Biogeochemical cycling of polyamines in a coastal marine environment

Abstract

Polyamines are a group of short-chain aliphatic compounds containing multiple primary or secondary amine groups. They are ubiquitous in aquatic environments with concentrations in the sub-nanomolar range, while concentrations in cells are at millimolar levels. They are commonly regarded as important osmolytes, synthesized or assimilated by osmotrophs, like bacterioplankton. They play a key role in cellular growth and synthesis of nucleic acids and proteins. However, few studies have examined the role of polyamines in oceanic N cycling. In this study, I hypothesized that polyamines contributed a significant amount of nitrogen to bacterial N production because of greater N: C ratio in polyamines than other labile organic nitrogen compounds. I determined turnover rates of three polyamines (putrescine, spermidine and spermine) in water samples using 3H-labeled compounds and measured their concentrations by HPLC in the South Atlantic Bight (SAB) and in salt marsh estuaries. The data showed that polyamines were rapidly assimilated by bacterioplankton, especially in samples from salt marshes and the inner-shelf of the SAB. However, low ambient concentrations of polyamines limited their contribution to bacterioplankton C and N demands compared to dissolved free amino acids that were used as controls. My study of polyamine metabolism in phytoplankton suggested that bacterial uptake of polyamines was limited by sources of polyamines. I found low concentrations (nmol L-1) of dissolved polyamine pools in media of phytoplankton cultures, similar to concentrations measured in the field. My data suggested that the composition and concentration of dissolved polyamines was a result of low diffusion from intracellular pools, selective release and uptake by healthy phytoplankton cells, as well as modifications by bacterioplankton uptake. Thaumarchaeota have been shown to oxidize polyamine nitrogen. This dissertation investigated the spatial and seasonal variability of abundances and activity of Thaumarchaeota in the SAB by quantitative PCR of Thaumarchaeota 16S rRNA and Archaea ammonia monooxygenase subunit A (amoA) genes and simulated in situ incubations with 15NH4Cl. I found a peak in Thaumarchaeota abundance (>1000-fold increase) in mid-summer at inshore and nearshore stations, but rapid ammonia oxidation was constrained to the inner-shelf at the mouth of the estuaries, probably driven by the substrate availability.