The effects of aqueous extract from watermelon (Citrullus lanatus) peel on the growth and physiological characteristics of Dolichospermum flos-aquae

Abstract

Nowadays, the increasing Dolichospermum (Anabaena) blooms pose a major threat to the aquatic environment and public health worldwide. The use of naturally derived chemicals from plants to control cyanobacteria blooms has recently received a tremendous amount of attention. This study investigates the possibility of transforming watermelon peel (WMP) into a biological resource to allelopathically inhibit Dolichospermum flos-aquae blooms. The results demonstrated that the growth of D. flos-aquae was efficiently restricted by the aqueous extract of watermelon peel (WMPAE) in a concentration-dependent manner. Cell viability decreased quickly, intracellular structural damage occurred, chlorophyll a in algal cells degraded, and photosynthesis was clearly inhibited. At the same time, the levels of reactive oxygen species in viable cells increased significantly, as did malondialdehyde levels, indicating that WMPAE elucidated strong oxidative stress and corresponding damage to D. flos-aquae. Capsular polysaccharide (CPS) levels increased in all treatment groups, which represents an adaptive response indicative of the development of resistance to WMPAE stress and oxidative damage. Despite this, WMPAE had clear inhibitory effects on D. flos-aquae. These findings provide fundamental information on an allelopathic system that could be a novel and attractive approach for suppressing D. flos-aquae blooms in small aquatic environments, especially aquaculture ponds.

Figure 1

Growth curves (A) and algal growth inhibition efficiency (B) of the Dolichospermum flos-aquae after treatment with different aqueous extract of watermelon peel (WMPAE) concentrations. The data in the figures are shown as mean ± standard deviation (n = 3).

Figure 2

Cell viability of Dolichospermum flos-aquae treated with aqueous extract of watermelon peel (WMPAE): (A) Quantitative measurement results (Data are shown as mean ± standard deviation for n = 3, * indicates p < 0.05); (B) Light images and fluorescent images of algal cells in 0 g L⁻¹ and 5.4 g L⁻¹ WMPAE group: B1, cells in 0 g L⁻¹ group; B2, cells in 5.4 g L⁻¹ group for 24 h; B3, cells in 5.4 g L⁻¹ group for 48 h.

Figure 3

Representative TEM images of Dolichospermum flos-aquae cells treated with 0 g L⁻¹ and 5.4 g L⁻¹ aqueous extract of watermelon peel (WMPAE). (A) (B), normal cells in 0 g L⁻¹ WMPAE group. (C,D), cells in 5.4 g L⁻¹ WMPAE group after 24 h exposure. (E,F), cells in 5.4 g L⁻¹WMPAE group after 48 h exposure. (Abbreviations and arrows number indicate: plasma membrane, (PM); cell wall, (CW); photosynthetic lamellae, (PL); nucleoid zone, (NZ); Thylakoid, (Th); 1, vacuolation; 2, disconnection between cells in filaments; 3, pyknosis formation; 4, the separation of cell wall and plasma membrane; 5, blurring of photosynthetic lamellae).

Figure 4

MDA level (A), ROS level (B), and ROS level in viable cells (C) of the Dolichospermum flos-aquae treated with aqueous extract of watermelon peel (WMPAE) at different concentrations. (Data are shown as mean ± standard deviation for n = 3, * indicates p < 0.05).

Figure 5

Chlorophyll a (Chl a) and carotenoid in Dolichospermum flos-aquae cells under the stress of aqueous extract of watermelon peel (WMPAE) (Data are shown as mean ± standard deviation for n = 3).

Figure 6

In vivo single-cell Raman spectra of Dolichospermum flos-aquae under aqueous extract of watermelon peel (WMPAE) stress.

Figure 7

The $\text{Fv}/\text{Fm}$, ${\text{F}}_{\text{v}}^{\prime }/{\text{F}}_{\text{m}}^{\prime }$ and $\text{NPQ}$ of Dolichospermum flos-aquae treated with aqueous extract of watermelon peel (WMPAE) at different concentrations. (Data are shown as mean ± standard deviation for n = 3, * indicates p < 0.05).

Figure 8

Capsular polysaccharide (CPS) levels in Dolichospermum flos-aquae treated with aqueous extract of watermelon peel (WMPAE) at different concentrations. (Data in the figure are shown as mean ± standard deviation for n = 3, *indicates p < 0.05).