Effects of Ocean Acidification on Molting, Oxidative Stress, and Gut Microbiota in Juvenile Horseshoe Crab Tachypleus tridentatus.

Ximei Liu,Quangang Zhu,Kai Xiong,James Kar-Hei Fang,Caoqi Zhang,Jie Song,Menghong Hu,Zongguang Tai,Jiani Liu,Youji Wang

doi:10.3389/fphys.2021.813582

Abstract

Anthropogenic elevation of atmospheric carbon dioxide (CO2) drives global-scale ocean acidification (OA), which has aroused widespread concern for marine ecosystem health. The tri-spine horseshoe crab (HSC) Tachypleus tridentatus has been facing the threat of population depletion for decades, and the effects of OA on the physiology and microbiology of its early life stage are unclear. In this study, the 1st instar HSC larvae were exposed to acidified seawater (pH 7.3, pH 8.1 as control) for 28 days to determine the effects of OA on their growth, molting, oxidative stress, and gut microbiota. Results showed that there were no significant differences in growth index and molting rate between OA group and control group, but the chitinase activity, β-NAGase activity, and ecdysone content in OA group were significantly lower than those of the control group. Compared to the control group, reactive oxygen species (ROS) and malondialdehyde (MDA) contents in OA group were significantly increased at the end of the experiment. Superoxide dismutase (SOD), catalase (CAT), and alkaline phosphatase (AKP) activities increased first and then decreased, glutathione peroxidase (GPX) decreased first and then increased, and GST activity changed little during the experiment. According to the result of 16S rRNA sequencing of gut microbiota, microbial-mediated functions predicted by PICRUSt showed that “Hematopoietic cell lineage,” “Endocytosis,” “Staphylococcus aureus infection,” and “Shigellosis” pathways significantly increased in OA group. The above results indicate that OA had no significant effect on growth index and molting rate but interfered with the activity of chitinolytic enzymes and ecdysone expression of juvenile horseshoe crabs, and caused oxidative stress. In addition, OA had adverse effects on the immune defense function and intestinal health. The present study reveals the potential threat of OA to T. tridentatus population and lays a foundation for the further study of the physiological adaptation mechanism of juvenile horseshoe crabs to environmental change.

Highlights

The ongoing increase of anthropogenic carbon dioxide (CO2) in the atmosphere due to human fossil fuel combustion and deforestation causes an accumulation of CO2 in the oceans
Three glass tanks of juvenile horseshoe crabs were randomly selected as the acidification treatment [pH 7.3, Meilana et al (2021) found that estuarine waters inundated the intertidal area with pH fluctuation of 6.99–8.62, and pH 7.3 is the extreme acidified pH expected to be reached by 2300 in the ocean (Caldeira and Wickett, 2005)], and the other three tanks of juvenile horseshoe crabs were set as the control group
Molting occurred in six juvenile horseshoe crabs in the acidification group, with a molt rate of 1.7%, and seven juvenile horseshoe crabs in the control group, with a molting rate of 1.9%

Summary

Introduction

The ongoing increase of anthropogenic carbon dioxide (CO2) in the atmosphere due to human fossil fuel combustion and deforestation causes an accumulation of CO2 in the oceans. It is predicted that the maximum pH reduction at the ocean surface will reach 0.77 units by 2,300 (Fabry et al, 2008). This global-scale OA driven by the excessive emission of anthropogenic CO2 is changing the hydrochemical environment on which marine organisms depend. It may reduce the biodiversity, leading to the extinction of some key species in the food chain, affecting the interaction between species and affecting the stability of marine ecosystems (Rossoll et al, 2012; Gaylord et al, 2015)

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