Plastics aplenty in paddy lands: incidence of microplastics in Indian rice fields and ecotoxicity on paddy field phytoplankton

Abstract

Occurrence of microplastics (MP) in natural paddy fields and its impact are less studied. This study reports the abundance of MP in two paddy fields of Kerala, India, cultivating rice crops, 'Pokkali' and 'Uma' crops, which are vital to Kerala's food security and climate resilience. Fourier transform infrared spectroscopy (FTIR) analyses confirmed the presence of polyethylene (PE) and polypropylene (PP) fragments as major MP in the surface water of paddy fields during vegetative (transplantation) and ripening (near harvesting) phases. MP density in the vegetative phase of 'Pokkali' (1370 ± 468.51 fragments/m3) and 'Uma' (1110 ± 304.96 fragments/m3) was thrice more than the ripening phase concentrations (400 ± 196.85 and 370 ± 57.00 fragments/m3, respectively). Subsequently, ecotoxicity of MP and plastic leachates (PL) on phytoplankton that are naturally found in rice fields was examined. Microalga, Chlorococcum sp., and cyanobacterium, Synechococcus sp., were grown in modified BG11 and BG11 media, respectively, and tested with paddy field concentrations for PE-MP and PE-PL. MP bestowed a significant hormetic effect on the specific growth rate of the microalga (121% of the control) whereas the cyanobacterial growth was negatively impacted (70% of the control). Both phytoplankton exhibited a similar response when exposed to PL, but results were neither dose-dependent nor significant. Further, increased catalase activity and compromised superoxide dismutase machinery in the cyanobacterium corroborated the toxic impact on growth (p ≤ 0.05), which indicates reactive oxygen species (ROS) generation in MP-treated groups. ROS generation indicates oxidative stress following MP exposure in the studied phytoplankton perhaps through surface contact or by leaching of toxic intermediates into the medium. The distinctive responses of paddy field phytoplankton to MP and PL stress suggest that MP pollution may enrich certain resilient species over others leading to a possible change in phytoplankton community structure. Pollution load indices suggest that even environmental concentrations of MP and PL may affect the rice productivity as paddy field phytoplankton play a significant role in sustaining and enhancing crop health. Therefore, the presence of MP at alarming concentrations in the paddy fields signifies the emergence of a global environmental and food security concern.