Diatom-derived oxylipins, including polyunsaturated aldehydes (PUA), are considered to have infochemical, allelochemical and bacteriostatic properties, with plausible roles as grazing deterrents and regulators of inter- and intraspecific competition. However, the extent and mechanisms of how PUA influence diatom–bacteria interactions remain unresolved. In this study, impacts on the diversity of the associated bacterial communities (microbiota) of two contrasting Skeletonema marinoi strains (a PUA and a non-PUA producer) were investigated under three nitrate conditions in batch culture. Further, the response of the culture microbiota was studied when spiked with PUA at ecologically relevant concentrations (86nM octadienal and 290nM heptadienal). Of the 741 identified OTUs, Proteobacteria was the most abundant phylum (62.10%), followed by Bacteroidetes (12.33%) and Firmicutes (6.11%). Escherichia/Shigella were the most abundant genera for all treatments. Similar communities were present in both spiked and non-spiked cultures suggesting they can tolerate PUA exposure at realistic concentrations. This study suggests that PUA are not major drivers of diatom–bacteria interactions in laboratory cultures.

A series of mass mortalities (wash-ups) of marine life were documented along England's north east coastline with peaks in September and October 2021, coincident with a programme of intensified maintenance dredging of the River Tees. Decapod crustaceans were the worst affected fauna, with brown crab (Cancer pagurus), European lobster (Homarus gammarus, L.), green shore crab (Carcinus maenas, L.) and velvet swimming crab (Necora puber, L.) populations severely affected. Moribund animals presented with twitching behaviours and paralysis. A potential release of the industrial pollutant pyridine was forwarded as one explanation; however, toxicology data for pyridine in decapods is lacking. In this study, we address this knowledge gap by executing a programme of immersion exposure experiments (pyridine at 2 - 100 mg L-1) using C. pagurus, measuring toxicity effects at the individual (survival) and cellular levels (cellular, mitochondrial, and lipid peroxidation reactive oxygen species (ROS) formation in the gills, hepatopancreas and claw muscle). Highest mortality rates were seen after 72 hours of exposure, returning an LC50 value of 2.75 mg L-1. Exposed crabs presented with patterns of convulsions, limb twitching, paralysis, and death. Crabs exposed to the lowest pyridine dose (2 mg L-1) were noticeably more docile than controls. Concentration was a significant factor influencing mitochondrial ROS formation at low concentrations, with tissue type, time, and their interaction all significant at 100 mg L-1. Computer simulations were used to model the transport of any pyridine released from the dredging work, demonstrating the potential for a pyridine plume to extend from Seaham to the north of the Tees to Whitby and Robin Hood's Bay to the south. This range corresponds well with the reported wash-ups and subsequent declines in catch rates.

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