Community changes of gut microbes highlight their importance in the adaptation of copepods to toxic dinoflagellates

Zooplankton grazers, like copepods, can feed on toxic microalgae and live normally. We hypothesize that gut microbial communities (GMCs) may contribute to the detoxification of the host by changing their compositions and recruiting more beneficial bacteria. Here, we measured the physiological responses of two copepod species ( Acartia sp. and Paracalanus sp.) fed with toxic ( Alexandrium tamarense ) and non-toxic ( Alexandrium andersonii ) dinoflagellates, respectively. Both copepods maintained consistently high survival rates but slightly reduced ingestion rates when feeding upon the toxic dinoflagellate (when compared to the non-toxic one), suggesting a compensatory mechanism. The compositional variation of copepod GMCs, at the amplicon sequence variant (ASV) level, was mostly significantly different among copepod host species (R = 0.83, by ANOSIM test), while diet type played minor but significant roles. Under the toxic diet, Acartia sp. enriched only five ASVs while Paracalanus sp. recruited a wide range of taxa (38 ASVs) mostly belonging to Alphaproteobacteria (e.g., Rhodobacteraceae) and Gammaproteobacteria (e.g., Alteromonadaceae). In contrast, when clustering GMCs by predicted functions, diet type was the key regulating factor, suggesting the functional convergence of copepod GMCs in response to algal toxins. This can be explained by the fact that most of the enriched bacteria under the toxic diet have similar functions on detoxification and maintaining the host homeostasis. This study deepens our understanding of the roles of GMC in the detoxification and adaptation mechanisms of copepods during harmful algal blooms.