Reply on RC1

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> 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 <em>Synechococcus</em> 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 <em>Synechococcus</em> 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 (&lt;50 &micro;m) compared to crystals formed in exponential phase-EPS, which were less and larger (&gt; 50 &micro;m). 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).