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JOURNAL OF PLANKTON RESEARCH | VOLUME 20 | NUMBER 1 | PAGES 43-59 | 1998
© Oxford University Press
research-article |
The colonial cyanobacterium Trichodesmium as a physical and nutritional substrate for the harpacticoid copepod Macrosetella gracilis
Horn Point Environmental Laboratory, University of Maryland Cambridge, MD 21873, USA
1Correspondence address: School of Marine Sciences, University of Queensland, Brisbane, Qld 4072, Australia
Received on November 17, 1996; accepted on August 28, 1997 The pelagic harpacticoid copepod Macrosetella gracilis uses the colonial cyanobacterium Trichodesmium not only as a physical substrate for juvenile development, but also as a food source. By associating itself with a buoyant colonial cyanobacterium, M.gracilis has developed a successful mode of life for existence in the plankton. Further evidence of M.gracills' dependence on Trichodesmium as a physical substrate is demonstrated by previously undescribed microscopic observations of a gravid M.gracilis female attaching eggs to a Trichodesmium colony. Shipboard experiments investigating the ingestion and assimilation of Trichodesmium carbon (C) were conducted in September 1991 and January/February 1992 in waters of the Bahamas and the Caribbean, respectively. Macrosetella gracilis not only ingested, but rapidly incorporated, cyanobacterial organic matter into its own cellular material. Utilization of ingested Trichodesmium by M.gracilis was investigated by assessing the metabolic partitioning and incorporation of 14C-labelled Trichodesmium into copepod lipids, proteins, polysaccharides and low-molecular-weight (LMW) compounds using sequential biochemical fractionation techniques. Despite variations in grazing rates between the two sites and times (September 1991,0.017 µg C* µg–1 C h–1; January 1992, 0.134 µg C * µg–1 C h–1, the partitioning of incorporated C into the different biochemical fractions was relatively consistent. There was rapid assimilation of ingested C into the LMW ({small tilde}60%) and polysaccharide fractions ({small tilde}30%) in the first few hours, with a subsequent increase in the percent C incorporated into protein. On average, {small tilde}21% of the Trichodesmium C ingested by M.gracilis was assimilated. Therefore, M.gracilis is an important secondary link in the food web of oligotrophic waters where Trichodesmium is abundant.
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