Plant pigments as biomarkers of high‐molecular‐weight dissolved organic carbon

Abstract

We used plant pigments as tracers of high‐ molecular‐weight dissolved organic C (HMW DOC) (<0.2 µm and > 1,000 Da). Water samples were collected from four stations along a transect on cruises in June 1992 and January 1993 in the Gulf of Mexico. Samples were also collected from three stations on the continental shelf of Cape Hatteras for comparison. Chlorophylls a and b were at detectable levels in HMW DOC; concentrations of the carotenoids zeaxanthin and fucoxanthin in HMW DOC indicated that cyanobacteria, prochlorophytes, and diatoms contributed to the total HMW DOC pool across the continental margin in the gulf. The extremely low concentrations of pheopigments (except for chlorophyllide) in particulate organic C and HMW DOC suggested that direct exudation from phytoplankton and sloppy feeding by zooplankton were the major mechanisms of release. However, significant and selective pigment loss, due to light and thermal degradation in the reservoir, did occur in the absence of any cooling or light‐shielding precautions. This study demonstrated that pigments can be used to acquire qualitative information on the sources of HMW DOC and to gain insight on its relative age—based on pigment turnover.

Importance of acid polysaccharides for 234Th complexation to marine organic matter

“…From the experimental titration data that used the same COM material (Murphy 2000), we can calculate the concentration of strong acid sites with a pKa of 3 (or lower) as 1.3 mmol g Ϫ1 of COM. This value is of the same magnitude as the total exchangeable proton concentration (which includes that of all the acid functional groups) of 1.4 mmol g Ϫ1 for COM reported by Santschi et al (1995) and the content of ϪCOOH sites of aquatic polysaccharides (0.3-1.3 meq g Ϫ1 ) reported by Buffle (1990). Thus, a conditional stability constant for COM consisting of 100% polysaccharide (pH 8) can be estimated as 10 7.8 M Ϫ1 .…”

“…No data are available for the total Chl a concentration of that water, however. This value of 2 g L Ϫ1 of Chl a in the colloidal fraction is three orders of magnitude higher than has been observed elsewhere for GOM COM samples (Bianchi et al 1995), which likely indicates not only high phytoplankton activity but also cell lysis having occurred in the water before collection. Because of this high extracellular concentration of Chl a, it is likely that cell lysis rates could have been unusually high.…”

“…Its isotopes, especially 234 Th, have been widely used as tracers to quantify the fluxes of particulate organic carbon (POC) in the upper ocean (e.g., Buesseler 1998 and references therein; Santschi et al 1999) and to constrain the cycling of dissolved organic carbon (DOC) in the marine environment (e.g., Baskaran et al 1992;Santschi et al 1995;Guo et al 1997). However, interactions of Th(IV) with marine organic matter are still poorly understood.…”

Interactions between mercury and phytoplankton: Speciation, bioavailability, and internal handling

“…Recently, net Hg methylation in open oceans has been shown to be promoted by the presence of small‐sized nano‐ and picophytoplankton, whose Hg content was efficiently remineralized by bacteria . Higher organisms will graze natural algal communities, leading to the transfer of Hg to the consumers and possibly to Hg recycling to the dissolved phase by “sloppy feeding.” As far as we know, this latter phenomenon has not yet been demonstrated for Hg, but it has been described for dissolved organic matter and nutrients as well as for other trace elements such as Cd and Zn . In the latter study concerning metals, the metals accumulated by the cyanobacterium Synechococcus were regenerated into the dissolved fraction by the grazing of natural microzooplankton.…”

The role of nutrient loading and eutrophication in estuarine ecology.

“…A significant fraction of the DOC in estuarine waters is composed of colloidal or macromolecular OM (9-11), which plays an important role in the carbon cycle, trace metal scavenging (12), and biogeochemical processes (13)(14)(15). DOM sources in coastal systems include production through phytoplankton exudates (mucopolysaccharides) or phytodetritus (16)(17)(18), inefficient zooplankton feeding (19), fecal pellet decomposition, sedimentary inputs through benthic exchange (20), urban/agricultural runoff (16), and sewage inputs. The pool of organic substances is dynamic.…”