Chromophoric signatures of microbial by-products in the dark ocean

Detailed examination of the absorption spectra from dark ocean samples allowed us to identify and deconvolve two distinct chromophores centered at 302 nm (UV) and 415 nm (visible) from the exponential decay curve characteristic of humic substances. The UV chromophore was ubiquitous in intermediate and deep waters, and it has been proposed as the secondary absorption peak of nitrate. The visible chromophore was prominent at the central and intermediate water masses of the North Pacific, and it has been proposed as cytochrome c. Subtraction of themodeled absorption spectra of the two chromophores from themeasured absorption spectrumof the samples leads to a spectral slopeoverestimationby 13.3 ± 6.0% for S275–295 and 14.8 ± 10.6% for S350–400. To only consider the chromophoric fraction of DOM, the absorption spectra of nitrate should be subtracted in samples with a [NO3 ]:a302 ratio> 70μMm.