Decomposition dynamics of the bloom forming macroalga Ulva rigida C. Agardh determined using a super(14)C-carbon radio-tracer technique

The short-term decomposition dynamics of Ulva rigida C. Agardh by a natural bacterial consortium were studied in the laboratory using a super(14)C radio-tracer technique. Following an initial rapid loss of approximately 15% of the particulate organic carbon (POC) by leaching of dissolved organic carbon (DOC), there was an exponential loss of POC and an accompanying increase in the labelling of the inorganic carbon (IC) pool. However, losses of labelled POC were significantly greater than the summed accumulation of radiolabel in the IC and DOC pools over the course of the experiments. We propose that this discrepancy may be due to the production of volatile organic compounds (VOC) during decomposition, which were stripped from the medium during the extraction of IC, by acidification and sparging with helium, but were not retained by the KOH traps for IC. The compatible solute dimethylsulphonioproprionate (DMSP) which is accumulated to high concentrations by Ulva spp. and is metabolised by bacteria to volatile dimethylsulphide (DMS) and potentially methane, could be one of the major sources of VOCs during our incubations. Modelled decomposition rates based on the loss of label from the particulate fraction were high, with calculated half times of 8.1 and 7.6 days. However, these rates greatly overestimate true mineralisation rates, since only approximately 50% of the label lost from the POC pool was recovered as IC. Overall these data indicate that previously determined decomposition rates based on loss of dry weight of macroagal biomasses would also greatly overestimate carbon mineralisation rates and hence the oxygen demand for this mineralisation. This aspect may be particularly important for environmental management or modelling of bloom impacted environments, since the most severe impacts of macroalgal blooms are mediated by the hypoxic or anoxic conditions induced when the blooms collapse and hence are strictly related to the oxygen demand for mineralisation of the macroalgal biomasses.