Salts in general, and particularly sodium chloride, have played an important role in human history, not only because of their nutritional value but also because of their applications in several industrial processes and their impact on agricultural practices. Hypersaline lakes, salt marshes, hot springs, saline ponds and salt deposits are distributed throughout the world and constitute natural resources from which salt has been extracted for many centuries. For geologists, some of these environments represent a model system to study salt precipitation and related biogeochemical processes in shallow evaporitic environments [1
In warm and arid areas, salt can be recovered from seawater thanks to several procedures that involve solar salterns. The solar salterns are filled with seawater that is concentrated gradually by the effects of the wind and temperature. Thus, solar salterns are restricted to areas such as Mediterranean regions, where the climate is characterized by periods during which evaporation exceeds precipitation. This process is especially rapid in summertime when highly salted water flows through an increasingly concentrated pool until ponds crystallize.
In addition to a high salt concentration, the environment of solar salterns is also characterized by its pH (range from 6 up to 11). Although halophilic microorganisms living in these environments are distributed among all three domains of life, it has been extensively reported that members of the Halobacteriaceae
family constitute the dominant microbial population, especially in those environments where the NaCl concentration ranges from 20% (w/v) up to halite saturation (< 32% (w/v)) [2
]. In fact, the red color associated with hypersaline lakes and ponds is mainly due to the pigmentation of halophilic archaea and the eukaryote Dunaliella
]. However, this does not mean that these are the only inhabitants contributing to the red color of these environments. Populations of halophilic bacteria and other members of Eukarya are also present in saltern ponds [7
]. To thrive under these conditions, halophilic archaea accumulate potassium ions inside their cells to balance the high salt content of the environment. This ability differentiates them from halophilic bacteria that usually accumulate compatible solutes (betaine, ectoine) to counteract the high external salt concentrations [9
]. Nevertheless, it has been found that some halophilic bacteria such as Salinibacter
use the same strategy as haloarchaea to cope with osmotic stress [7
]. Because halophilic archaea are the predominant microorganisms in hot and hypersaline environments, it is possible that they sustain key metabolic cycles under these conditions; indeed the understanding of life under such extreme circumstances has become a key area of research recently [12
By amplifying 16S rDNA sequences directly from environmental samples, halophilic archaea have been detected in different hypersaline environments such as the crystallizer ponds of a marine saltern [10
], alkaline soda lakes [15
] or sediments of hypersaline Antarctic lakes and coastal salt marshes [16
]. Microbial research in saline environments is important for several reasons: i) there are potential biotechnological applications of halophilic microorganisms [18
]; ii) knowledge about microbial diversity in terrestrial saline environments may shed light on the properties of salt deposits and saline environments found on Mars [20
]; and iii) primitive life on earth might have started in this kind of extreme environment, so these systems are ideal to understand the evolution of the biosphere on Earth [22
Regarding the biodiversity of these halophilic environments, it is also very interesting to analyze the nature of the plant communities because of the various mechanisms that they have developed to cope with such extreme saline conditions. Plant growth is highly limited by salty soil conditions. The soils surrounding solar salterns are characterized by a high salt concentration. This places limitations on plant growth because the soil's osmotic potential is low, making it difficult for the plants to extract water from the soil (hydric stress situations) and in order to get water throughthe roots, the plant's internal osmotic potential must be lower than that of the soil. Consequently, plants have developed mechanisms to adapt to high salt levels in soil.
The exploitation of saltworks has become a very successful industry, with large scale production in southeastern Spain where Solvay developed a method to produce caustic soda from NaCl in 1817 [23
]. Salt obtained from plants has been used since the second half of the nineteenth century for the production of glass, dyes and soap. Although these plants are no longer used in industry today, they still have a great environmental value. Some irrigation methods, along with increased desertification in arid and semiarid regions, resulting in increases soil salinization. In response to these phenomena, some scientists have studied halophilic plants as a tool to manage soils that have undergone the salinization processes [24
The United Nations' environmental program focuses on desertification processes and has drawn attention to the use of halophilic vegetation to restore degraded lands, provide food for livestock and fix some atmospheric carbon dioxide. In most cases, the aim is not to remove the salt from the ground so much as it is to allow for some tracts of land to be useful again. Planting halophilic species could be very useful in areas where salinization has been caused by human activities (thus making it impossible to sustain traditional agricultural activities). The most interesting aspect of these halophilic plants is that they can be irrigated even with sea water. In any case, a good vegetation cover protects the underlying soil against erosion, improving its structure and assisting drainage processes against capillary ascent [24
In Alicante County (Spanish Mediterranean coast) there are many examples of well-characterized coastal solar salterns from which salt is extracted for human consumption [30
]. However, the inland solar salterns located in this county remain poorly described to this day [31
]. In the inland solar salterns, the groundwater interacts with underground salt deposits, resulting in an increase in water salinity. The saline water is then pumped into the ponds, where salt is extracted in a manner similar to that used in coastal solar salterns.
The aim of this study was to describe general environmental aspects of two of the three inland solar salterns located in the municipality of Villena (northwest Alicante County, Figure ). We have paid special attention to the ecological parameters and the properties of the collected brine; we have also assessed the Archaeal diversity of the crystallization ponds using (PCR)-based molecular phylogenetic approaches. Properties of the plant communities surrounding these inland solar salterns have also been studied in detail. Recently, this area has been catalogued as a "Wetland of interest" in the Alicante province http://www.cth.gva.es/areas/espacios/zonas_humedas/zon/Ficha-35.PDF
, as it constitutes an optimum environment for a number of birds and salt marsh plant communities.
Geographical location of the Alto Vinalopó Valley, Alicante, Spain (A). The three salterns are located at the margins of the old lagoon, called "La Laguna": 1, Fortuna; 2, Redonda; and 3, Penalva (B). Scale in panel B is 1:25,000.