Abstract: Populations of the benthic diazotrophic cyanobacterium Hydrocoleum Kützing (1843) are widely distributed in coral reefs and lagoons of tropical oceans, where they contribute to benthic nitrogen fixation. We compared the morphological, molecular and physiological features of 31 freshwater and marine Hydrocoleum populations from geographically different localities and five specimens from herbaria. Our aims were to compare the features of Hydrocoleum with the morphologically and phylogenetically related genera Trichodesmium Ehrenberg ex Gomont 1892 and Blennothrix Kützing 1843, and to provide the basis for the validity of the genus Hydrocoleum. The Hydrocoleum populations were distributed over a wide range of sizes, showing no consistent inter-population grouping and leaving no gaps between groups that were traditionally attributed to different species. Transmission electron microscopy showed regular cell division rhythms in all Hydrocoleum populations with each cell division completed before the next one was initiated, similar to that described for Trichodesmium. In contrast, Blennothrix populations exhibited multiple initiation of cross-wall formation. Most Hydrocoleum populations fixed nitrogen at consistently higher nightly rates. The phylogenetic analysis of 16S rRNA and nifH gene sequences placed all morphologically distinct Hydrocoleum and Trichodesmium populations within a tight cluster. Neither of the analysed genes brought any further resolution of phenotypically and ecologically distinguished forms. However, freshwater morphospecies of Blennothrix together with the type species Blennothrix vermicularis Kützing were separated from the Trichodesmium/Hydrocoleum cluster, which further supported the genotypic distinction between the compared taxa, in addition to the fact that they also differ in morphology and ecology. We concluded that the genus Hydrocoleum possesses unique features that warrant its maintenance in the botanical classification system and that its introduction in bacteriological manuals as a valid cyanobacterial genus is acceptable.
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