Dynamics of Heterotrophic Bacterial Assemblages within Synechococcus Cultures

Abstract

Interactions between photoautotrophic and heterotrophic microorganisms are central to the marine microbial ecosystem. Lab cultures of one of the dominant marine photoautotrophs, <i>Synechococcus</i>, have historically been difficult to render axenic, presumably because these bacteria depend upon other organisms to grow under these conditions. These tight associations between <i>Synechococcus</i> and heterotrophic bacteria represent a good relevant system to study interspecies interactions. Ten individual <i>Synechococcus</i> strains, isolated from eutrophic and oligotrophic waters, were chosen for investigation. Four to six dominant associated heterotrophic bacteria were detected in the liquid cultures of each <i>Synechococcus</i> isolate, comprising members of the <i>Cytophaga</i>-<i>Flavobacteria</i>-<i>Bacteroides</i> (CFB) group (mainly from <i>Flavobacteriales</i> and <i>Cytophagales</i>), <i>Alphaproteobacteria</i> (mainly from the <i>Roseobacter</i> clade), <i>Gammaproteobacteria</i> (mainly from the <i>Alteromonadales</i> and <i>Pseudomonadales</i>), and <i>Actinobacteria</i> The presence of the CFB group, <i>Gammaproteobacteria</i>, and <i>Actinobacteria</i> showed clear geographic patterns related to the isolation environments of the <i>Synechococcus</i> bacteria. An investigation of the population dynamics within a growing culture (XM-24) of one of the isolates, including an evaluation of the proportions of cells that were free-living versus aggregated/attached, revealed interesting patterns for different bacterial groups. In <i>Synechococcus</i> sp. strain XM-24 culture, flavobacteria, which was the most abundant group throughout the culture period, tended to be aggregated or attached to the <i>Synechococcus</i> cells, whereas the actinobacteria demonstrated a free-living lifestyle, and roseobacters displayed different patterns depending on the culture growth phase. Factors contributing to these succession patterns for the heterotrophs likely include interactions among the culture community members, their relative abilities to utilize different compounds produced by <i>Synechococcus</i> cells and changes in the compounds released as culture growth proceeds, and their responses to other changes in the environmental conditions throughout the culture period.<b>IMPORTANCE</b> Marine microbes exist within an interactive ecological network, and studying their interactions is an important part of understanding their roles in global biogeochemical cycling and the determinants of microbial diversity. In this study, the dynamic relationships between <i>Synechococcus</i> spp. and their associated heterotrophic bacteria were investigated. <i>Synechococcus</i>-associated heterotrophic bacteria had similar geographic distribution patterns as their "host" and displayed different lifestyles (free-living versus attached/aggregated) according to the <i>Synechococcus</i> culture growth phases. Combined organic carbon composition and bacterial lifestyle data indicated a potential for succession in carbon utilization patterns by the dominant associated heterotrophic bacteria. Comprehending the interactions between photoautotrophs and heterotrophs and the patterns of organic carbon excretion and utilization is critical to understanding their roles in oceanic biogeochemical cycling.