No abstract

Marine dissolved organic matter (DOM) is a key source of carbon and nutrients to microbial life in the oceans, but rapid biological utilization of labile DOM confounds its compositional characterization. In order to characterize potentially bioavailable DOM produced by phytoplankton, DOM from axenic cultures of Thalassiosira pseudonana cultivated in phosphorus (P) replete and low P conditions was extracted using high‐recovery electrodialysis (ED) techniques, which resulted in an average dissolved organic carbon (DOC) recovery of 76% ± 7% from all cultures. Low P concentrations resulted in greater cell‐normalized production of DOC relative to P replete culture controls at the same growth phase. Despite the different nutrient conditions, DOC composition and DOM molar ratios of carbon to nitrogen (C : N) were similar in all cultures. In contrast, low P concentrations influenced DOM molar carbon to phosphorus (C : P) ratios and dissolved organic phosphorus (DOP) composition. Under P replete and low P conditions, DOM C : P ratios were 130 (± 22) and 2446 (± 519), respectively. 31P Nuclear Magnetic Resonance (NMR) spectroscopy identified P esters (> 90% of DOP) as the dominant P species in DOM produced under P replete conditions, with small or negligible contributions from phosphonates or glycerol P and polyphosphates. However, based on direct fluorometric analysis, DOP from low P cultures was greater than 8 times enriched in dissolved polyphosphate compared to DOP from replete cultures, which is consistent with the growing evidence that polyphosphate is a dynamic component of total P in low P ocean regions.

The composition of dissolved organic phosphorus (DOP), which directly impacts its biological and chemical reactivity, remains poorly constrained due to methodological and operational challenges. A series of compound‐specific isolation experiments and bulk estuarine seawater isolations were conducted to assess the ability of coupled electrodialysis (ED) and reverse osmosis (RO) to isolate DOP from seawater for molecular characterization using solution 31P‐NMR analysis. These isolations were carried out using a recently developed small‐volume (24 L) ED/RO system. Compound‐specific isolation experiments (n = 7) averaged 64.1% ± 9.2% for final DOP recovery, while recoveries for estuarine seawater (n = 7) were on average 83.8% ± 10.7%. Nearly complete mass balance of DOP was observed for the compound‐specific isolations (88.1% ± 11.2%) with ED responsible for the majority of DOP loss (16% ± 5%). Isolation recovery was impacted by compound molecular weight rather than charge‐to‐size ratio. Isolation bias between bulk dissolved organic carbon and DOP was negligible. Solution 31P‐NMR confirmed that ED/RO isolation does not impact molecular integrity or DOP composition. In addition, solution 31P‐NMR accurately estimated several components (phosphonomethyl‐glycine, D‐glucose‐6‐phosphate, L‐α‐phosphatidylcholine) of an artificial complex DOP matrix. However, we suggest further refinements for ED/RO operation and significant caveats remain for 31P‐NMR sample preparation and the interpretation of 31P‐NMR spectra. Hydrolysis from NaOH dissolution was found to impact the ability to accurately resolve diester composition (ca. 40% underestimation) and discrimination between terminal and central phosphoanhydride atoms. Nonetheless, our results describe a viable option for the isolation and molecular characterization of marine DOP.