What constitutes Gomphonema parvulum? Long-term culture studies show that some varieties of G. parvulum belong with other Gomphonema species

Abstract

Background and aims – Gomphonema parvulum sensu lato contains a number of morphologically distinguishable varieties and forms that also show variation in their distribution and ecological ranges. As part of a study to investigate the ecophysiology of this taxon in relation to its use as a bio-indicator, clones from a range of sites in the UK were established and maintained in culture, monitoring their growth and morphology over extended periods of time. Methods – Clonal cultures of strains of G. parvulum were isolated and maintained in liquid medium under standard conditions, harvesting old cultures after each sub-culturing event. Wall morphology was monitored by light microscopy of permanent preparations, and a number of morphometric measurements obtained. Results – Four clones identified as belonging to two different varieties of G. parvulum, underwent autogamous sexual reproduction in culture, producing auxospores and initial cells, and then continuing to reproduce vegetatively. The two varieties gave rise to morphologically different cells that were identified as different Gomphonema species, i.e. G. gracile and G. hebridense . Continued culturing resulted in cell size reduction and changes in morphology towards that of the original isolates. Conclusions – Cells identified as part of G. parvulum sensu lato represent one end of the morphological spectra of taxa that also exhibit morphologies that would be assigned to other Gomphonema species. The morphological range of these taxa has previously been inadequately documented and species definitions must be re-visited. Any nomenclatural revision must follow priority rules, but it will be necessary to check type material of all the relevant species before making any nomenclatural changes. The ecological tolerances of the taxa involved should also be re-evaluated; it is improbable that cells at the opposite ends of the morphological spectra have different ecological responses.