Attached bacterial community dynamics with changes in core phytoplankton species based on the phycosphere concept

Abstract

Variations in the attached bacterial communities according to changes in core phytoplankton species were analyzed in samples collected from the southern coastal waters of the Jangmok Bay, South Korea, from November 2016 to June 2017 (total investigation periods: 210 days). Samples were examined under microscopes (light microscope and Scanning electron microscope) and analyzed using metagenomic amplicon sequencing (16S rDNA V3-4 regions) methods. Seven core phytoplankton species were focused on during the investigation period. When the Dinophyceae Akashiwo sanguinea bloomed, Rickettsia rhipicephali (Alphaproteobacteria) was dominant and showed a significant correlation with A. sanguinea (r = 0.92, p < 0.001). An increase in Cryptomonas sp. (nanosized Cryptophyceae) was associated with ‘Candidatus Limnoluna rubra’ (Actinomycetia) (r = 0.50, p < 0.05). When the Bacillariophyceae Skeletonema marinoi-dohrnii complex and Pseudo-nitzschia delicatissima were dominant, large populations of Gilvibacter sediminis (Flavobacteriia) and Lewinella nigricans (Saprospiria) were observed, but there was no significant correlation between the dominant phytoplankton and bacteria. Moreover, when the three diatom species, Chaetoceros curvisetus, Leptocylindrus danicus, and Thalassiosira tenera, were found together, the attached bacteria Planktomarina temperata (Alphaproteobacteria), Crocinitomix catalasitica, and Lishizhenia caseinilytica (Flavobacteriia) showed a high relative abundance. In particular, L. danicus showed a strong correlation with C. catalasitica (r = 0.92, p < 0.001). Therefore, we speculate that the advantages of attached bacteria living in the phycosphere are protection from outer environments and the opportunity to utilize the extracellular polysaccharide secreted by phytoplankton as an energy source. Thus, attached bacteria may contribute to the biochemical cycling between seawater and phytoplankton, and have a competitive relationship with phytoplankton for nutrients from seawater.