Using the tag-encoded 454 pyrosequencing approach, we described extremely diverse fungal communities, with several hundreds OTUs generated in each sample. Such values were not anticipated, as they are orders of magnitude higher than any of fungal species richness reported previously from mangroves. For instance, Schmit & Shearer (2004) identified in a meta-analysis 75 fungal species on A. marina and 32 on R. stylosa and an overall fungal diversity of 163 species collected on 16 tree host species. In a similar fashion, based on intensive search of literature records of xylophilous Basidiomycetes in mangroves, a list with 112 species were presented by Baltazar et al. (2009), which is less than the 278 OTUs assigned to the Basidiomycota phylum in the ITS1F data set (ITS4: 227 OTUs; nu-ssu-0817: 227 OTUs; nu-ssu-1536: 187 OTUs). Furthermore, the latest review of the described marine fungi (Jones et al., 2009) report 530 species, compared with 2048 OTUs we described in the inundated part of the mangrove (ITS4 data set, data not shown), indicating a large gap in our knowledge of the mangrove colonization by fungi.
Our observations also indicate that only a very small number of fungal OTU dominate each sample, while the majority of the diversity is present in minute quantities (Table S1). This seems to confirm previous analysis concerning microbial eukaryote species, revealing that numerous environments contain a few functionally active species and a large ‘seed bank’ of species able to survive under different conditions (Finlay, 2002), corresponding to a potential functional reserve essential for the ecosystem's resilience in response to environmental disturbances.
The analysis of the OTUs spatial distribution reveals that the fungal communities found on A. marina and R. stylosa are markedly different. The vast majority of the OTUs are found almost exclusively in a single microhabitat (AB, AH, RB or RH). When considering the dominant OTUs, we note that none of them is ubiquitous, 80% are found majoritarily in a single microhabitat and 20% of them are found in similar abundance in two microhabitats. Interestingly, these two microhabitats always share a common tree host or a common sea level. These data suggest a strong host specificity of the fungal colonization of both the aerial and intertidal parts of the mangrove. This is relatively surprising, as we assumed that the presence of seawater would create a continuous, homogeneous media, recycled cyclically by the tides, in which fungal dispersion and propagation between trees would be promoted. Host specificity in the aerial part was expected, as it has been shown to happen in plant pathogens fungi, mycorrhizas, endophytes and saprobes (Zhou & Hyde, 2001). Several studies have described this effect in terrestrial ecosystems, in soils and in phyllospheres where it occurs to varying degrees depending on the climate and the fungal lineage considered (Ferrer & Gilbert, 2003; Buée et al., 2007; Tedersoo et al., 2008). However, there was little evidence of this phenomenon in marine fungi or in intertidal mangrove fungi. Jones (2000) pointed out that some fungi occurred more readily on test blocks of one wood type than any another. Meanwhile, another research group (Hyde, 2007) indicated that certain fungal species were only associated with a host species or genera. In the same way, some authors reported that fungal endophytes show some degree of host specificity at least for families of host trees and that such specificity may influence endophytes distribution more than geographical location of the host plants (Petrini & Carroll, 1981; Suryanarayanan and Kumaresan, 2004). On the other hand, Schmit & Shearer (2004) revealed through a meta-analysis of published data that mangrove trees that are close phylogenetically do not necessarily harbour microfungal communities that are distinctly different from less closely related hosts. In our study, given the spatial proximity of the A. marina and R. stylosa sampled at each site (less than a few metres), it is unlikely that the variations observed are linked to differing environmental conditions. It is probable that the host specificity observed in this study is enhanced by the high phylogenetic differences of the two host species studied, respectively, refereed to the asterids and rosids clades. We also note that the variability of the fungal community between the three sampling sites was limited. This is probably caused by the very small variations of micro-environmental conditions between these locations. Indeed, despite being over 150 m apart from each other, the sampling sites are exposed to similar temperatures, humidity, pH and tide level. The most noticeable difference is that the α site is more exposed to waves and spray than β or θ.
Furthermore, given the fact that the samples were collected in the transition zone between the two tree stands, it is probable that our data are not totally representative of the fungal diversity in this mangrove. The environmental conditions encountered in the A. marina zone (higher salinity, shorter exposure to seawater, higher exposure to wind and higher light exposure) and the R. stylosa zone (higher tree density, higher tide level and lower exposure to light) mean that further fungal diversity could be retrieved from these loci.