TRICHODESMIUM is a filamentous, colonial nitrogen- fixing cyanobacterium, ubiquitous in tropical and subtropical regions of the world's oceans. As a nitrogen-fixing organism,TRICHODESMIUM has the potential to introduce "new" nitrogen (N) to oligotrophic systems which are generally regarded to be N-limited. TRICHODESMIUM often forms extensive blooms and can comprise a significant fraction of total primary production in oceanic surface waters. Therefore, the consumption and fate of TRICHODESMIUM has important consequences for understanding carbon (C) and N cycling in the open ocean. The one confirmed grazer of TRICHODESMIUM is the pelagic harpacticoid copepod MACROSETELLA GRACILIS. M. GRACILIS has a pan-global distribution and has evolved a very specific association with TRICHODESMIUM. M.GRACILIS uses TRICHODESMIUM as a physical substrate for juvenile development, as well as a food source.

Experiments were conducted to assess TRICHODESMIUM ingestion, assimilation and ammonium regeneration by M. GRACILIS and other pelagic copepods. Experiments were conducted aboard ship at sites within the Atlantic, Pacific and Indian Oceans. Several different types of pelagic copepods (calanoids, harpacticoids and a poecilostomatoid species), were tested for ingestion of TRICHODESMIUM by labelling the cyanobacteria photosynthetically with C-14. The only copepods capable of ingesting TRICHODESMIUM were the three pelagic harpacticoid species tested: M. GRACILIS , MIRACIA EFFERATA and OCULOSETELLA GRACILIS. These harpacticoids were able to graze >75% of their weight in C/day in TRICHODESMIUM. There were no significant differences at the p < 0.05 level (ANOVA) between ingestion rates determined in the Atlantic, Pacific and Indian Oceans. In the Caribbean, M. GRACILIS showed higher ingestion rates of T. ERYTHRAEUM than T. THIEBAUTII. This may be due to the presence of a neurotoxin in T. THIEBAUTII which may be an important factor in determining trophodynamic interactions. M. GRACILIS and M. EFFERATA showed resistance to the T. THIEBAUTII toxin at concentrations well above those lethal to calanoid and poecilostomatoid species tested.

M. GRACILIS not only ingested, but rapidly incorporated ingested TRICHODESMIUM. Approximately ~22-38% of the TRICHODESMIUM C ingested was incorporated. This was determined by following the metabolic partitioning and incorporation of C-14 labelled TRICHODESMIUM into copepod lipids, polysaccharide and low molecular weight compounds using sequential biochemical fractionation. The rapid transfer of N fixed by TRICHODESMIUM to M. GRACILIS was demonstrated in the Caribbean by use of 15N2-labelled T. thiebautii. Rates indicated that M. GRACILIS could consume 33-45% of total colony N per day and >100% of new N fixed/colony /day. Experiments investigating ammonium regeneration by M. GRACILIS using N-15 isotope dilution, indicated that the rate of ammonium release increased as both copepod numbers and food availability increased. M. GRACILIS, through excretion and possible mechanical breakage of cells while grazing, appears to provide a direct link between atmospherically derived "new" N from TRICHODESMIUM and regenerated ammonium in tropical and subtropical oceans.