Diatoms as epiphytes on macroalgae from Terra Nova Bay (Ross Sea, Antarctica)

Epiphytic diatoms are one of the key biotic element of the Antarctic marine ecosystem. Their unusually high metabolic flexibility gives them a crucial role in the Antarctic trophic chain. Despite being fundamental to the ecosystem functioning, diatom communities associated with marine macroalgae are frequently ignored in the floristic studies of the area. Few attempts to investigate substrate preferences have been reported. The present study provides the first examination and description of the epiphytic diatom flora from Terra Nova Bay (Ross Sea, Antarctica). Material was collected during Italian Antarctic campaigns in the years 1989-2005. Samples of thalli of Iridaea cordata, Phyllophora Antarctica, and Plocamium cartilagineum were taken from different depths at five sampling sites localized along the shore. All observations have been carried out under the scanning electron microscope. The genus Cocconeis was the most common. Two identified, previously described and well known from the Antarctic waters species - Cocconeis antiqua and C. fasciolata, as well as small naviculoid species (most likely Navicula perminuta), appeared in abundance. The Cocconeis component of the assemblage was estimated to approach 105 cells per cm2 on foliose hosts such as Iridaea. Melosira adelie was the only non-pennate diatom to achieve any degree of dominance, although Paralia sol and Trigonium arcticum were common on some of the filamentous parts of host macroalgae. Our results indicate a strong relation between the diatom assemblages and the sampling site. There is no clear evidence of diatoms host specificity. However, the filamentous forms as well as topographic anomalies (e.g. colonies of hydroids or bryozoans) can provide a point of attachment for a wider range of taxa.