Filamentous cyanobacterial associates of the marine planktonic cyanobacterium Trichodesmium

Three types of filamentous microorganisms were associated with colonies of the marine planktonic cyanobacterium Trichodesmium collected in the Sargasso Sea. The filaments were categorized according to the cell diameter into large (LTF; about 1 μm), medium (MTF; about 0.5 μm) and small (STF; about 0.2 μm)filaments. All three types were detected in fusiform colonies of both T. thiebautii Gomont and T. erythraeum Ehrenberg ex Gomont as well as in radially arranged Trichodesmium colonies. The MTF were most abundant and always present whereas the other two types appeared less frequently. The LTF and MTF were identified as non-heterocystous cyanobacteria using epifluorescence, scanning and transmission microscopy as well as immunogold labelling and were tentatively ascribed to the ‘Lyngbya, Plectonema. Phormidium group’. The LTF showed oscillatory movements and distinct cross walls, and possessed phycoerythrin. The MTF were often seen below the extracellular sheath of Trichodesmium filaments, and although no cross walls were detected, the ultrastructure and phycobiliproteins were characteristic of cyanobacteria. MTF were also shown immunochemically to contain nitrogenase. The possible rôle of and benefit for these filaments, one of which is an additional putative nitrogen-fixing cyanobacterium, within Trichodesmium colonies is discussed.