Gut Symbiotic Microbial Communities in the IUCN Critically Endangered Pinna nobilis Suffering from Mass Mortalities, Revealed by 16S rRNA Amplicon NGS

Abstract

Mass mortality events due to disease outbreaks have recently affected almost every healthy population of fan mussel, Pinna nobilis in Mediterranean Sea. The devastating mortality of the species has turned the interest of the research towards the causes of these events. After the haplosporidan infestation and the infection by Mycobacterium sp., new emerging pathogens have arisen based on the latest research. In the present study, a metagenomic approach of 16S rRNA next generation sequencing (NGS) was applied in order to assess the bacterial diversity within the digestive gland of diseased individuals as well as to carry out geographical correlations among the biodiversity of microbiome in the endangered species Pinna nobilis. The specimens originated from the mortalities occurred in 2019 in the region of Greece. Together with other bacterial genera, the results confirmed the presence of Vibrio spp., assuming synergistic effects in the mortality events of the species. Alongside with the presence of Vibrio spp., numerous bacterial genera were detected as well, including Aliivibrio spp., Photobacterium spp., Pseudoalteromonas spp., Psychrilyobacter spp. and Mycoplasma spp. Bacteria of the genus Mycoplasma were in high abundance particularly in the sample originated from Limnos island representing the first time recorded in Pinna nobilis. In conclusion, apart from exclusively the Haplosporidan and the Mycobacterium parasites, the presence of potentially pathogenic bacterial taxa detected, such as Vibrio spp., Photobactrium spp. and Alivibrio spp. lead us to assume that mortality events in the endangered Fan mussel, Pinna nobilis, may be attributed to synergistic effects of more pathogens.

Mussel Mass Mortality and the Microbiome: Evidence for Shifts in the Bacterial Microbiome of a Declining Freshwater Bivalve

“…A previous study found strong shifts in unionid gut microbiome composition when comparing wild Ohio pigtoe ( Pleurobema cordatum ) to individuals held in mesocosms under conditions known to induce wasting syndromes and mortality, with shifts to dominance by Proteobacteria in the latter [ 34 ]. Proteobacteria were also the most abundant taxa in the gut bacterial communities of the marine bivalve P. nobilis during a mass mortality event in the Mediterranean Sea [ 26 ]. Based on both prevalence and percent composition, SV2 ( Yokenella/Salmonella ) and SV4 ( Aeromonas spp.)…”

“…[ 25 ], and opportunistic Vibrio spp. [ 26 ]. Infectious disease outbreaks in marine bivalves are most often driven by interactions among host factors, abiotic environmental conditions, and pathogens [ 27 , 28 ].…”

Pathophysiological Responses of Pinna nobilis Individuals Enlightens the Etiology of Mass Mortality Situation in the Mediterranean Populations

“…Disease aetiology of mass mortality events in P. nobilis species is considered a complex situation, involving many microorganisms in relation to rapid changes in abiotic factors due to ongoing climate change [ 21 ]. Infection by H. pinnae was adopted as a main causative agent in many cases without taking into consideration the pathobiome of the species [ 21 ].…”

“…belonging to the Splendidus and Mediterranei clades were detected during mortality events in P. nobilis populations. Experimental laboratory conditions detecting the infection of P. nobilis by Vibrio spp have transformed the phenomenon of mortalities into a complicated matter [ 8 , 19 , 20 , 21 , 22 ]. Investigation of P. nobilis mortalities continued and more potential threats were implicated in the devastation of its natural populations [ 23 , 24 ].…”

The Implication of Vibrio Bacteria in the Winter Mortalities of the Critically Endangered Pinna nobilis

“…[ 17 ], were analyzed, and revealed the presence of 14 different bacterial OTUs, among which were bacterial genera such as Vibrio sp., Mycoplasma spp., Pseudoalteromonas spp., Mycobacterium sp., Aliivibrio spp., Photobacterium spp., and Psychrilyobacter spp. [ 31 ]. The aforementioned genera can be symbiotic or pathogenic genera for bivalves, supporting the complexity of mass mortality events.…”