Two of the most learn more frequently used general bacterial PCR primers, targeting the 16S rRNA gene around E. coli positions 8-27 and 338-355, contain mismatches against planctomycete sequences [27, 28]. This may have caused planctomycete abundances to be underestimated in many

habitats, leading investigators to turn their attention towards bacterial groups that appear more abundant. Despite awareness of this problem, the literature and the sequence databases probably reflect a tradition of neglect towards the planctomycetes. In the light of this, it is difficult to say whether the dominance of planctomycetes on Laminaria hyperborea surface biofilms represents a unique feature of this habitat, or if other planctomycete-dominated bacterial communities mTOR inhibitor have been overlooked until now. For example, Staufenberger and co-workers

[29] did not detect planctomycetes in surface biofilms of another species of kelp (Saccharina latissima) using general bacterial primers for cloning and DGGE analysis. Yet, use of different primers has let to the detection of planctomycetes on both the kelps S. latissima and Laminaria digitata (Bengtsson, unpublished results). A possible explanation for the suitability of kelp as a habitat for planctomycetes is its content of sulfated polysaccharides, a class of molecules that some marine planctomycetes are known for being able to degrade [10]. For example, Laminaria hyperborea contains fucoidan, a class of complex brown algal sulfated polysaccharides. however These substances are secreted to the surface of L. hyperborea via mucilage channels [30]. It is reasonable to assume that planctomycetes living on kelp surfaces utilize substances produced by the kelp, for example fucoidan, as carbon sources. However, the presence of suitable carbon sources appears insufficient to explain the observed dominance of planctomycetes, as they must not only be able to grow and divide, but also outcompete other bacteria to be successful. Another contributing factor to the success of planctomycetes on kelp

surfaces may be resistance to chemical antimicrobial defense compounds produced by the kelp. Antibacterial activity has been detected in extracts from many species of kelp, yet the substances responsible for the activity have often not been identified [31]. The lack of peptidoglycan in planctomycete cell walls makes them resistant to conventional cell wall targeting antibiotics like ampicillin. Resistance to other antibiotics, targeting for example protein synthesis (streptomycin) has also been reported in some marine planctomycetes [32, 33]. In many cases the reference sequences that are the most closely related to kelp surface planctomycetes are obtained from other marine eukaryotes such as for example red and green seaweeds, corals, crustaceans and sponges (Figure 4). The frequent association of planctomycetes to eukaryotes has previously been noted [34].