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The capacity of marine sponges to cope with environmental changes is often attributed to the specific composition of their bacterial communities. In this study, we assessed the bacterial microbiome of two dominant sponges, Rhabdastrella globostellata (Rg) and Hyrtios erectus (He), living in the Bouraké lagoon (New Caledonia), where abiotic conditions daily fluctuate according to the tide. Sponge specimens, sediment and seawater samples were collected at 2–3 m depth. The bacterial communities were assessed using 16S rRNA metabarcoding, and variations between the two sponges were compared using PCA biplots. Chloroflexi was the dominant phyla in both He and Rg with an average relative abundance of 41.2% and 53.2%, respectively, while it was absent in sediment and seawater. Among the phylum Chloroflexi, SAR202 clade was dominant in both sponges, reaching an average relative abundance of 53.2% (He) and 78.7% (Rg). Principal component analysis (PCA) was used to identify the main variables driving the bacterial community structure in both sponges. The results indicated that the bacterial community structure in both sponges was strongly associated with Chloroflexi (70.9% of the phyla variance) and SAR202 clade (86.6% of the clade variance). The high relative abundance of the phylum Chloroflexi and the SAR202 clade observed in this study is the highest reported so far in the literature in marine sponges. Such a high relative abundance of these bacteria could suggest their involvement in the well-being of sponges in the extreme environmental conditions of Bouraké.

Nickel (Ni)-tolerant ectomycorrhizal Pisolithus albus was isolated from extreme ultramafic soils that are naturally rich in heavy metals. This study aimed to identify the specific molecular mechanisms associated with the response of P. albus to nickel. In presence of high concentration of nickel, P. albus Ni-tolerant isolate showed a low basal accumulation of nickel in its fungal tissues and was able to perform a metal efflux mechanism. Three genes putatively involved in metal efflux were identified from the P. albus transcriptome, and their overexpression was confirmed in the mycelium that was cultivated in vitro in the presence of nickel and in fungal tissues that were sampled in situ. Cloning these genes in yeast provided significant advantages in terms of nickel tolerance (+ 31% Ni EC50) and growth (+ 83% u) compared with controls. Furthermore, nickel efflux was also detected in the transformed yeast cells. Protein sequence analysis indicated that the genes encoded a P-type-ATPase, an ABC transporter and a major facilitator superfamily permease (MFS). This study sheds light on a global mechanism of metal efflux by P. albus cells that supports nickel tolerance. These specific responses to nickel might contribute to the fungal adaptation in ultramafic soil. (Resume d'auteur)