Anabaena gerdii sp nov., a new planktonic filamentous cyanobacterium from the South Pacific Ocean and Arabian Sea

A new cyanobacterium, Anabaena gerdii E.J. Carpenter & Janson sp. nov. (Hor mogonales, Cyanophyta), has been collected from the western South Pacific O cean, Arabian Sea and Arafura Sea. This species appears to be one of the ve ry few free-living heterocystous cyanobacteria existing planktonically in t he open ocean. Cells are 6-7.5 mum in diameter, and trichomes grow as long as 2.7 mm. Generally, about 15-20 vegetative cells separate the heterocysts . in both oceans where A. gerdii was found, abundance was relatively low, r anging from 0.13 to 4.7 and 0.92 to 3.7 trichomes l(-1) in the South Pacifi c and Indian Oceans, respectively. In the South Pacific it was present at 7 of 51 stations examined and in the Indian Ocean at 2 of 10 stations. It wa s collected at one station in the Arafura Sea. Water temperatures where it was observed in the South Pacific ranged from 26.2 degreesC to 28.7 degrees C and in the Indian Ocean from 28.7 degreesC to 29.4 degreesC; in the Arafu ra Sea, it occurred at 28.7 degreesC. The hetR sequences from the three fil aments of A. gerdii analysed were identical. Phylogenetic analysis of this sequence together with other hetR sequences from a wide span of cyanobacter ial genera suggests that A. gerdii is the sister group to Anabaena spp. and Aphanizomenon sp. KAC 15.