The role of culture conditions on the production of exopolymeric substances (EPS) by Synechococcus strain PCC7942 was investigated. Carbonate mineral precipitation in these EPS was assessed in forced precipitation experiments. Cultures were grown in HEPES-buffered medium and non-buffered medium. The pH of buffered medium remained constant at 7.5, but in non-buffered medium it increased to 9.5 within a day and leveled off at 10.5. The cell yield at harvest was twice as high in non-buffered medium than in buffered medium. High molecular weight (>10 kDa) and low molecular weight (3–10 kDa) fractions of EPS were obtained from both cultures. The cell-specific EPS production in buffered medium was twice as high as in non-buffered medium. EPS from non-buffered cultures contained more negatively charged macromolecules and more proteins than EPS from buffered cultures. The higher protein content at elevated pH may be due to the induction of carbon-concentrating mechanisms, necessary to perform photosynthetic carbon fixation in these conditions. Forced precipitation showed smaller calcite carbonate crystals in EPS from non-buffered medium and larger minerals in polymers from buffered medium. Vaterite formed only at low EPS concentrations. Experimental results are used to conceptually model the impact of pH on the potential of cyanobacterial blooms to produce minerals. We hypothesize that in freshwater systems, small crystal production may benefit the picoplankton by minimizing the mineral ballast, and thus prolonging the residence time in the photic zone, which might result in slow sinking rates.

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Abstract. Extracellular polymeric substances (EPS) are an important organic carbon reservoir in many pelagic and benthic environments. The production of EPS is intimately associated with the growth of phyto- and picoplankton. EPS plays a critical role in carbonate precipitation through the binding of cations and by acting as a nucleation site for minerals. Large-scale episodes of fine-grained calcium carbonate precipitation in the water column (whiting events) have been linked to cyanobacterial blooms, including of Synechococcus spp.,. The mechanisms that trigger these precipitation events are still debated. We pose that the cyanobacterial EPS, produced during exponential and stationary growth phases plays a critical role in the formation of whitings. The aim of this study was to investigate the production of EPS during a two-month cyanobacterial growth, mimicking a bloom. We further evaluated the potential role of EPS in carbonate precipitation. The production and properties of EPS produced at different Synechococcus spp. growth stages were investigated and carbonate mineral formation within these EPS matrices was determined in forced precipitation experiments. EPS produced during the early and late stationary phase contained a larger amount of negatively charged groups than present in EPS produced during the exponential phase. Consequently, a higher Ca2+ binding affinity of the stationary phase-EPS led to the formation of a larger amount of smaller carbonate minerals (<50 µm) compared to crystals formed in exponential phase-EPS, which were less and larger (> 50 µm). These findings were used to establish a conceptual model for picoplankton bloom-mediated CaCO3 precipitation that can explain the role of EPS in whitings (see graphical abstract).

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