Abstract
Production, transformation, and degradation are the principal components of the cycling of dissolved organic matter (DOM) in marine systems. Heterotrophic Bacteria (and Archaea) play a large part in this cycling via enzymatic decomposition and intracellular transformations of organic material to inorganic carbon (C), nitrogen (N), and phosphorus (P). The rate and magnitude of inorganic nutrient regeneration from DOM is related to the elemental composition and lability of DOM substrates as well as the nutritional needs of the mediating organisms. While many previous efforts have focused on C and N cycling of DOM, less is known in regards to the controls of organic P utilization and remineralization by natural populations of bacteria. In order to constrain the relative time scales and degradation of select dissolved organic P (DOP) compounds we have conducted a series of experiments focused on (1) assessment of the short-term lability of a range of DOP compounds, (2) characterization of labile DOP remineralization rates, and (3) examination of temperature sensitivities of labile DOP remineralization for varying bacterial populations. Results reinforce previous findings of monoester and polyphosphate lability and the relative recalcitrance of a model phosphonate: 2-aminoethylphosphonate. High resolution time-series of P-monoester remineralization indicates decay constants on the order of 0.67–7.04 day−1 for bacterial populations isolated from coastal and open ocean surface waters. The variability of these rates is predictably related to incubation temperature and initial concentrations of heterotrophic bacteria. Additional controls on DOP hydrolysis included seasonal shifts in bacterial populations and the physiological state of bacteria at the initiation of DOP addition experiments. Composite results indicate that bacterial hydrolysis of P-monoesters exceeds bacterial P demand and thus DOP remineralization efficiency may control P availability to autotrophs.
Highlights
The marine phosphorus (P) cycle is characterized by tight coupling between the uptake and decomposition of inorganic and organic P, respectively
Substantial remineralization of G6P (93 ± 5% decomposition of added dissolved organic P (DOP)) was observed in COASTAL incubations and minimal turnover of P3 was seen in North Pacific Subtropical Gyre (NPSG) incubations (Table 1). 2-AEP was not appreciably remineralized by either NPSG or COASTAL populations (Table 1; Figure 1)
Time lags between the addition of DOP and observation of remineralization in all cases was less than 24 h (Figure 1)
Summary
The marine phosphorus (P) cycle is characterized by tight coupling between the uptake and decomposition of inorganic and organic P, respectively. In a landmark work, Kolowith et al (2001) used 31P nuclear magnetic resonance (NMR) spectrometry to show that the high molecular weight (HMW) fraction of dissolved organic matter (DOM) is dominated by P-linked esters (75%) and phosphonates (25%) occurring in unchanging proportions throughout the water column. Both components are known to have a biological origin: P-esters occur in both nucleic acids and membrane phospholipids while phosphonates are found as peptide, glycan, or lipid conjugates, typically in cell membranes (Hilderbrand, 1983; Quinn et al, 2007). The relative absence of phosphonates in POM and their abundance in HMW DOM coupled with the observation of the conserved proportionality of P-esters to phosphonates throughout the water column suggests that phosphonates are less biologically reactive and become selectively www.frontiersin.org
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