Fatty acid and stable isotope biomarkers suggest microbe‐induced differences in benthic food webs between depths

Abstract

Benthic marine consumers inhabiting the subphotic zone rely on subsidies of energy synthesized by macrophytes and phytoplankton in the photic zone. The effects of this energy subsidy on the trophic ecology of deep invertebrates are generally unknown. We used fatty acids (FA) and multiple stable isotopes (MSI) as trophic biomarkers to compare tissues from conspecifics of primary and secondary consumers in photic and subphotic habitats (15 and 100 m depth) at three sites in the San Juan Archipelago, Washington. FA composition differed across depths for all five species and MSI differed across depths for six of seven species. We found a general pattern of enrichment in d 13 C from shallow to deep for all consumers. d 15 N was consistently enriched in deep herbivores and suspension feeders, but did not differ in predators. Total v-3 FA were lower in deep primary consumers, whereas predator v-3 FA did not differ between depths. Total bacterial marker FA were lower in deep suspension feeders but higher in deep predators. The results suggest a possible mechanism for the differences in FA and enrichment between habitats: deep consumers potentially ingest detritus that has been biochemically altered by microbes during transport. We found support for this hypothetical mechanism in an algal aging experiment. Aged algae colonized by microbes responded with increases in bacterial FA, and decreases in v-3 FA. This study highlights the power of combining FA and MSI biomarkers, and provides evidence for the importance of organic matter degradation to food web studies.

Factors affecting trophic compositions of offshore benthic invertebrates at a sub‐Antarctic archipelago

“…The trend of rising δ 15 N with increasing depth is likely to be a feature of many ecosystems. For instance a similar effect has been recorded by Polunin et al () and Fanelli et al (), working on demersal species and brittle stars respectively in the Mediterranean Sea, by Galloway et al () for benthic invertebrates in the San Juan Archipelago and by Bergmann et al () for suspension feeders from the deep‐sea observatory HAUSGARTEN.…”

“…For instance, SPM from coastal areas has usually a higher δ 13 C ratio than offshore waters due to the presence of macroalgae and detritus, which have higher δ 13 C values than phytoplankton (approximately −15‰ and −22‰ for macroalgae and phytoplankton, respectively), the predominant contributor to the SPM of offshore waters (Hill et al ; Miller et al ; Renaud et al ; Puccinelli et al a ). An increase of δ 13 C with depth in benthic invertebrates and SPM, especially in the upper part of the water column below the light extinction zone, generally reflects a decrease in phytoplankton abundance, the main food source for filter feeder species (Nerot et al ; Galloway et al ). The lack of depth effect on the carbon isotope composition of all trophic groups investigated in this study suggests that the relationship between consumer δ 13 C and depth may be influenced by several factors, as for instance differences in turnover rates between animal tissue and SPM.…”

“…In addition, ∑n‐3 decreased with depth. Such decreases, together with an increase in δ 15 N, have been associated with a change in food availability with depth, with deeper consumers eating food that has been more heavily modified by microbial activity (Chen et al ; Galloway et al ). In addition, relatively high levels of PUFA have been used as an indicator of good food quality (Brett and Müller‐Navarra ; Dalsgaard et al ).…”

Broad sampling and diverse biomarkers allow characterization of nearshore particulate organic matter

“…The increase of bacterial FA biomarkers in winter and spring samples corresponds to a significant increase in C:N and slight enrichment of δ 15 N, consistent with patterns of depleted δ 13 C and enriched δ 15 N in experimentally degraded material (Degens et al ). Other studies have attributed depletion of ω‐3 to bacterial degradation as evidenced by increases of bacterial FA and enrichment of δ 15 N in consumer tissue (Galloway et al ). While potentially confounded by seasonal fluctuations of ω‐3 sources, proportional bacterial FA were observed to increase seasonally with decreased ω‐3 FA.…”

Assessing the dietary sources of two cichlid species in River Nile sub-branches: Stomach contents, fatty acids and stable isotopes analyses

“…Extraction, methylation and quantification of samples were done using modified Folch method (Taiple et al, 2011 andGalloway et al, 2013) as described in detail by Abd El-Karim et al (2016). Fatty acids were measured by gas chromatography-mass spectrometry (GC-MS) using Agilent 7890 Series GC system interfaced to 5975 inert MSD with tripleaxis detector MS.…”