Divalent metal ions are essential components of DNA polymerases both for catalyzing nucleotide transfer reaction and base excision. All DNA polymerases require Mg
2+ or Mn
2+ for primer extension and for removal of incorrectly incorporated dNTPs (
Vashishtha et al., 2016). Metal occupy sites A and B for DNA synthesis. A metal is in an octahedral complex composed of two highly conserved acidic residues, water molecules, and the 3-hydroxyl group of the primer. Its catalytic function is to reduce the hydroxyl pKa, making it a highly efficient nucleophilic cell, which can attack the phosphorus atoms of the incoming dNTPs. B metal helps to stabilize the transition state for nucleotide excision (
Vashishtha et al., 2016). Substitution of Mg
2+ with other divalent metal tend to reduce the fidelity of DNA replication (
Vaisman et al., 2005). The results of this investigation showed that the chelate agent EDTA inhibited the DNA polymerase activity of POLD1, indicating that POLD1 requires divalent metal ions for primer extension. The fluids around DSHV where vent shrimp
R. exoculata lives are enriched in metals, such as Na
+, K
+, Mg
2+, Ca
2+, Mn
2+, Cu
2+, Ba
2+, Zn
2+, and Co
2+ (
Hutnak et al., 2008). Metal Ba
2+, Cu
2+ or Zn
2+ is not able to support the polymerase activity (
Vashishtha et al., 2016). The results showed that the presence of K
+, Ba
2+, Cu
2+ or Zn
2+ has no obvious effect on the enzymatic activity, indicating that POLD1 of shrimp
R. exoculata is stable under metal stress and its activity is not significantly affected by metal ions in the DSHV environment. Previous studies showed that Co
2+ and Mn
2+ could effectively replace Mg
2+ but caused an increase in misincorporation (
Seal et al., 1979;
Hays and Berdis, 2002). The present results also showed the activity of the enzyme was significantly enhanced by Mn
2+, indicating that Mn
2+ may be a good substitute for Mg
2+ in the terms of enzymatic activity. Due to limited opportunities and logistical difficulties in deep-sea sampling, the knowledge of metal bioaccumulation of deep-sea organisms and their relationships with environmental characteristics is quite scarce. Although there is no study on the concentration of metal ions in shrimp tissues from hydrothermal vents, studies from mussels show that despite quite different metals loads among the geochemical environment settings, Mn, Zn, and Cu concentrations varied over a smaller range across the sampling sites, implying biological regulation by deep-sea mussels for these elements (
Zhou et al., 2020). In deep-sea hydrothermal vents, the vent shrimp
R. exoculata are found to swarm around hydrothermal black smoker chimneys. This species maintains close proximity to the hydrothermal fluids, where steep thermal and chemical gradients are expected (
Desbruyères et al., 2001). They inhabit the chimney wall on the gradient between hydrothermal fluid and cold oxygen environment seawater, and the temperature ranges from 3°C to 25°C (
Le Bloa et al., 2017). Therefore, the species must evolve molecular mechanisms to cope with extreme temperature fluctuations. However, the understanding of the heat tolerance of this species and other deep-sea vent species is still limited due to the complexity of access to the vents and the difficulty of sampling and in vivo experiments on deep-sea animals under good physiological conditions. Deep-sea shrimp
R. exoculata shows a higher thermal tolerance than the coastal shrimp
Palaemonetes varians (
Ravaux et al., 2019). This study showed that the polymerase activity of vent shrimp POLD1 was decreased when the temperature was higher than 30°C. The optimum temperature of vent shrimp POLD1 was 25°C, which was comparable to those of other marine shrimp DNA polymerases. The results also indicate that in a short period of time, such as 10 minutes, the effect of temperature on enzyme activity was not significant. This result has significant ecological implications, since this would mean that
R. exoculata POLD1 is not affected by short temperature variations in its natural environment, which is an advantage in a thermal environment that is highly variable on short time scales. As reported, most types of DNA polymerases keep high activity under the condition of weak acid (
Biertümpfel et al., 2010), which was consistent with the findings in this investigation.