Within the context of local impacts, the process of environmental contamination has been raising deep concerns in the scientific community [
98]. As a reflection of that, a great number of studies have aimed to understand how pollutants could affect the carbonic anhydrase activity and its applicability as a pollution biomarker. As the reader will see, the majority of these studies showed that this enzyme activity can be inhibited by a multitude of contaminants. For example, El-Gendy et al. (2019) [
99] demonstrated that the land snail Theba pisana orally exposed to three organic contaminants (abamectin, thiamethoxam, and acrylamide) for 2 weeks displayed reduced carbonic anhydrase activity. Similarly, Lionetto et al. (2006) [
78] demonstrated that the mussel Mytilus galloprovincialis exposed to cadmium chloride, both in vitro and in vivo (incubation for 1 h or exposure for 14 days, respectively), had a remarkable inhibition in carbonic anhydrase activity. Similarly, the oysters C. giga and C. angulata also showed reduced activity of this enzyme when exposed to 2.78 mg/L arsenic for 28 days [
93]. Moreover, Santini et al. (2011) [
100] showed that the freshwater bivalve Anodonta anatina exposed to low levels of copper for 15 days displayed reduced carbonic anhydrase activity. Similarly, this enzyme was inhibited when the corals Acropora cervicornis and M. faveolata were exposed to this metal for 5 weeks [
101]. Additionally, Marangoni et al. (2019) [
83] showed that the coral M. harttii exposed for up to 35 days to 2.3 or 3.2 μg/L copper also experienced inhibition of this enzyme. Similarly, Fonseca et al. (2019) [
102] demonstrated that the same coral species exposed to many copper concentrations (4.6–19.4 μg/L) for 96 h displayed reduced carbonic anhydrase activity. In opposition, Caricato et al. (2010) [
103] showed that the mussel M. galloprovincialis exposed to cadmium elevated the carbonic anhydrase protein concentration and activity in both laboratory and field conditions. Similarly, the coral A. milepora and the calcareous algae Halimeda opuntia exposed to the herbicide diuron (0–30 μg/L) had this enzyme activity enhanced (in preparation). On the other hand, some studies also evaluated the effects of environmental pollution on carbonic anhydrase in terms of gene expression; for example, Balbi et al. (2017) [
104] showed that the mussel M. galloprovincialis exposed for 48 h to polystyrene nanoplastics had a down-regulation of about 40% in this gene transcription. Similarly, Capolupo et al. (2018) [
105] demonstrated that the same species also experienced a reduction in carbonic anhydrase gene expression following exposure to polystyrene microplastics for 48 h.
At this point, it is clear that carbonic anhydrase responds to environmental pollution in a very predictable way, both in terms of gene transcription and enzymatic activity. As it was demonstrated, a great diversity of pollutants can reduce this enzyme activity and/or gene transcription in many calcifying animals. Therefore, there is sufficient evidence to sustain the idea that carbonic anhydrase is a good biomarker of environmental pollution, despite confounding factors such as phylogeny, type of contaminant, exposure period, and/or concentration. As a matter of fact, this enzyme has already been used in the context biomonitoring studies. Interestingly, the pattern of response of carbonic anhydrase in ecotoxicological studies performed under field conditions was opposite to that observed in laboratory experiments; for example, Santos et al. (2017) [
106] showed that this enzyme was enhanced in the oyster Crassostrea rhizophorae collected in a highly impacted zone (Paraíba Estuary, Paraíba state, Brazil), in comparison to an area of environmental preservation (Mamanguape Estuary, Paraíba state, Brazil). Similarly, Azevedo-Linhares and Freire (2015) [
107] demonstrated that the same oyster species also showed elevated activity of this enzyme when animals from impacted zones were compared to control areas. Moreover, Caricato et al. (2010) [
103] demonstrated in a translocation study that the mussel M. galloprovincialis displayed an elevation in the carbonic anhydrase activity when exposed for 30 days to a polluted site (Mar Grande of Taranto, Taranto, Italy) in comparison to a reference site (S. Maria of Leuca, Lecce, Italy).