Heterotrophic Utilisation of Particulate Matter from the Kelp Laminaria pallida

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Abstract
The microbial community, which colonizes particulate debris from the kelp L. pallida, shows a succession from bacteria (through flagellates, ciliates, amoebae and choanoflagellates) over a period of up to 34 days incubation at 10.degree. C. The biomass of bacteria reaches 16.5 mg g-1 of added organic matter compared with 42 mg g-1 dry weight of dissolved organic matter added from kelp mucilage, despite the fact that the combined biomass of grazing protozoans in both cases was 4.2-4.6 mg g-1 of added organic matter. This suggests that the dissolved components of mucilage, comprising principally the acyclic polyol D-mannitol and other carbohydrates, are more readily utilizable as a substrate than the particulate components of kelp debris. The C in particulate debris is converted into bacterial biomass with an efficiency of approximately 11% compared with values of up to 29% reported for mucilage incubated with seawater during the summer. Differences in conversion efficiency of soluble compounds and particulate debris from L. pallida are reflected in the time taken for utilization by microheterotrophs under culture conditions. The time taken for 50% utilization of particulate debris at 10.degree. C was 240 h compared with reported values of 144 h for sugars and alginates, and 48 h for mannitol incubated at 10.degree. C. Estimates of the relative significance of dissolved and particulate components released during fragmentation of kelp suggest that out of an annual average C production of 820.5 .times. 104 kg from a kelp bed of 700 ha, a dry bacterial biomass of 72.20 .times. 104 kg is supported by particulate production and 29.9 .times. 104 kg from mucilage; this yields a total bacterial biomass of 102.10 .times. 104 kg. Because of the rapid turnover time of dissolved components, it is suggested that mucilage production supports mainly the dense population of free-living bacteria which are characteristic of kelp beds; the bacteria associated with the more refractory particulate matter may be exported to adjacent communities.