Role of bacteria and bacterial exopolymer in the attachment of Achnanthes longipes (Bacillariophyceae)

The attachment of diatoms to surfaces is an important and poorly understood step in the development of biofouling communities. Experiments were performed in vitro on a common fouling diatom (Achnanthes longipes) to determine the influence of the base material and bacterial conditioning on diatom attachment. The first series of experiments compared attachment of A. longipes to four different base materials, and the influence of a bacterial film on attachment to these materials. A. longipes preferentially attached to polystyrene, a hydrophobic surface, but was inhibited by the presence of a bacterial biofilm. On other surfaces, bacteria either facilitated or had no effect on algal attachment. The second series of experiments found no difference in the attachment of A. longipes to a surface covered with bacterial exopolymer compared to a surface with a film of living bacteria. Attachment of A. longipes was found to vary depending on the conditions under which the bacterial film developed and the species of bacteria within the film. These results help to illustrate the complexity of the relationship between surfaces and attaching organisms and show that bacteria may either facilitate, have no effect or inhibit attachment by diatoms. The mechanisms underlying these patterns require further investigation

Eunotia amazonica

A new stalk-forming diatom was observed at high relative abundances on periphytic samples collected in the Rio Negro hydrographic basin (Brazilian Amazon) and is here described using light and scanning electron microscopy. We also present ecological preferences of the new species. Eunotia amazonica sp. nov. co-dominated periphytic samples collected during the rainy seasons of 2013 and 2014 near the city of Manaus (Amazonas, Brazil). The main diagnostic feature of the new species is the stalk-forming growth form (‘Cymbella-like’) illustrated here for the first time in the genus Eunotia Ehrenberg. Previous reports on growth forms in common European databases for this genus included colonial (‘ribbon-like’) but also ‘mobile’, ‘fixed by pads’ or ‘without structures of fixation’. We demonstrated that the three dominant Eunotia species in periphytic samples from the Rio Negro all exhibited distinct ‘growth forms’ or habits: Eunotia amazonica sp. nov. (stalked), Eunotia intricans Metzeltin & Lange-Bertalot (ribbon-like) and Eunotia rabenhorstiana (Grunow) Hustedt (branched-arborescent). The high diversity of species, many of which are unknown to science, as well as the poorly known life-forms and habits of the Eunotiaceae in the region, partially prevent the use of metrics based on life-form to assess ecological changes. Generic assumptions based on robust statistical methods may obscure reality and lead to biased conclusions of diatom community changes in poorly explored regions such as Neotropical freshwater rivers, where the Eunotiaceae are largely dominant, particularly in the Amazon basin. © 2018 British Phycological Society