Colloidal size distribution of humic- and protein-like fluorescent organic matter in the northern Gulf of Mexico

Abstract

The colloidal size distributions (0.5–400nm) of humic-like and protein-like fluorescent and UV-absorbing marine dissolved organic matter (DOM) have been determined using flow field-flow fractionation (FlFFF) coupled to UV-absorbance and fluorescence detectors. Surface seawater samples were taken monthly between November 2008 and September 2009, from eight stations along a transect across the Mississippi Sound and Mississippi Bight in the northern Gulf of Mexico. DOM was also isolated from cultures of two phytoplankton species. The concentration of dissolved organic carbon (DOC) in the study area was largely controlled by river inputs, but the specific UV-absorbance and fluorescence index indicated that the importance of marine DOM increased from nearshore to offshore stations. In the seawater samples, only a small fraction (3–16%) of the ‘humic-like’ fluorescent DOM (Ex/Em wavelengths at 240/440nm, 310/400nm, 320/440nm and 350/450nm) was found in the colloidal size range (>0.5nm), mainly associated with small colloids (mean hydrodynamic diameter 2–3nm). For the ‘protein-like fluorescent DOM’ (Ex/Em at 275/305nm and 275/340nm) a larger fraction (21–100%) was found in the colloidal size range, including both small (2–3nm) and larger (mean hydrodynamic diameter 6–7nm, ~75nm, and >400nm) colloids. A culture of the diatom Chaetoceros muelleri produced mainly humic-like fluorescent DOM, while the fluorescent DOM produced by the dinoflagellate Prorocentrum sp. was mostly protein-like. In both plankton cultures, the protein-like DOM was largely associated with colloids around 6nm, 12–14nm and larger, while only a small fraction of humic-like DOM was found in the colloidal size range. After five months of starving and aging of the cultures, both the humic-like and protein-like DOM became increasingly associated with small colloids having a mean hydrodynamic diameter of 2–4nm. We hypothesize that the larger (6–7nm, 12–14nm, ~75nm and >400nm) protein-like colloids in the seawater samples and phytoplankton cultures were freshly produced by phytoplankton or bacteria. The smaller (2–4nm) colloids in the aging phytoplankton cultures probably represented phytoplankton-derived organic material that had been transformed by microbial degradation, while the small (2–3nm) humic-like colloids in the seawater samples were most likely dominated by fulvic acids from terrestrial sources.