# Biomonitoring of Heavy Metals: The Unexplored Role of Marine Sessile Taxa
## 4 Discussion
This peculiarity makes the coral skeleton an important biomonitoring tool for marine pollution, allowing to analyze and detect possible variations in the concentrations of HMs over different time periods. However, together with scleratinians, other groups of cnidarians, e.g., octocorals and stylasterids, showed the presence of growth bands in their skeletons [50; 51], making these taxa additional organisms suitable for biomonitoring studies. Regarding the other well-studied group, Porifera have been mainly collected in the Temperate Northern Atlantic realm [29]. Especially interesting are the few studies analyzing the role of their associated bacteria [52; 53; 54; 55; 56; 57; 58]. Many sponges, in fact, are characterized by the presence of complex communities of microorganisms composing the associated microbiota, being named \"bacterisoponges\" or \"high microbial abundance sponges\" [59]. Since the microbiota can contribute up to 40% of sponge biomass and cover a fundamental role in their secondary metabolism [60; 61], it has been suggested that it can actively participate in the bioaccumulation process. Many bacteria isolated from sponges, e.g., _Fasciospongia cavernosa_, have been found to be resistant to different antibiotics and pollutants, including Persistent Organic Pollutants (POPs) and various heavy metals, as Cu, Pb, Co, Cd, Zn, Ni, and Hg together with their organic compounds [55]. Different researches showed how also the bacteria associated with polychatees and ascidians can be important in the sequestration of HMs from water [62; 63; 64; 65; 66; 67]; therefore, increasing study of the effective role of the microbiota is necessary to fully understand the dynamics of the process. Besides, we want to stress the importance of works on the possible negative physiological effects of HMs in organisms (e.g., apoptosis of sponge tissues, bleaching in corals, decreasing of sperm density in polychaetes, etc.) [68; 69; 70], but also the \"positive\" consequences that HMs can trigger, e.g., in the moon jellyfish _Aurelia_ spp., which polyps were observed to increase their asexual reproduction if exposed to Ag and Hg [4; 71]. However, what we call a \"positive\" response can have severe environmental consequences. Referring to the example of _Aurelia_ spp., an increase in the asexual reproduction can be followed by increase in polyp strobilation and ephra release and, thus, intense jellyfish blooms [72], with multiple ecological and socio-economic consequences on human health, marine trophic chains, fishery, and aquaculture [73; 74]. Moreover, an increase in this kind of studies will help to identify different physiological traits known as \"biomarkers.\" Biomarkers are biological responses of suitable sentinel species to pollutants, which can act as early-warning tools able to detect pollutant-induced stress syndromes in organisms before the occurrence of severe habitat and communities' alterations [75]. From our review emerged clearly that the most studied HMs have been Cd, Cu, Pb, and Zn, both due to their natural toxicity (Cd and Pb) (WFD 2000/60/EC) and both for being used by organisms as micronutrients (Cu and Zn). In fact, essential elements can be considered HMs since they can become toxic, for an organism, above specific thresholds [28], as it was observed, e.g., in _Aurelia aurita_ polyps [71] or in the sponge _Crambe crambe_[76]. Many groups of invertebrates are known to synthetize metallothionee or metallothionee-like proteins, including the taxa considered in this review [77; 78; 79; 80], except for Cnidaria and Bryozoa, for which there is still no information available [81; 82; 83]. Metallothioneeins are cysteine-rich metal-binding proteins, which can sequester metals, being involved in metal resistance and in the detoxification processes [82]. If the detoxification mechanism is not overwhelmed, organisms can increase body metal concentrations without showing toxic effects or signs of suffering.