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, but it’s quite happy with abundant phosphorus…

  Rhopalodia gibba in valve and girdle view (scalebars = 10 µm; from Hebron Swamp, Cheboygan Co, MI; photos by P. Kociolek www.westerndiatoms.colorado.edu); (top right) a specimen of the lunate R. gibberula (www.fcelter.fiu.edu); R. gibba with cyanelles (inset above), its endosymbiotic cyanobacteria: https://microscopesandmonsters.wordpress.com; cell length ~ 100 µm & max width 20 µm).

 

…such as in some areas of the Everglades, where R. gibba can tolerate high phosphorus (P) concentrations (> 800 μg P g1) in the periphyton mats4. This species has been found to dominate primary production in restored prairie wetlands in South Dakota that had persistent high P levels.5 Moreover, it is well adapted to high conductivity and alkalinity, such as in boreal wetlands in Wood Buffalo National Park (in Northeast Alberta, Canada); here R. gibba lives in yellow diatom ponds (!), nesting sites of whooping cranes6. The diatom ponds are shallow spring-fed, alkaline wetlands with bicarbonate, sulphate, calcium and magnesium. In these open habitats, cranes can see predators well, and they find their favored nesting material, bulrush6. So preserving the habitats of these glass-celled algae that fix nitrogen thanks to cyanobacteria living in their cells, we end up conserving large charismatic species too (and viceversa), from the bottom to the top of the food chain.


                        Ponded wetland in Wood Buffalo National Park (source: www.emaze.com).


A whooping crane (source: http://cranetrust.org); this large bird uses diatom ponds as nesting habitats.


* Postdoctoral Associate in Dr. Evelyn Gaiser's lab at Florida International University.

3. Round, F. E., R. M. Crawford, and D. G. Mann. 1990. The Diatoms: Biology and Morphology on the Genera.       Cambridge University Press, Cambridge, U.K.
4. Gaiser E.E., McCormick P.V., Hagerthey S.E. & Gottlieb A.D. (2011) Landscape Patterns of Periphyton in the Florida Everglades, Critical Reviews in Environmental Science and Technology, 41(S1), 92–120.
5.   Mayer P.M. & Galatowitsch S.M. (2001) Assessing ecosystem integrity of restored prairie wetlands from species production–diversity relationships Hydrobiologia, 443, 177–185.
6.   Tilmoney K., Zoltai S.C. & Goldsborough L.G. (1997) Boreal diatom ponds: a rare wetland associated with
    nesting whooping cranes. Wetlands, 17, 53951.

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