Spectroscopic detection of a ubiquitous dissolved pigment degradation product in subsurface waters of the global ocean

[摘要] Measurements of light absorption by chromophoric dissolved organic matter(CDOM) from subsurface waters of the tropical Atlantic and Pacific Oceansshowed a distinct absorption shoulder at 410–415 nm. This indicates anunderlying absorption of a pigment whose occurrence is partly correlatedwith the apparent oxygen utilization (AOU) but also found in the deepchlorophyll maximum. A similar absorption maximum at ~415 nm was alsofound in the particulate fraction of samples taken below the surface mixinglayer and is usually attributed to absorption by respiratory pigments ofheterotrophic unicellular organisms. In our study, fluorescence measurementsof pre-concentrated dissolved organic matter (DOM) samples from 200–6000 m confirmed a previous study suggesting that the absorption at ~415 nm was related tofluorescence at 650 nm in the oxygen minimum zone. The absorptioncharacteristics of this fluorophore was examined by fluorescenceemission/excitation analysis and showed a clear excitation maximum at 415 nmthat could be linked to the absorption shoulder in the CDOM spectra. Thespectral characteristics of the substance found in the dissolved andparticulate fraction did not match with those of chlorophyll a degradationproducts (as found in a sample from the sea surface) but can be explained bythe occurrence of porphyrin pigments from either heterotrophs or autotrophs.Combining the observations of the fluorescence and the 415-nm absorptionshoulder suggests that there are high concentrations of a pigmentdegradation product in subsurface DOM of all major oceans. Mostpronouncedly we found this signal in the deep chlorophyll maximum and theoxygen minimum zone of tropical regions. The origin, chemical nature,turnover rate, and fate of this molecule is so far unknown.

[学科分类] 地球化学与岩石