Showing posts from May, 2016

Diatom of the month – May 2016: Rhopalodia gibba

by Luca Marazzi*
Rhopalodia gibba belongs to the epithemioid group, that is to say it has valves symmetric about a line (bilateral symmetry), but asymmetrical to the apical (longitudinal) axis, the raphe system is well developed, enclosed within a canal, and positioned near the valve margin1. It is longer and more slender than other species such as the lunate R. gibberula, and the more arched R. musculus; it is found in the benthos, attached to substrata such as plant (epiphytic), or gliding more freely and opportunistically2. Interestingly, R. gibba seems to have declined significantly in the United Kingdom due to agricultural intensification and associated large use of nitrogen fertilizers. In fact, this pennate (bilaterally symmetric) diatom species lives in nitrogen poor habitats, but it has evolved an endosymbiosis with cyanobacteria that fix nitrogen3. So it does not like when there is too much nitrogen around, and loses its competitive edge against other fellow diatoms and algae…