## 3 Results
### Microbial Diversity of Chill-Stored Meagre Products  
According to HTS analysis, a total of 375,507 raw reads were obtained and after quality checking 269,884 of them were retained, with an average of 16,867 per sample (Table S1). Those high-quality reads were assigned to 936 observed features (range from 28 to 96). The rarefaction to 5000 sequences for estimation of bacterial diversity was quite enough satisfied, since, for example, the Shannon-Wiener Index curves plot (Figure S1) reached a plateau at approximately 500 sequences, indicating a sufficient sequencing depth to characterize microbial diversity.  
Figure 1: Microbial population changes of whole and filleded chill-stored meagre of January (**a,c**) and July (**b,d**), respectively. (\(\bullet\)) Total Viable Counts (TVC), (o) _Pseudomonas_ spp., (\(\blacksquare\)) H\({}_{2}\)S producing bacteria, (\(\blacktriangle\)) LAB—pH 6.4, and (\(\Delta\)) LAB—pH 8 (\(\bullet\)). Each point is the mean of six replicates. Error bars represent the standard error. The dashed vertical line indicates the end of shelf life.  
The results of bacterial diversity at Phylum and Family level are given in Figure S2. Metata taxonomic analysis revealed the presence of three main bacterial phyla (Firmicutes, Proteobacteria and Actinobacteria) detected in high relative abundances and other five in traces (Acidobacteria, Spirochaetes, Cyanobacteria, Chlamydia and Bacteroides), during the whole storage time. In all samples, Proteobacteria (mainly Pseudomonasaceae and to a lesser extent Moraxellaceae) was the most abundant phylum at the end of storage, while at the first stage of storage, this bacterial phylum co-existed with Actinobacteria (mainly Propionibacteriaceae) and Firmicutes (Clostridiaceae). It is crucial to mention that Firmicutes was the most abundant phylum in all but one sample (ArW1_D3) at the first stage of storage.  
At genus level, _Ralstonia_ (37.5%), _Novosphingobium_ (24.2%), _Sphingomonas_ (16.8%) and _Clostridium_ (10.1%) were found initially (D 3) to dominate in the whole meagre of January, while other bacterial genera were found at lower abundances (Figure 2). At advanced period of storage (D 12) the presence of _Arthobacter_ is noteworthy (58%), while at the end of shelf-life (ArW1_D15) potential spoilage bacteria of fish like _Pseudomonas_ (46.6%) and _Psychrobacter_ (42.5%) clearly dominated. Oppositely, in the whole fish of July, _Clostridium_ (43.5%) and _Propionibacterium_ (25.9%) were the most abundant bacterial genera at the initial stage (D 3), while at the end of fish shelf-life (ArW2_D12), _Pseudomonas_ (66.6%) was found to dominate, followed by _Shewanella_ (10.5%). For the fillets, _Pseudomonas_ was the most abundant genus at the end of shelf-life (D 7) for both months (Figure 2).  
Figure 2: Relative abundance (%) of top-20 bacterial genera of whole (ArW) and filleted (ArF) chillstored meagre caught in January (1) and July (2), obtained through metabarcoding analysis of 16S rRNA gene at intervals of storage time.  
Finally, to evaluate potential differences based on samples' microbiota profile, a PCoA plot was generated based on the weighted UniFrac distance (Figure 3). The analysis indicated a clear separation of samples in advanced storage level and the rejection time point (D 7 for fillets and 12, 15 for whole fish) from the rest. Furthermore, storage temperature (0 and 4 \({}^{\circ}\)C for whole and filleted fish, respectively) did not affect samples' distribution at all. The principal coordinates explained about 91.27% of total variance (vectors 1, 2 and 3 explained 73.05%, 13% and 5.22%, respectively).