Bathymodiolus azoricus mussels thrive 840 to 2300 m deep at hydrothermal vents of the Azores Triple Junction on the Mid-Atlantic Ridge.Although previous studies have suggested a mixotrophic regime for this species, no analysis has yet yielded direct evidence for the assimilation of particulate material.In the present study, tracer experiments in aquaria with 13 C-and 15 N-labelled amino acids and marine cyanobacteria demonstrate for the first time the incorporation of dissolved and particulate organic matter in soft tissues of vent mussel.The observation of phytoplanktonic tests in wild mussel stomachs highlights the occurrence of in situ ingestion of sea-surface-derived material.Particulate organic carbon fluxes in sediment traps moored away from direct vent influence are in agreement with carbon export estimates from the surface ocean above the vents attenuated by microbial degradation.Stable isotope composition of trapped organic matter is similar to values published in the literature, but is enriched by + 7 ‰ in 13 C and +13 ‰ in 15 N, relative to mussel gill tissue from the Menez Gwen vent.Although this observation suggests a negligible contribution of photosynthetically produced organic matter to the diet of B. azoricus, the tracer experiments demonstrate that active suspension-feeding on particles and dissolved organic matter could contribute to the C and N budget of the mussel and should not be neglected.
Photosynthetic microphytobenthic activity has increasingly been examined using pulse‐amplitude‐modulated (PAM) fluorescence techniques. Nevertheless, estimating carbon production rates from fluorescence measurements implies the establishment of reliable relationships. The aim of this study was to determine such a relationship from field measurements of both PAM fluorescence and CO 2 fluxes. Three study sites of varying sedimentary features were investigated in different seasons. Both linear and with plateau relationships were obtained between the fluorescence parameter (relative electron transport rate [rETR]) and the community‐level carbon‐fixation rate (gross community primary production rate [GCP] in mg C · m −2 · h −1 ). The correlation calculated from the whole data set (i.e., all sites and all seasons) was very strong ( n = 106; r = 0.928). Significant correlations were also obtained for light‐curve parameters assessed with the two methods: P m ( n = 8; r = 0.920) and I k ( n = 8; r = 0.818). Total community‐level carbon fixation for the emersion period was calculated from fluorescence measurements according to the relationship established between GCP and rETR, and between light‐curve parameters, and the results were compared to the estimation obtained directly from GCP measurements. The agreement between the two estimations was quite good for both ways of calculation (with a mean discrepancy of 30% for the first one and −2% for the second one). These results suggest the potential application of PAM measurements to calculate carbon‐fixation rates at large spatial and temporal scales, provided that a set of experiments coupled with CO 2 ‐flux measurements are performed.
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