Temperature shapes the response of marine nitrogen-fixing cyanobacteria to nanoplastics

Nanoplastic (NP) pollution is an escalating global environmental issue, and the concurrence of increasing sea temperatures and NP pollution is expected to intensify in the coming decades. Although extensive research has explored the effects of NPs on marine organisms, their combined impact with temperature, particularly on marine nitrogen-fixing cyanobacteria, which are critical for nitrogen cycling in the ocean, remains largely unexplored. Here, we present one of the first investigations into this specific interaction for marine nitrogen-fixing cyanobacteria by examining the responses of Crocosphaera watsonii, a key species in this group, grown under NP-free and NP-exposed conditions at 25, 28, and 31°C. Our results showed that NP exposure significantly inhibited growth rates across all temperatures, with more pronounced effects at both low (25°C) and high (31°C) temperatures. Notably, nitrogen fixation rates declined by over 80% at all temperatures, while photosynthesis was particularly compromised at 31°C. Nanomechanical analysis using atomic force microscopy revealed that at 25°C, the cells exhibited reduced mechanical robustness, increasing their susceptibility to NPs and resulting in greater membrane damage. Conversely, at 31°C, the cells showed increased adhesion due to elevated extracellular polysaccharide production, promoting aggregation that may shade the cells and hinder light harvesting. Our findings suggest that temperature affects the nanomechanical properties of C. watsonii, which, along with its impacts on photosynthesis and nitrogen fixation under NP exposure, explains the temperature-dependent effects of NPs on growth. This research highlights the potential for NP pollution to significantly reduce new nitrogen inputs, ultimately threatening productivity in a warming ocean.