Antioxidant Responses in Copepods Are Driven Primarily by Food Intake, Not by Toxin-Producing Cyanobacteria in the Diet.

Elena Gorokhova,Rehab El-Shehawy

doi:10.3389/fphys.2021.805646

Abstract

The association between oxidative processes and physiological responses has received much attention in ecotoxicity assessment. In the Baltic Sea, bloom-forming cyanobacterium Nodularia spumigena is a significant producer of various bioactive compounds, and both positive and adverse effects on grazers feeding in cyanobacteria blooms are reported. To elucidate the effect mechanisms and species sensitivity to the cyanobacteria-dominating diet, we exposed two Baltic copepods, Acartia bifilosa and Eurytemora affinis, to a diet consisting of toxin-producing cyanobacteria N. spumigena and a high-quality food Rhodomonas salina at 0–300 μg C L−1; the control food was R. salina provided as a monodiet at the same food levels. The subcellular responses to food type and availability were assayed using a suite of biomarkers – antioxidant enzymes [superoxide dismutases (SOD), catalase (CAT), and glutathione S-transferases (GST)] and acetylcholinesterase (AChE). In parallel, we measured feeding activity using gut content (GC) assayed by real-time PCR analysis that quantified amounts of the prey DNA in copepod stomachs. As growth and reproduction endpoints, individual RNA content (a proxy for protein synthesis capacity), egg production rate (EPR), and egg viability (EV%) were used. In both toxic and nontoxic foods, copepod GC, RNA content, and EPR increased with food availability. Antioxidant enzyme activities increased with food availability regardless of the diet type. Moreover, CAT (both copepods), SOD, and GST (A. bifilosa) were upregulated in the copepods receiving cyanobacteria; the response was detectable when adjusted for the feeding and/or growth responses. By contrast, the diet effects were not significant when food concentration was used as a co-variable. A bimodal response in AChE was observed in A. bifilosa feeding on cyanobacteria, with up to 52% increase at the lower levels (5–25 μg C L−1) and 32% inhibition at the highest food concentrations. These findings contribute to the refinement of biomarker use for assessing environmental stress and mechanistic understanding of cyanobacteria effects in grazers. They also suggest that antioxidant and AChE responses to feeding activity and diet should be accounted for when using biomarker profiles in field-collected animals in the Baltic Sea and, perhaps other systems, where toxic cyanobacteria are common.

Highlights

The health conditions in biota are influenced by numerous environmental factors, with oxidative stress, i.e., the imbalance between pro-oxidant and antioxidant homeostatic cellular conditions, mediating many disorders
Feeding experiments with the copepods fed with cyanobacteriumrich diet and adequate non-toxic food control were conducted to assess the effects of N. spumigena on suborganismal responses [antioxidant enzymes (SOD, CAT, and glutathione S-transferases (GST)) and neurotoxicity biomarker AChE], food intake, and growth
GST activity significantly increased with food availability (Figures 1E,F; Table 1); in A. bifilosa fed Nodularia, this increase occurred only below150 μg C L−1, followed by a decrease at 300 μg C L−1 (Figure 1E)

Summary

Introduction

The health conditions in biota are influenced by numerous environmental factors, with oxidative stress, i.e., the imbalance between pro-oxidant and antioxidant homeostatic cellular conditions, mediating many disorders. Pro-oxidant conditions occur due to either increased generation of reactive oxygen species (ROS) or their poor quenching/scavenging into the body, potentially inducing damage to macromolecules, including proteins lipids, and DNA (Sies et al, 2017). While naturally occurring at low levels, ROS can be induced by environmental conditions, both natural (e.g., hypoxia, UV radiation, and bioactive compounds) and anthropogenic (e.g., chemical contaminants). The essential antioxidant enzymes (Pisoschi and Pop, 2015) are (i) ROS eliminating enzymes [e.g., superoxide dismutases (SOD), catalase (CAT), and peroxidases], (ii) enzymes that eliminate internal lipid peroxidation products [e.g., glutathione peroxidases], and (iii) those eliminating toxic secondary radical oxidation products [e.g., glutathione S-transferases (GST)]. A balance between ROS production and antioxidants to quench and/or scavenge them can be tipped by any additional burden of free radicals either from the environment or produced within the body, leading to oxidative stress manifested as fitness penalties

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Conclusion