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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 primarily by the relatively rapid loss of soluble and ester-bound phenolic acids. Therefore, we took a new approach to quantify lignin-derived phenols by subtracting soluble and ester-bound phenols from total CuO oxidation yields. When these losses were taken into account, lignin-derived phenol yields were similar from freshly senescent and highly degraded detritus. Cutin, 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, hemicellulose, lignin and cutin) was similar in freshly senescent and highly degraded tissues suggesting that the compounds which comprise the ultrastructure of H. wrightii are degraded at similar rates. Photobleached H. wrightii blades were characterized by a much higher neutral sugar content, slmilar levels of cutin acids and a nearly complete absence of phenols compared 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.

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