Retinoic Acids Can Hinder Molting and Development of the Marine Copepod Tigriopus Japonicus in the Coastal Environment.

Retinoic acids (RAs) and their oxidative metabolites (i.e., 4-oxo-RAs) are teratogenic to aquatic animals at elevated levels, while their toxic mechanisms in invertebrates remain largely unknown. This study aimed to determine whether marine microalgae could release RAs and 4-oxo-RAs, and to elucidate their toxic mechanisms to the marine copepod Tigriopus japonicus using transcriptomics. We observed elevated levels of RAs and 4-oxo-RAs in natural coastal seawater during algal blooms, and their releases from microalgae. A 21-day chronic exposure experiment showed that the widely distributed all-trans-RA (at-RA), at environmentally relevant concentrations (including worst-case scenarios, i.e., 10-2400 ng/L), delayed molting and development in both nauplius and copepodite stages of the copepod, with significant correlations between at-RA concentrations and developmental times. Transcriptomic analyses suggested that the toxic mechanisms may involve the down-regulation of calcium and nitric oxide synthase signaling in the cyclic guanosine monophosphate pathway by at-RA, leading to excessive formation of ecdysteroid binding to its receptor and thereby disrupting the normal ecdysis process. This was further supported by the over-expression of genes related to the ecdysteroid receptor and its heterodimer ultraspiracle, as well as by increased 20-hydroxyecdysone content in exposed copepods. Such new discoveries advance our understanding of the ecotoxicity of RAs, particularly in aquatic ecosystems that suffer from eutrophication and frequent algal blooms.