In this study we focused on an archaeological Sambaqui site located in the Boguaçú River which is part of the Guaratuba bay estuary (Fig. 1). The soil of the Sambaqui Boguaçú is arranged in three visually distinct layers, with varying composition, that are likely to reflect the time of occupation of this site by different groups (Fig. 1C). To assess the microbial diversity within the Sambaqui-Boguaçú samples of the three visually different layers were collected and subjected to microbial cultivation approach and to metagenome microbial biodiversity survey.
3.1 Prokaryotic biodiversity using 16S rDNA amplicon sequencing
Total DNA was extracted from each of the three Sambaqui layers (Fig. 1C). The medium layer, namely CM, did not yield enough DNA for this analysis and was not considered for further analysis. The extracted metagenomic DNA from the top layer, namely S, and the bottom layer, namely B, were used as template for PCR amplification of the V4 region of the 16S rRNA gene and subjected to next generation Illumina sequencing using primers and methodology described previously (Huergo et al., 2018).
The amplicon sequence variants (ASV) were taxonomically classified; 13.8% and 9.3% of the ASV were classified as Archaea in S and B samples, respectively. Strikingly, despite being visually different (Fig. 1C), the samples S and B had similar biodiversity profiles at Phylum level (Fig. 2). The most represented Phylum were Acidobacteria, Rokubacteria, Proteobacteria and the Archaea Thaumarchaeota (Fig. 2). The most represented identified family was the methylotrophic Methylomirabilaceae (∼10% of the reads in both samples) (Figure S1). This family was reported to be abundant in paddy fields, an anaerobic habitat typically rich in methane (Ghashghavi et al., 2019). The Sambaqui site explored in this study was built over a mangrove which is also considered a habitat with high methane production, this may explain the abundance of the methylotrophic Methylomirabilaceae in our samples. Most of the remaining sequences could not be classified at Family (Figure S1) or lower taxonomic levels. These data support that the Sambaqui site hosts unique poorly characterized microorganisms.
3.2 Bacteria isolation and selection for growth in recalcitrant polysaccharides
All the three different layers of Sambaqui samples were diluted in saline and the 10− 2 dilution was plated in LB agar medium. The samples from the layers B and CM did not yield any CFU, while sample S produced 8 CFU, namely S1 to S8. The isolates S3 and S8 showed a similar colony morphology and produced of a dark pigment in LB agar plates or LB liquid medium (Figure S2). The isolates were evaluated for their ability to growth in M9 minimum medium containing glucose, agar, colloidal chitin, microcrystalline cellulose (MCC), carboxymethyl cellulose (CMC) or xylan as carbon sources. All the isolates were able to grow using glucose as carbon source while using ammonium as nitrogen source (Table 1). Interestingly, the isolates S3 and S8 showed little growth in M9-glucose agar plates without a nitrogen source (Table 1). Furthermore, the isolates S3 and S8 grew well in LB agar prepared using sea water indicating halophilic properties (not shown).
Table 1
Growth of isolates in M9 solid minimum medium with different carbon sources.
Isolate | Carbon Source1 |
Glucose | Agar | Chitin | MCC | CMC | Xylan | Glucose without nitrogen |
S1 | +++ | - | - | - | - | - | - |
S2 | + | - | - | - | - | - | - |
S3 | +++ | + | ++ | + | ++ | +++ | + |
S4 | + | - | - | - | - | - | - |
S5 | +++ | - | - | - | - | - | - |
S6 | +++ | - | - | - | - | - | - |
S7 | + | - | - | - | - | - | - |
S8 | +++ | + | ++ | + | ++ | +++ | + |
1all media contain agar 15g/L. + indicates the visual amount of biomass observed after growth (+ low, ++ medium, +++ high). |
The isolates S3 and S8 were able to growth in M9 agar containing different recalcitrant carbohydrates as carbon source including agar, chitin, CMC, MCC and xylan (Table 1 and Figure S3). Given that S3 and S8 were able to grow using agar as sole carbon source, we consider that the growth detected in the presence of other polysaccharides could be sustained by agar degradation instead. However, the degradation of chitin, CMC and xylan could be confirmed by staining the agar plates with congo red which showed typical polysaccharide degradation halo (Figure S3C).
To provide a taxonomic classification for the isolates the 16S rRNA gene of the isolates S3, S5, S6, and S8 were PCR amplified and subjected to partial sequencing. All sequences showed 100% identity to various Streptomyces strains when searched against specialized 16S rRNA NCBI database. Multiple sequence alignment of the 16S rRNA gene revealed two groups of sequences, one includes the sequences of S3 and S8 which were identical in pair wise comparison. The sequences of isolates S5 and S6 were also identical in pair wise comparison. The two different groups of isolates (S3-S8 and S5-S6) showed 4 base differences within the 16S rDNA V4/V5, 98.5% identity (Figure S4). Despite belonging to Streptomyces genus these two groups of isolates have completely different capacity to use recalcitrant polysaccharides as carbon source in M9 media (Table 1).
