in four estuarine rivers in the Chesapeake Bay (Yonkos et al., 2014) and 0–7.0 × 10⁻¹ mg/L in the Danube River (Lechner et al., 2014). The highest abundance of microplastics in freshwater systems worldwide, which was observed in Taihu Lake, was in the range of 30–50 mg/L (Su et al., 2016). Nevertheless, the majority of studies that have sought to explain the impacts of microplastics on biochemical processes were carried out with microplastics that far exceeded its environmental levels and even reached 250–1000 mg/L (Besseling et al., 2014; Sjollema et al., 2016). Therefore, toxicity studies with excessive microplastics concentrations could cause misinterpretation of environmental influence of microplastics (Lenz et al., 2016).

Generally, algal growth exhibits four phases (lag, logarithmic, stationary and death phases) (Tsai et al., 2017). Despite microplastic toxicity at environmental relevant dosage had been performed recently (Bergami et al., 2017; Zhang et al., 2017), these studies were mostly conducted when the phytoplankton reached its logarithmic phase, during which the phytoplankton generally showed the strongest ability to maintain its activities under abiotic or biotic stressors (Tian et al., 2014). Hence it is possible that the published literature has underestimated the adverse influence of microplastics on phytoplankton growth. To our best knowledge, limited information is available concerning the effects of microplastics on freshwater algal physiology across its entire growth period, but these are imperative for understanding the real role of microplastics.

Against this background, Chlorella pyrenoidosa, a freshwater algae species that is widely distributed in fresh water systems all over the world, was employed as the testing phytoplankton. Polystyrene (PS) microplastics at a dosage ranging from 10 to 100 mg/L (reaching or exceeding the upper limit of environmentally relevant concentrations) were used as the hazardous materials. The specific purpose of this study was to 1) evaluate Chlorella pyrenoidosa growth in response to PS microplastics exposure under the upper limit of environmentally relevant concentration across its whole growth period, 2) identify morphological and physiological characteristics of the algae attributed to PS microplastics damage, and 3) elucidate how the algae reacted to microplastics exposure during its growth cycle as well as the mechanism underlying it. The results from this study are expected to extend our knowledge regarding the impact of microplastics on the primary producer of aquatic systems.