Acid-base properties of Synechococcus-derived organic matter

Abstract

Marine cyanobacteria play an important role in trace metal cycling in the oceans. However, the influence of bacterial detritus following cell lysis – that includes both particulate organic matter (POM) and dissolved organic matter (DOM) – is less understood. Previous investigations have shown that the ratio of bacterial detritus to living cells increases significantly with ocean depth, indicating that the former plays a central role in trace metal cycling and eventual sequestration in deep marine sediments. Thus, in this study we produced detritus from the mechanical lysis of the common marine cyanobacterium Synechococcus sp. PCC 7002 (referred to as cyPOM and cyDOM), and then measured their buffering capacity through proton adsorption and release via potentiometric acid-base titrations. Both cyPOM and cyDOM were best modelled using a 2-site non-electrostatic protonation model. The cyPOM fraction possessed ligands with pKa values of 5.78 (±0.07) and 9.01 (±0.29) with corresponding site concentrations of 41.8 (±8.15) µmol g−1 and 41.2 (±5.28) µmol g−1, while cyDOM had pKa values of 4.89 (±0.22) and 6.80 (±0.22) and corresponding site concentrations of 42.8 (±14.6) µmol g−1 and 37.3 (±16.5) µmol g−1. As a comparison, intact Synechococcus cells were previously reported to be best modelled by invoking 3-sites with overall ligand densities more than double that of cyPOM and cyDOM. Trace metal (Cd, Co, Cu, Ni, and Zn) adsorption experiments indicate that cyPOM has a lower affinity towards trace metals than intact Synechococcus cells, evidenced by lower trace metals adsorption per gram of dry mass and therefore less trace metal sequestration potential. This implies that intact Synechococcus cells are chiefly responsible for trace metal sequestration, even at high cyPOM proportions (10:1 – cyPOM:Synechococcus) that are more representative of POM that reaches the seafloor. The cyDOM-trace metal association could not be determined experimentally; thus we applied existing thermodynamic data from modern marine DOM studies acquired via potentiometric titrations and metal-binding constants. Unlike cyPOM, metal speciation is almost entirely accounted for in the dissolved fraction (>99.9%). These results further the view that DOM is the most important microbial fraction in controlling metals speciation in the water column.