Significant decomposition of riverine humic-rich DOC by marine but not estuarine bacteria assessed in sequential chemostat experiments

Abstract

Humic substances (HS) are the most abundant natural organic compounds in aquatic and terrestrial environments. However, the bacterial degradation of HS in the estuarine salinity gradient and in coastal regions as a sink for HS, entering the open sea, is not well understood. Therefore, we studied the bacterial degradation of humic-rich dissolved organic carbon (DOC) at increasing salinities between 1 and 30 in an estuary of the southern North Sea (Weser). Three-stage chemostats, inoculated with natural brackish (salinity 5 and 15) and coastal marine (salinity 30) bacterial communities fed 0.1 µm filtered natural humic-rich freshwater adjusted to the respective salinity, were run for up to 51 d at dilution rates of 0.1 and 0.15 d -1. DOC concentrations, bacterial numbers, and production were assessed in each stage over the incubation time. In addition, the fulvic acid (FA), humic acid (HA), and hydrophilic acid (HPA) fractions of the HS were determined by XAD fractionation. At a salinity of 30, the humic-rich DOC was decomposed to more than 60%, whereas in the other 2 experiments at salinities of 15 and 5, no detectable decomposition occurred. At the highest salinity condition, all 3 HS fractions were reduced substantially, whereas at a salinity of 5, only the HA and HPA fractions decreased, and at 15, only the HA fraction decreased. In the latter experiments, concentrations of the FA and HPA fractions in some chemostat stages increased despite unchanged DOC concentration. Our study provides evidence that the main bacterial decomposition, i.e. conversion into bacterial biomass and remineralization, of humic-rich DOC occurs in the polyhaline estuarine region and that at lower salinities, only some transformation of the humic fractions takes place, modifying the adsorptive properties of the HS, but no bacterial DOC decomposition occurs. The sequential chemostats were a useful approach to study the complex process of bacterial HS decomposition. KEY WORDS: Humic substances · DOC · Bacteria · Chemostat · Estuary · Marine Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 53: [151][152][153][154][155][156][157][158][159][160] 2008 the lowest molecular weight, i.e. < 600 Da. Riverine inputs of dissolved organic carbon (DOC), of which HS comprise a substantial fraction, constitute the major flux of such compounds into the coastal zone.Flocculation at increasing salinity is one important process of DOC removal during the estuarine passage (Sholkovitz 1976, Sholkovitz et al. 1978, Kerner et al. 2003, but photochemical and bacterial degradation also contribute to this removal (see below). Present estimates imply that terrestrial and thus humic DOC, after mixing with seawater, is degraded rapidly in the coastal zone (Hedges et al. 1997, Cauwet 2002, Mannino & Harvey 2004. Hence, estuaries, the transition zone between freshwater and coastal marine environments, presumably are of prime importance for the degradation and thus for losses of riverine and terrestrial DOC.Deg...

Bacterial community composition and colored dissolved organic matter in a coastal upwelling ecosystem

“…By comparison, little is known about the patterns of utilization of allochthonous carbon sources, such as those derived from continental runoff, in marine environments. A recent study by Kisand et al (2008) suggests that bacterial decomposition of riverine humic-rich DOM might be significantly higher in marine than in estuarine waters.…”

“…By comparison, little is known about the patterns of utilization of allochthonous carbon sources, such as those derived from continental runoff, in marine environments. A recent study by Kisand et al (2008) suggests that bacterial decomposition of riverine humic-rich DOM might be significantly higher in marine than in estuarine waters.The northwest Iberian coastal transition zone is an area affected by a marked seasonal cycle of coastal winds, which divides the annual cycle in an upwelling season (March to September) with short relaxation intervals that enhance productivity, and a downwelling season (from October to March), characterized by low phytoplankton biomass and primary production (Álvarez-Salgado et al 2003). Continental runoff is most intense during this low productivity period, potentially transporting large amounts of allochthonous carbon into this coastal zone.…”

“…The relative abundance of Betaproteobacteria in our study was not correlated to salinity, which was higher overall than 35.5, indicating an important dilution of the freshwater inputs along the coastal transition zone. Kisand et al (2008) demonstrated that the bacterial degradation of terrestrial humic substances might preferentially occur at high salinities (in polyhaline estuarine regions and coastal adjacent seas), with only a minor fraction of riverine DOC degradation occurring during the estuarine mixing. …”

Effects of stratification depth and dissolved organic matter on brackish bacterioplankton communities

“…Although bacterial production is often heavily subsidized by allochthonous DOM, the dominant fraction of this largely humic material is biologically recalcitrant, with only about 14 to 19% readily available for bacterial use (Søndergaard & Middelboe 1995). However, the degradation of riverine humic substances may accelerate under estuarine conditions (Kisand et al 2008), and since the inputs of allochthonous material is substantial for many coastal areas, including the Baltic Sea, increased allochthonous inputs might shift the entire system towards net heterotrophy, thereby uncoupling microbial growth from phytoplankton primary production (Kuparinen et al 1996).…”

Composition of humic acid-degrading estuarine and marine bacterial communities

“…Surprisingly, only scarce information is available on the relative partitioning of each of the three fractions to total humic substances in freshwater as well as marine systems. Kisand et al (2008) reported that HA constitute~30% of total humic substances in a humicrich stream, and Gebhardt (2005) found that HA constitute 16-37% of total humic substances in a drainage channel discharging into the Wadden Sea, southern North Sea. Humic substances in total represent 50-80% of dissolved organic matter (DOM) in freshwaters (Thurman, 1985;McKnight & Aiken, 1998), but only a small fraction (0.7-2.4%) of DOM in the ocean (Opsahl & Benner, 1997).…”