Bacteria-mediated aggregation of the marine phytoplankton Thalassiosira weissflogii and Nannochloropsis oceanica

Nhan-An T Tran,Bojan Tamburic,Christian R Evenhuis,Justin R Seymour

doi:10.1007/s10811-020-02252-8

Abstract

The ecological relationships between heterotrophic bacteria and marine phytoplankton are complex and multifaceted, and in some instances include the bacteria-mediated aggregation of phytoplankton cells. It is not known to what extent bacteria stimulate aggregation of marine phytoplankton, the variability in aggregation capacity across different bacterial taxa or the potential role of algogenic exopolymers in this process. Here we screened twenty bacterial isolates, spanning nine orders, for their capacity to stimulate aggregation of two marine phytoplankters, Thalassiosira weissflogii and Nannochloropsis oceanica. In addition to phytoplankton aggregation efficiency, the production of exopolymers was measured using Alcian Blue. Bacterial isolates from the Rhodobacterales, Flavobacteriales and Sphingomonadales orders stimulated the highest levels of cell aggregation in phytoplankton cultures. When co-cultured with bacteria, exopolymer concentration accounted for 34.1% of the aggregation observed in T. weissflogii and 27.7% of the aggregation observed in N. oceanica. Bacteria-mediated aggregation of phytoplankton has potentially important implications for mediating vertical carbon flux in the ocean and in extracting phytoplankton cells from suspension for biotechnological applications.

Highlights

The ecological interactions between marine phytoplankton and bacteria can vary from mutualistic to parasitic (AminElectronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.One of the ways that bacteria can influence phytoplankton is by increasing cell stickiness and causing aggregation (Decho 1990; Heissenberger and Herndl 1994; Grossart et al 2006b)
Twenty different species of bacteria were screened for their ability to stimulate aggregation of the marine phytoplankton T. weissflogii and N. oceanica
The degree of aggregation varied significantly (p < 0.05) across bacterial isolates, and the Sphingomonadales and Erythrobacter citreus caused the highest levels of aggregation of both T. weissflogii (Fig. 1a) and N. oceanica cells (Fig. 1b)

Summary

Introduction

Physical attachment of bacteria to phytoplankton cells has been shown to influence aggregation in several species (Rodolfi et al 2003; Grossart et al 2006b; Gärdes et al 2010), which has potential significance for biogeochemical cycling in the ocean, because the aggregation of phytoplankton and subsequent sinking of organic matter in the form of marine snow increases the flux of organic carbon from surface waters to the deep ocean (Alldredge and Gotschalk 1989; Grossart and Ploug 2001; Grossart et al 2006b), meaning these microscale interactions can have ecosystem-level implications (Amin et al 2012, 2015; Landa et al 2016). Given the emerging knowledge of phytoplankton-bacteria interactions within natural environments, a logical extension is to take advantage of these processes for biotechnological applications

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