Effects of mercuric chloride and protease inhibitors on degradation of particulate organic matter from the diatom Thalassiosira pseudonana

Abstract

Mercuric chloride (HgCl 2) and broad-spectrum protease inhibitors are often used to inhibit bacterial or enzymatic activity in environmental samples. In this study, we investigated their effects on degradation of particulate organic matter derived from a culture of the diatom, Thalassiosira pseudonana, and from sediment traps, with emphasis on compositional changes over one month. In control (untreated) samples, concentrations of particulate organic carbon (POC) and nitrogen (PN) and particulate amino acids (PAAs) decreased by 50%, dissolved combined amino acids (DCAAs) decreased by 34% and dissolved free amino acids (DFAAs) decreased by 64% relative to time zero. Production of γ-aminobutyric acid after one month indicated biological degradation. Fatty acids, but not neutral lipids, were also degraded substantially. Chlorophyll a (Chl- a) increased after one month by about 30%, suggesting the release of free Chl- a from Chl–protein complexes via decomplexation or degradation of proteins. Concentrations and compositions of samples treated with protease inhibitor (PI) were similar to untreated controls over time, indicating that PI alone did not stop bacterial activity. For samples treated with HgCl 2, 10–20% of the initial organic carbon and nitrogen, PAAs and lipids were lost. DCAAs and DFAAs increased by about the same amount as PAAs decreased, suggesting that dissolution was a major mechanism for loss of particulate organic compounds. Particulate arginine, tyrosine and threonine were slightly enriched, and 16:3, 18:3 and 18:4 fatty acids depleted, in Hg-treated samples. Moreover, Chl- a decreased by a factor of 4 after one month, compared to controls. Simultaneously, a significant amount of Chl- a O-allomer was produced. Although Chl- a allomerization in oceanic sediment trap samples treated with HgCl 2 was not as great as in Hg-treated diatom cultures, we recommend summing Chl- a and the allomer peak areas to avoid underestimating Chl- a concentrations in natural samples, or reporting the allomer separately. HgCl 2 is an excellent agent for preventing microbial decomposition of marine particulate materials in both field and experimental use, but the changes in composition it causes must be considered in interpreting analytical results.