Molecular Pathogens 2024, Vol.15, No.1, 17-29 http://microbescipublisher.com/index.php/mp 18 of marine pathogens and contributes to the development of effective strategies to mitigate the impacts of marine diseases on ecosystems and human well-being. 2 Types of Marine Pathogens 2.1 Bacteria Bacterial pathogens are significant contributors to marine diseases, affecting a wide range of hosts including macroalgae, corals, fish, and other marine organisms. Bacteria within the phylum Bacteroidota, particularly the genera Tenacibaculum and Aquimarina, are known to cause widespread disease outbreaks in marine eukaryotic hosts. These bacteria often exhibit opportunistic lifestyles, acting as secondary pathogens or participating in polymicrobial diseases, with key virulence traits such as the production of adhesins and tissue-degrading enzymes. Additionally, bacterial pathogens can have a substantial impact on macroalgae, with evidence suggesting that environmental changes may increase the occurrence of bacterial diseases in these hosts (Egan et al., 2014). 2.2 Viruses Viruses are the most abundant entities in marine ecosystems and play crucial roles in oceanic processes through their interactions with all types of marine organisms. They infect a wide range of hosts, from bacteria to whales, and are major drivers of mortality and global geochemical cycles (Suttle, 2015). Marine viruses can manipulate the life histories and evolution of their hosts in remarkable ways, challenging our understanding of their ecological roles (Rohwer and Thurber, 2019). They are also implicated in the termination of algal blooms and can move between marine and terrestrial reservoirs, raising concerns about emerging pathogens (Gleason et al., 2017). 2.3Fungi Fungal pathogens are increasingly recognized as important agents of disease in marine animals. A comprehensive review identified 225 fungal species causing infections in 193 marine animal species, with Chordata and Arthropoda being the most frequently reported hosts (Pang et al., 2021). Microsporidia, Ascomycota, Mucoromycota, Basidiomycota, and Chytridiomycota are the primary fungal groups involved, with Microsporidia being the most prevalent. These fungi can cause a range of diseases, from respiratory infections in marine mammals to infections in fish and crustaceans. Environmental factors such as global warming and marine pollution are likely to exacerbate fungal disease outbreaks (Pang et al., 2021). Additionally, anamorphic ascomycetes, such as Aspergillus sydowii, are known to cause diseases in corals and molluscs, with their prevalence increasing due to changing environmental conditions (Gleason et al., 2017). 2.4 Parasites Parasites play a significant role in marine ecosystems, affecting a variety of hosts including algae, fish, and invertebrates. Parasitic infections can have profound impacts on host population dynamics and ecosystem functioning. For instance, parasitic dinoflagellates of the genus Amoebophrya and the newly described Perkinsozoa, Parvilucifera infectans, are widely distributed in coastal waters and can significantly impact host physiology, behavior, and bloom dynamics (Park et al., 2004). Parasites are also increasingly recognized for their role in algal diseases, contributing to sudden extinctions, regime shifts, and the spread of alien species (Gachon et al., 2010). The complex interactions between parasites, hosts, and environmental conditions underscore the need for further research to understand their full impact on marine ecosystems. 3 Pathogenic Mechanisms 3.1 Toxin production Toxin production is a common strategy used by marine pathogens to damage host tissues and disrupt normal cellular functions. For instance, Vibrio vulnificus biotype 3 secretes a multifunctional autoprocessing repeats-in-toxin (MARTX) toxin, which is a critical virulence factor. However, this biotype secretes a less virulent form of the toxin, which allows it to persist longer in host reservoirs by reducing its virulence (Choi et al., 2021). Similarly, Bacillus thuringiensis produces crystal toxins that enable it to invade host tissues and access nutrients,
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