Carrageenophyte-attached and planktonic bacterial communities in two distinct bays of Vietnam: Eutrophication indicators and insights on ice-ice disease

Abstract

The composition of the bacterial community of carrageenophyte-attached and planktonic bacteria was studied in two bays of Vietnam with contrasting anthropogenic inputs to assess their role as ecological indicators. Clear differences (~73% dissimilarity) between carrageenophyte-attached bacteria and bacterioplankton were detected in terms of genus composition: mainly Agaribacter, Ruegeria, Alteromonas, the Pir4 lineage and Vibrio for the carrageenophytes and Candidatus Actinomarina, HIMB 11, NS groups and SAR clades for the bacterioplankton. The copiotrophic nature, potential for complex-polymer degradation, and ability to form and defend biofilms were common features inferred for the carrageenophyte-attached microbiome. Significant differences between the bays were detected in the concentration of most inorganic nutrients. More eutrophic conditions and presumptive wastewater pollution in Cam Ranh (CR) bay were primarily indicated by the dominance of Rubripirellula, Leptobacterium, Hypnocyclicus and Porphyrobacter and their correlations with phosphate. In terms of bacterioplankton, the influence of intensive aquaculture in CR bay was suggested by the dominance of the NS5 and NS4 marine groups, the SUP05 cluster, Flavobacteriaceae unclassified and SAR 11 clade III as well as their strong correlations with ammonium and phosphate. The link between silicate and other inorganic nutrients suggests freshwater input in CR bay. Arenicellaceae unclassified and Formosa were also potential indicators of eutrophication. Operational taxonomic units (OTUs) of Marinagarivorans, Cobetia, Vibrio, Alteromonas and Pseudoalteromonas were typical of the carrageenophytes showing ice-ice disease symptoms. Vibrio and Alteromonas were also common among healthy macroalgae, and differences at the OTU level suggested potential succession of species from the healthy to the diseased state. The probable beneficial roles of some bacteria, such as Ruegeria, Cutibacterium and unidentified members of the family Rhizobiaceae, were discussed. This study provides pioneering insights into the bacterial community composition of carrageenophytes and highlights their ecological value as strong indicators of the sources of organic matter, anthropogenic impacts and health status of marine systems.