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HTS screening for Prochlorococcus and Synechococcus in sea water and sediment impacted by mariculture

Cyanobacterial indicators are less represented in marine environment than in more eutrophic freshwater ecosystems, where Cyanobacteria often form extensive blooms. Furthermore, they are more scarce when investigating mariculture impacted sites, even though aquaculture is the fastest growing food industry and generates additional pressure on coastal areas. Sampling of sea water and sediment was conducted seasonally in 2017 and 2018, on the fish farm of European sea bass and nearby control site. Sampling sites are situated in the southern Eastern Adriatic Sea, in semi-enclosed bay under the influence of the freshwater river. Sea water parameters were measured in situ (oxygen, temperature, conductivity, turbidity, pH, TDS) and in laboratory (total P, total N and SiO2). Sediment granulometric analysis was performed. eDNA was extracted from 16 sea water and 16 sediment samples. Highthroughput sequencing was performed targeting V1-V3 variable region of 16S rRNA gene. Data was processed in QIIME 2 2019.4, using taxa filtering to include only cyanobacterial ASVs and excluding chloroplast and mitochondrial sequences. Phylogenetic tree was constructed in iTOL 4.4.2., and statistical analysis performed in RStudio 1.2.1335. In sea water was revealed predominance of eutrophic, coastal strain Synechococcus CC9902 (20-70% ASVs) in all seasons during both years. Synechococcus CC9902 was recorded as potential bloom-forming strain which can furthermore negatively impact fish behavior. Prochlorococcus was found to be low-light adapted ecotype Prochlorococcus MIT9313. Moreover, this strain is known to utilize organic nitrogen compounds (urea and amino acids) generated by the fish in the farming process. Prochlorococcus MIT9313 had inconsistent presence in samples of different years (up to 29% in 2017 and 1, 4%-10, 1% in 2018, respectively), although totally absent from winter samples 2017 and summer 2018. In sediment, strain adaptable to light/oxygen deprivation was found to be Synechococcus PCC-7336. It was mostly present in sandy gravel type of sediments on control site, in relative abundances of 1, 6 – 14, 6% in 2017 and 2, 3 – 18% in 2018. Other ever-present genus in water samples was Cyanobium PCC-6307, whereas in sediments total of 13 genera was found in 2017 sampling and 9 genera in 2018. Predominant in all sampling points were benthic genera Pleurocapsa PCC-7319 and Xenococcus PCC-7305. Discovered Prochlorococcus and Synechococcus strains are indicating more eutrophic conditions then implied by water quality parameters and sediment type. This opens new questions in monitoring eutrophication indicators in the mariculture.

Synechococcus ; Prochlorococcus ; mariculture ; HTS ; indicators ; Adriatic Sea

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