Predominance of deterministic microbial community dynamics in salterns exposed to different light intensities

Abstract

Summary While the dynamics of microbial community assembly driven by environmental perturbations have been extensively studied, our understanding is far from complete, particularly for light‐induced perturbations. Extremely halophilic communities thriving in coastal solar salterns are mainly influenced by two environmental factors—salt concentrations and high sunlight irradiation. By experimentally manipulating light intensity through the application of shading, we showed that light acts as a deterministic factor that ultimately drives the establishment of recurrent microbial communities under near‐saturation salt concentrations. In particular, the stable and highly change‐resistant communities that established under high‐light intensities were dominated (>90% of metagenomic reads) by Haloquadratum spp. and Salinibacter spp. On the other hand, under 37‐fold lower light intensity, different, less stable and change‐resistant communities were established, mainly dominated by yet unclassified haloarchaea and relatively diverse photosynthetic microorganisms. These communities harboured, in general, much lower carotenoid pigment content than their high‐irradiation counterparts. Both assemblage types appeared to be highly resilient, re‐establishing when favourable conditions returned after perturbation (i.e. high‐irradiation for the former communities and low‐irradiation for the latter ones). Overall, our results revealed that stochastic processes were of limited significance to explain these patterns.

Distinct ecotypes within a natural haloarchaeal population enable adaptation to changing environmental conditions without causing population sweeps

“…Six adjacent crystallizer ponds, each of about 15 m 3 in volume, were used in the year 2012 to observe the effect of different environmental pressures on the hyperhalophilic microbial communities. The ponds subjected to different light intensities were described elsewhere [1]; here, we focus primarily on the pond subjected to a dilution treatment and the control ponds. All ponds had been recently constructed and were used to concentrate salt for the first time.…”

“…To reach the salt saturation, crystallizers were refilled weekly to increase the concentration of salts from 17.2% that was the brine salt content of the inlet source in May to saturation, or near saturation (33.6-38.4%), in early August (Table S1). Pond E1, with saturating salt conditions (38.4%) in early August, was used as a control for all experiments [1] and just experienced the natural environmental changes during the experiment. On the other hand, the main experiment pond, Pond E6, which was diluted as described below, was let to reach only near saturation conditions (i.e., 33.6% of salts) 2 weeks prior the experimental treatment (dilution) as opposed to salt-saturated conditions in order to minimize the effort required to dilute a salt-saturated pond with precipitated crystals.…”

“…), which are dynamic throughout the year [18]. Nonetheless, the microbial communities of these saltern systems are resilient to dilution and other stresses as evidenced by the fact that the community composition has remained similar during the 20 years that the Es Trenc brines have been studied [1,17,19,20]. This stability indicates that these highly adapted assemblages have developed intrinsic strategies to maintain their diversity throughout the year in this dramatically fluctuating system.…”

“…Prokaryotic species are very diverse, e.g., some of them harbor more unique genes than the human genome [1,2], while the number of distinct species on Earth has been estimated between several millions to a trillion [3]. Understanding the mechanisms generating and/or maintaining this biodiversity is one of the important challenges in microbial ecology.…”

“…To test these hypotheses, it is also important to study the populations in situ, where the corresponding genes that underlie the mechanisms and/or speciation events can be better elucidated in response to perturbations or natural stimuli compared to the (typically) artificial conditions under laboratory settings. We have recently demonstrated that crystallizer ponds of solar salterns are excellent seminatural systems at the mesocosm scale for use in testing ecological theories on resilience and resistance [1], and demonstrated that microbial communities highly resistant and resilient to change driven by irradiation and salinity deterministically establish in these ponds. An additional advantage of the solar salterns is their relatively low microbial diversity that appears to be at equilibrium, meaning that the less-adapted species have been outcompeted [15].…”