## 7 Haslea ostrearia as a New Model Species for Marine Ecotoxicology  
The pollution of aquatic environments is a very complex issue, resulting from the interaction of chemical and physical factors. This pollution affects all life in these environments by causing damage and functional disturbances in ecosystems. To estimate physiological and molecular modifications induced by these toxins and contaminants, many studies use model species. Diatoms, as photosynthetic organisms, are important bio-sentinels in most aquatic environments. Being the main primary producers [138], they are an essential component in studies that aim to understand the alterations of the whole trophic network. For marine ecotoxicology studies, only a few diatom species (_Asteroplanus karianus_, _Chaotecors_ spp., _Coscinodiscus eccentricus_, _P. tricornutum_, _Skeletonema_ spp., _Nitzschia closterium_, _Pseudo-nitzzchia_ spp., _Thalassioira_ spp.) have been used to estimate the negative effects of pollutants on primary photosynthetic producers. Preliminary bibliographic analysis in PubMed up to 2021 shows that 35.29% of the publications focused on marine toxicology are based on pennate diatoms (64.71% for centric diatoms), but only a few of them are benthic organisms. The diatoms used for these studies are mainly _P. tricornutum_, _Skeletonema_ spp. and _Thalassioira_ spp. The genomes of _P. tricornutum_, _S. costatum_ and _T. pseudonana_ have been sequenced and are available, allowing the identification of many genes involved in cellular mechanisms of detoxification. It has been shown that _H. ostrearia_ is a marine diatom with a large geographic distribution, encountered in the Atlantic Ocean, the North Sea, the Indian Ocean, the Mediterranean Sea, and the Pacific Ocean. This microalga is mainly a benthic and epiphytic organism, which sometimes constitutes large biofilms when it blooms on marine sediment or seaweeds, as in Corsica (Mediterranean Sea, Figure 7a,c), in Croatia (Adriatic Sea, Figure 7b), or in North Carolina (western Atlantic Ocean, Figure 7d). Thus, this widely distributed benthic diatom can be considered a representative of subtidal environments. The recent annotation of the _H. ostrearia_ strain HoB4 genome and transcriptome has allowed the identification of several biomarker sequences. Thus, data are now available to characterize the complete coding sequences of multiple markers involved in: the antioxidative responses (Ascorbate Peroxydase, 1197 bp; Catalase, 1581 pb; Fe-Mn SOD, 720 pb; Glutathione Synthase, 1584 bp); the general response to stress (Hsp60, 1740 bp; Hsp70, 1980 bp; Hsp90, 2220 bp); the biotransformation of xenobiotics (Cytochrome P450, 2201 bp; Glutathione Reductase, 1575 bp); and the apoptotic response (metacaspases, 984 and 2124 bp) (Figure 8). Some housekeeping genes, which may be used as references in qPCR investigations, can also be identified: actin (1134 bp), glyceraldehyde-3-phosphate dehydrogenase  
Figure 8: Molecular biomarkers involved in the defense of cellular mechanisms characterized in _Haslea estrearia_ HoB4. Figure 7: In situ blue _Haslea_ species blooms, observed in natural environments in (**a**,**c**) the Mediterranean Sea (Calvi, Corsica, France), (**b**) the Adriatic Sea (Dubrovnick, Croatia, B), and (**d**) the Atlantic Ocean (Morehead City, NC, USA). (1137 bp), and histone H2A (369 bp). This biomarker set can be used to evaluate the effects of various emerging pollutants in the marine environment (e.g., pesticides, nanoplastics, pharmaceutical compounds) on the gene expression of _H. ostrearia_ as a new model species in ecotoxicology. In vitro, it has been shown that high, environmentally relevant concentrations of marennine, such as in culture supernatants, can display toxicological effects on marine organisms--for instance, delayed embryonic development and mortality in _Mytilus edulis_ (mussel) and _Sphaerechinus granularis_ (sea urchin), with decreased fertilization success and larval development [139].