Bacteria from the “Saline di Tarquinia” marine salterns reveal very atypical growth profiles with regards to salinity and temperature

Résumé

Several thousand bacterial colonies were isolated from “Saline di Tarquinia” (Italy) marine salterns; after dereplication, thirty-one strains were obtained, confirming the low culturable bacterial diversity attributed to similar hypersaline environments. Strains were identified by 16S ribosomal DNA sequencing and phylogenetic analysis; most strains belonged to the class Gammaproteobacteria, with minorities from Actinobacteria, Bacilli and Alphaproteobacteria. Halomonas and Salinivibrio were predominant genera. To profile growth preferences, strains were cultivated at different salinities and temperatures. Non-metric multi-dimensional scaling (nMDS) analysis on growth data at different salinities identified four groups that were significantly correlated with the ecological categories conventionally used to describe halophiles. The majority were moderate halophiles with optimal growth at 40-80‰ salinity. Some were markedly euryhaline and grew from 40-80‰ to 240-280‰.

Very uncommon behaviour was observed for some strains, with a clear optimum growth in the slight to moderate halophilic range, but also the ability to grow without salt. These bacteria could barely be included in the mentioned traditional definitions and we suggest to consider them as “norm-tolerant halophilic”, being somehow in-between the slight halophiles and the halotolerants. As for temperature, nMDS groups were less defined and uncorrelated to ecological categories. Most strains were mesophilic-psychrotolerant or mesophilic, and some psychrotolerants were also recorded. Generally, diffused eurythermism was observed and various strains displayed unusual “flat growth profiles” that showed a broad range of thermal optima. Overall, the majority of strains appeared to be well adapted to the “Saline di Tarquinia” environment and showed great temperature and salinity variations compared to species observed in other salterns.