Absorption and fluorescence of dissolved organic matter in submarine hydrothermal vents off NE Taiwan

The role of hydrothermal vents as either a source or a sink for chromophoric and fluorescent dissolved organic matter (CDOM and FDOM) in the oceans is unknown, since DOM absorption and fluorescence have not been reported for submarine hydrothermal vents. Water samples were collected from two shallow submarine hydrothermal vents off NE Taiwan, the white vent and the yellow vent, during two cruises in August, 2010. Absorption and excitation-emission-matrix fluorescence spectroscopy were used to characterize the optical properties of DOM from such extremely special environments. The absorption coefficients at wavelength 300 nm (a300) were much higher at the white vent mouth and 1 m below it (2.52 ± 0.88 m-1) than in the background (0.34 ± 0.12 m-1). This indicated that the white vent was a source of CDOM for seawater. Three fluorescent components were identified using parallel factor analysis: humic-like C1, tyrosine-like C3, and C2 as a combination of tryptophan-like and marine humic-like components. Both C1 and C3 (but not C2) had their highest fluorescence intensity at the white vent mouth and 1 m below it, suggesting the role of the vent as a source for both humic-like and tyrosine-like DOM. Samples from the yellow vent mouth also had a higher a300 than the ambient seawater in our first cruise, but had fluorescence intensities of C(1–3) similar to the ambient seawater. Overall, the low humification index (HIX: 1.40 ± 0.30) and the high autochthonous index (BIX: 1.27 ± 0.63) indicated that the DOM likely had low humic contents and was mainly autochthonous of biological or bacterial origin in the study area. A biplot of HIX and BIX showed that DOM from the hydrothermal vents had a characteristic similar to terrestrial cave and spring waters, but was distinct from isolated humics.