The isolate Streptomyces sp. S3 was subjected to further characterization. This bacterium was able to growth aerobically in M9 liquid medium containing the soluble polysaccharides xylan and CMC as the only carbon source. Streptomyces sp. S3 formed cell pellets in these media (Fig. 3A and B). Scanning electron microscopy analysis of these pellets showed the formation of typical Streptomyces mycelium (Fig. 3). Interestingly, the morphology of the mycelium was different in medium containing xylan or CMC. When using CMC as carbon source, the mycelium was covered with an apparent extracellular matrix (Fig. 3). We suspect that this cell surface adhering material may be formed by an extracellular complex of CMC and enzymes involved in CMC degradation.
Qualitative analyses for presence of active extracellular enzymes degrading xylan and CMC were carried out by placing 10 µl of each cell culture supernatant over M9 solid medium containing either xylan or CMC. After 30 min incubation at 30oC, the plates were stained with congo red to detect substrate degradation. As expected, degradation of xylan was readily detected using the supernatant of cells cultured in xylan (Figure S5A, spot 1). On the other hand, and quite surprisingly, only a faint CMC degradation halo was detected using the supernatant of cells cultured in CMC (Figure S5A, spot 3). The amount of protein in the culture supernatants were below the limit of detection and could not be determined. Hence, at this point, we cannot infer if the differences in xylan vs CMC degradation halo are caused by differences in total enzymes load and/or specific enzyme activities.
The ability of Streptomyces sp. S3 to grow aerobically in M9 liquid medium carrying insoluble MCC or filter paper as sole carbon sources was also evaluated. Even though the cells did not appear as pellets as observed when growing with CMC or xylan, it was possible to confirm microbial biomass development adhered to the insoluble MCC by staining the remaining insoluble substrate for protein using the Bradford reagent (not shown). Furthermore, it was possible to detect active xylan and CMC degrading extracellular enzymes from the Streptomyces sp. S3 supernatant of cells cultured in MCC (Figure S5A, spot 3). The supernatant from MCC growing cells produced a more intense CMC degradation halo than those assayed with the supernatant of cells growing in CMC itself (Figure S5B, compare spots 3 and 4).
3.3 Genome analysis
A draft genome sequence of Streptomyces sp. S3 was obtained comprising 569 contigs (40558 N50), with a total of 10,992,946 bp with G + C content of 70.55%. Genome annotation revealed 9844 protein coding sequences. The Streptomyces sp. S3 genome was used to perform phylogenetic analysis against reference organisms using Automated Multi-Locus Species (auto MLST) (Alanjary et al., 2019). The phylogenetic tree revealed that Streptomyces antibioticus DSM40234 is the closet type strain to Streptomyces sp. S3 (Figure S6) with a MASH distance of 0.1064 and estimated average nucleotide identity of 0.8936 (Table S1). S. antibioticus was originally isolated from soil and is well known for its potential to produce several bioactive compounds, including several antibiotics. There are strains of S. antibioticus isolated form marine environment which may explain the halophilic properties of the Streptomyces S3 isolate (Sharma et al., 2019; Waksman and Woodruff, 1941; Wang et al., 2017; Xu et al., 2011). Automated genome blast-based phylogeny retrieved from NCBI indicated that soil environmental isolate Streptomyces sp. WM6386 (Blodgett et al., 2016) as the closest relative (Figure S7).
In order to identify biosynthetic gene clusters (BGC) the Streptomyces sp. S3 genome was subjected to antiSMASH analysis (Blin et al., 2019) which predicted 22 BGCs, totalling 464,995bp (4.2% of the genome), including: 4 terpene (76,408bp); 1 polyketide synthase (PKS; 32,694bp); 1 polyketide synthase (PKS)/nonribosomal peptide synthetase (NRPS) hybrid (68,067bp); 6 NRPS among other (220,742bp) (Table S2). Out of these, only 5 showed similarities ≥ 0.7 to known BGC including those involved in the biosynthesis of the terpenes albaflavenone, geosmin, ectoine and hopene. A BGC for melanin production was also found, suggesting that melanin be responsible for the black pigmentation of Streptomyces sp. S3 when cultured in LB medium (Figure S2). Indeed, two putative tyrosinases containing secretion N-terminal signal peptides were identified in the annotated genome (not shown). A BGC involved in ectoine production was identified (Table S2). Ectoine is produced by halophilic and halotolerant microorganisms to prevent osmotic stress in highly saline environments (Reshetnikov et al., 2011). The production of ectoine may explain the halotolerant behavior of Streptomyces sp. S3.
To assess the potential for carbohydrate degradation the predicted proteome of Stremptomyces S3 were compared with the enzymes deposited at the CAZy database using blastP. This analysis revealed a total of 459 enzymes comprising: 247 Glycosyl Hydrolase; 99 Glycosyl transferases; 25 Polysaccharide lyases; 31 Carbohydrate esterases; 5 Auxiliary activity; 121 Carbohydrate-binding modules (Table S3). 214 (46.6%) of these putative CAZy enzymes were annotated as hypothetical proteins by Prokka while some enzymes were annotated as putative Chitinases, xylanases and cellulases (Table S3). These data provide additional support that Stremptomyces S3 is well suited for recalcitrant polysaccharide degradation and a potential source of biotechnologically relevant enzymes.