Remineralization of phytoplankton-derived organic matter by natural populations of heterotrophic bacteria

Abstract

The relative lability, elemental stoichiometry, and remineralization rates of various particulate organic matter (POM) substrates by natural heterotrophic marine microorganisms were investigated. POM was harvested from laboratory cultures of a marine diazotroph (Trichodesmium IMS101), a cosmopolitan diatom (Thalassiosira weissflogii), a common marine cyanobacteria (Prochlorococcus MED4), and from surface waters off the Oregon coast. These POM resources were used as inoculants in a field experiment conducted at the Hawaii Ocean Time-series Station ALOHA in the North Pacific Subtropical Gyre. POM from these various sources was added to seawater collected from below the surface mixed layer, incubated in the dark, and remineralization rates were quantified via high-frequency measurement of soluble phosphorus (P) and nitrogen (N) concentrations over a 6-d period. Rapid solubilization and near complete remineralization of particulate P (PP) occurred in all treatments where cultured POM was used, with lesser relative mobilization of P from a ‘natural’ POM sample isolated from surface seawater off the Oregon coast. Soluble P pools, likely consisting of surface-adsorbed inorganic P and inorganic P liberated from cells during harvesting of biomass accounted for 28% of natural PP pools and 80±32% of cultured PP. 31P nuclear magnetic resonance (NMR) confirmed that PP was predominately present as orthophosphate in all POM types. By the end of the incubation period, all added P from cultured material had been converted to dissolved inorganic P. This finding may be a caveat of our utilization of laboratory cultures and natural POM which has been exposed to high inorganic P concentrations (0.8–5.0μmolL−1), albeit it is consistent with previous reports of significant contributions of surface-adsorbed P to total particulate P. In contrast, over the course of these experiments, only 37–40% of added N had been remineralized to ammonium (NH4+). In general, N remineralization rates of cultured material increased with the amount of N added (per gram of dry material). The net yield of bacterial cells was also positively correlated to the initial amount of C and N added. Most notably, when corrected for non-biological turnover (i.e. removal of soluble pools), the N:P remineralization ratio of cultured material (8.5±1.3) was independent of the N:P of added organic material (5–23).