What lies on macroalgal surface: diversity of polysaccharide degraders in culturable epiphytic bacteria

Abstract

Macroalgal surface constitutes a peculiar ecological niche and an advantageous substratum for microorganisms able to degrade the wide diversity of algal glycans. The degrading enzymatic activities of macroalgal epiphytes are of paramount interest for the industrial by-product sector and biomass resource applications. We characterized the polysaccharide hydrolytic profile of bacterial isolates obtained from three macroalgal species: the red macroalgae Asparagopsis taxiformis and Sphaerococcus coronopifolius (Rhodophyceae) and the brown Halopteris scoparia (Phaeophyceae), sampled in South Portugal. Bacterial enrichment cultures supplemented with chlorinated aliphatic compounds, typically released by marine algae, were established using as inoculum the decaying biomass of the three macroalgae, obtaining a collection of 634 bacterial strains. Although collected from the same site and exposed to the same seawater seeding microbiota, macroalgal cultivable bacterial communities in terms of functional and phylogenetic diversity showed host specificity. Isolates were tested for the hydrolysis of starch, pectin, alginate and agar, exhibiting a different hydrolytic potential according to their host: A. taxiformis showed the highest percentage of active isolates (91%), followed by S. coronopifolius (54%) and H. scoparia (46%). Only 30% of the isolates were able to degrade starch, while the other polymers were degraded by 55-58% of the isolates. Interestingly, several isolates showed promiscuous capacities to hydrolyze more than one polysaccharide. The isolate functional fingerprint was statistically correlated to bacterial phylogeny, host species and enrichment medium. In conclusion, this work depicts macroalgae as holobionts with an associated microbiota of interest for blue biotechnologies, suggesting isolation strategies and bacterial targets for polysaccharidases' discovery.

Keywords: Bioprospecting; Blue biotechnology; Cultivable microbiota; Epibiota; Polysaccharidases; Seaweeds.

Fig. 1

Bacterial collections. A Taxonomic composition of bacterial isolates at phyla and genera levels for each macroalga: Asparagopsis taxiformis, Sphaerococcus coronopifolius (both Rhodophycea) and the brown Halopteris scoparia (Phaeophyceae). B Venn diagram showing the bacterial genera unique for each or shared among the three macroalgae

Fig. 2

Hydrolytic activities of bacteria isolated from the macroalgae Asparagopsis taxiformis, Halopteris scoparia and Sphaerococcus coronopifolius on starch, pectine, alginate and agar. A Active (dark colors) and not-active (light colors) isolates considering the 3 macroalgal species and the different hydrolytic activities. B Number of active isolates for each macroalgal species. Percentages represent the fraction of active isolates in each algal sub-collection. Colors in the percentage boxes are related to the percentage values: higher percentages are reported in green, intermediate percentages in yellow and lower percentages are in red. Bars represent the numbers of active isolates

Fig. 3

Heat map of the tested activities (amylase, pectinase, alginate lyase, agarase) for each isolate in the macroalgae: A Asparagopsis. taxiformis; B Halopteris scoparia; and C Sphaerococcus coronopifolius. Colors indicate the activity level calculated as the ratio between the degradation halo diameter and the isolate growth diameter

Fig. 4

Activity levels (reported as the ratio between the degradation halo diameter and the isolate growth diameter) of the bacterial isolates from the macroalgae Asparagopsis taxiformis, Halopteris scoparia and Sphaerococcus coronopifolius with the highest activities. Specifically, isolate CA2-8 showed the highest amylase activity (value 15), while 11 isolates showed the highest activity on pectin (value 8); isolate CA28 had the highest alginate lyase activity (value 8) and isolate CA35 showed the highest measured agarase activity (value 13)

Fig. 5

Distribution of hydrolytic activities (i.e., amylase, pectinase, alginate lyase and agarase) in the 3 macroalgal sub-collections (A), and considering the halogenated compounds (1,2-DBE vs. 1,2-DCA, B) and isolation media (MB vs. ONR7a, C). Statistical analysis (Student’s t-test) was conducted with Calc Statistical Function of Microsoft^R Office. In bold: p < 0.05