Alteration of organic matter during infaunal polychaete gut passage and links to sediment organic geochemistry. Part II: Fatty acids and aldoses

Abstract

The activities of sediment-dwelling fauna are known to influence the rates of and pathways through which organic matter is cycled in marine sediments, and thus to influence eventual organic carbon burial or decay. However, due to methodological constraints, the role of faunal gut passage in determining the subsequent composition and thus degradability of organic matter is relatively little studied. Previous studies of organic matter digestion by benthic fauna have been unable to detect uptake and retention of specific biochemicals in faunal tissues, and have been of durations too short to fit digestion into the context of longer-term sedimentary degradation processes. Therefore this study aimed to investigate the aldose and fatty acid compositional alterations occurring to organic matter during gut passage by the abundant and ubiquitous polychaetes Hediste diversicolor and Arenicola marina, and to link these to longer-term changes typically observed during organic matter decay. This aim was approached through microcosm experiments in which selected polychaetes were fed with 13C-labelled algal detritus, and organisms, sediments, and faecal pellets were sampled at three timepoints over ∼6 weeks. Samples were analysed for their 13C-labelled aldose and fatty acid contents using GC–MS and GC–IRMS. Compound-selective net accumulation of biochemicals in polychaete tissues was observed for both aldoses and fatty acids, and the patterns of this were taxon-specific. The dominant patterns included an overall loss of glucose and polyunsaturated fatty acids; and preferential preservation or production of arabinose, microbial compounds (rhamnose, fucose and microbial fatty acids), and animal-synthesised fatty acids. These patterns may have been driven by fatty acid essentiality, preferential metabolism of glucose, and A. marina grazing on bacteria. Fatty acid suites in sediments from faunated microcosms showed greater proportions of saturated fatty acids and bacterial markers than those from afaunal controls. Aldose suite alterations were similar in faunated microcosms and afaunal controls, however the impact of faunal gut passage on sedimentary aldose compositions may be observable over longer timescales. Therefore this study provides direct evidence that polychaete gut passage influences OM composition both through taxon-specific selective assimilation and retention in polychaete tissues, and also through interactions with the microbial community.