Decomposition of senescent blades of the seagrass Halodule wrightii in a subtropical lagoon

Abstract

Senescent blades from the seagrass Halodule wrightii Aschers were suspended in the water column of Laguna Madre (Texas, USA) for a period of 419 d , representing the longest seagrass decomposition study to date. The initial stage of decomposition was characterized by a rapid loss of organic matter (36 % in 24 d ) attributed to leaching. A total of 76 % of the organic matter from seagrass tissues was lost by the end of the decomposition period. Of the major bulk constituents measured, neutral sugars were most abundant and accounted for 23 % of the ash-free dry wt of the initial senescent material. A complete and early loss of the cyclitol, myo-inositol, a reduction in glucose yields, and a relative enrichment in mannose were the most dynamic features of the neutral sugar fraction. An overall stability series: mannose > fucose > arabinose = rhamnose = galactose = xylose > glucose, reflects patterns of selective degradation of the polymers from which these sugars are derived. Soluble and ester-bound phenolic acids were lost from tissues at relatively high rates. Susceptibility of individual phenolic acids to removal from bulk tissue upon treatment with weak base correlated well to initial losses in the field. Early changes in total CuO oxidation phenol yields from H. wrightii were caused pnmarlly by the relatively rapid loss of soluble and ester-bound phenolic acids. Therefore, we took a new approach to quantlfy lignin-derived phenols by subtracting soluble and ester-bound phenols from total CuO oxldat~on ylelds. When these losses were taken Into account, lignin-derived phenol yields were similar from freshly senescent and highly degraded detntus. Cutln, although initially selectively preserved relative to bulk tissue, was also found at near initial yields In the highly degraded detritus. Overall, the distribution of polymeric constituents (cellulose, hem~cellulose, lignin and cutin) was similar in freshly senescent and highly degraded tissues suggesting that the compounds which comprise the ultrastructure of H, wrightij are degraded at similar rates Photobleached H. wriglitii blades were characterized by a much higher neutral sugar content, slmilar levels of cutin acids and a nearly complete absence of phenols con~pared to freshly senescent tissue. Photobleaching appears to be an important degradative mechanism which yields a polysaccharide-rich detritus that is devoid of the lignin signature characteristic of vascular plant tissues.

Transport and diagenesis of dissolved and particulate terrigenous organic matter in the North Pacific Ocean

“…We now know that source signals are more varied than originally thought (e.g., Opsahl and Benner, 1993), and that fractionation occurs during leaching (e.g., Benner et al, 1990), leading toward higher (Ad:Al)v in the dissolved phase. The latter is probably a significant factor in the size-dependent 3-fold variability of (Ad:Al)v in riverine OM Hernes and Benner, submitted), and interpretations of differences among samples of varying source and size should be done with care.…”

Assessment of terrigenous organic carbon input to the total organic carbon in sediments from Scottish transitional waters (sea lochs): methodology and preliminary results

“…The method for cupric oxide oxidation of sediment samples has been adapted from the following: Hedges and Ertel (1982); Ertel and Hedges (1984); Readman et al (1986); Goni and Hedges (1990, 1992; Gough et al (1993);Benner (1993 and and Reeves and Preston (1991).…”

Molecular composition of plant parts and sediment organic matter in a Mediterranean seagrass (Posidonia oceanica) mat

“…Knowledge on the in situ decomposition of P. oceanica detritus within the mats is limited. It involves an initial phase of abiotic leaching of fast-cycling components, consisting of low molecular weight (LMW) phenolic and carbohydrate compounds, followed by microbial and detritivorous breakdown of more refractory materials (Kenworthy and Thayer 1984;Harrison 1989;Peduzzi and Hernle 1991;Enríquez et al 1993;Opsahl and Benner 1993;Mateo and Romero 1996). The second (biotic) decomposition phase is a much slower process (Romero et al 1992), and allows the more refractory tissues to accumulate in mats.…”

“…Other approaches rely on the chemical breakdown, usually by CuO oxidation (Hedges and Mann 1979;Opsahl and Benner 1993), or thermal breakdown, such as pyrolysis (Sáiz-Jiménez 1988), of the organic matter followed by gas chromatography and mass spectrometry (GC-MS) for identification of the products. Here we will focus on analytical pyrolysis techniques, i.e.…”