Distribution, Sources, and Sinks of Cyanate in the Coastal North Atlantic Ocean

On the basis of reverse genomics and growth of cultured populations, it has been hypothesized that cyanate is utilized as a nitrogen source by ubiquitous groups of marine phytoplankton. Recently, a nanomolar method was developed to measure cyanate concentrations in marine and estuarine waters. Here we report the first measurements of cyanate distributions, biological utilization, and production from the coastal North Atlantic Ocean. Cyanate concentrations were highest below the chlorophyll maximum at many stations but were high throughout the water column on the shallow Georges Bank where chlorophyll concentrations were especially high down to the bottom, suggesting production by organic matter degradation or release by phytoplankton. Here we demonstrate that cyanate is produced in senescent algal cultures and through photochemical reactions at rates comparable to rates of production of other labile nitrogen compounds. Cyanate uptake accounted for up to 10% of total N uptake at an oligotrophic mid-Atlantic Bight station. Our results suggest that cyanate may be an important but hitherto overlooked component of the marine nitrogen cycle

Insights into the Photoproduction Sites of Hydroxyl Radicals by Dissolved Organic Matter in Natural Waters

The hydroxyl radical (^•OH) is the most reactive oxidant produced in natural waters. Photoproduction by chromophoric dissolved organic matter (CDOM) is one of its main sources, but the structures responsible for this production remain unknown. Here, a series of substituted phenol model compounds are examined to test whether these structures could act as a source of ^•OH. We find that many of these compounds do produce ^•OH with quantum yields (Φ) ranging from ∼10^–4 to ∼10^–2. In particular, two compounds that have hydroxy groups and carboxyl groups in a para relationship (4-hydroxybenzoic acid and 2,4-dihydroxybenzoic acid) exhibit relatively high Φ values, ∼10^–2. For 2,4-dihydroxybenzoic acid, the formation of ^•OH was confirmed through the use of competition kinetics and reaction with methane. We conclude that these types of structures, which may derive from polyphenolic source materials such as lignins, tannins, and humic substances, could be an important source of ^•OH in natural waters

Production of Black Carbon-like and Aliphatic Molecules from Terrestrial Dissolved Organic Matter in the Presence of Sunlight and Iron

Photochemical processing of dissolved organic matter (DOM) in natural waters can alter its composition and structure, supply particulate organic matter (POM) to sediments, and deliver modified terrestrial DOM to the ocean. Our studies show that terrestrial DOM exposed to simulated sunlight is altered to produce POM with a markedly different molecular composition enriched with newly formed aliphatic and condensed aromatic molecules. This process is closely tied to the chemistry of iron, which primarily exists as dissolved Fe­(II) and Fe­(III)–organic complexes in initial DOM and photochemically matures to Fe­(III) oxyhydroxides before coprecipitating out with POM. The newly formed condensed aromatic compounds resemble black carbon, which until now was thought to be produced by only combustion. These new molecules contribute a pool of Fe-rich, aliphatic, and black carbon-like organic matter to sediments as the terrestrial DOM is transported through rivers. We estimate that the annual global flux of this photoproduced black carbon, most of which may be preserved in sediments, is nearly equivalent to the estimated flux of dissolved black carbon to the ocean from all other sources