Desmid-bacterial associations in Sphagnum-dominated Wisconsin peatlands

In a survey employing epifluorescence microscopy with the DNA flourochrome DAPI, associations between bacteria and filamentous desmids were found to be common-place in acidic, Sphagnum-dominated Wisconsin peatlands. Bacteria were associated with all genera of filamentous desmids encountered including Desmidium, Hyalotheca, Onychonema, Spondylosium, and Teilingia. Although only associations involving filamentous desmids having mucilaginous sheaths are illustrated here, bacteria were also noted on ta,va lacking sheaths as well as some unicellular forms. Bacteria on Desmidium majus Lagerheim, D. grevillii (Kutz.) De Bary, and Hyalotheca dissiliens (Smith) Breb. ex Ralfs tended to be concentrated in small pockets in the sheath material located near the isthmus and in the region between adjacent cells in the filament, whereas those associated with Spondylosium pulchrum (Bail.) Archer were more evenly distributed throughout the sheath. Most bacteria were rod-shaped. Those associated with S. pulchrum, D. grevillii, and D. majus ranged from 1.1 to 11.2 mu m in length. Bacteria within the sheaths of H. dissiliens and D. grevillii were Gram-negative. A second morphologically distinct population of bacteria was found at the sheath margin in D. majus and D. grevillii. Field collections containing filamentous desmids were examined with scanning electron microscopy and bacteria associated with Desmidium majus were investigated with transmission electron microscopy.