María Cristina Casero

The cyanobacteria management in water bodies requires a deep knowledge of the community composition. Considering the reliable and thorough information provided by the polyphasic approach in cyanobacteria taxonomy, here we assess the cyanobacterial community structure of the Cheffia reservoir from Algeria. Cyanobacteria were identified on the basis of morphological traits and next-generation sequencing (NGS); toxins-related genes were localized in addition to the identification of toxins; temperature and nutrient level of water samples were also determined. The polyphasic approach was essential for cyanobacteria investigation; 28 genera were identified through 16S rRNA metabarcoding with the dominance of taxa from Microcystis (34.2%), Aphanizomenon (20.1%), and Planktothrix (20.0%), and morphological analysis revealed the association in this water body of five species within the genus Microcystis: M. aeruginosa, M. novacekii, M. panniformis, M. ichthyoblabe, and M. flos-aquae. The presence of mcyE genotypes was detected; moreover, HPLC–PDA and LC–ESI–MS/MS revealed the production of microcystin-LR. Results obtained in our study are very important since this ecosystem is used for water supply and irrigation; as a consequence, a good water management plan is essential.

Endolithic microhabitats have been described as the last refuge for life in arid and hyper-arid deserts where life has to deal with harsh environmental conditions. A number of rock substrates from the hyper-arid Atacama Desert, colonized by endolithic microbial communities such as halite, gypsum crusts, gypcrete, calcite, granite and ignimbrite, have been characterized and compared using different approaches. In this work, three different endolithic microhabitats are described, each one with a particular origin and architecture, found within a lithic substrate known as gypcrete. Gypcrete, an evaporitic rock mainly composed of gypsum (CaSO₄ ⋅ 2H₂O) and collected in the Cordón de Lila area of the desert (Preandean Atacama Desert), was found to harbour cryptoendolithic (within pore spaces in the rock), chasmoendolithic (within cracks and fissures) and hypoendolithic (within microcave-like pores in the bottom layer of rock) microhabitats. A combination of microscopy investigation and high-throughput sequencing approaches were used to characterize the endolithic communities and their habitats at the microscale within the same piece of gypcrete. Microscopy techniques revealed differences in the architecture of the endolithic microhabitats and the distribution of the microorganisms within those microhabitats. Cyanobacteria and Proteobacteria were dominant in the endolithic communities, of which the hypoendolithic community was the least diverse and hosted unique taxa, as a result of less access to sun radiation. These results show, for the first time, that the differences in the architecture of a microhabitat, even within the same piece of a lithic substrate, play an essential role in shaping the diversity and composition of endolithic microbial communities.

Cyanobacteria exposed to high solar radiation make use of a series of defense mechanisms, including avoidance, antioxidant systems, and the production of photoprotective compounds such as scytonemin. Two cyanobacterial strains of the genus Chroococcidiopsis from the Atacama Desert – which has one of the highest solar radiation levels on Earth- were examined to determine their capacity to protect themselves from direct photosynthetically active (PAR) and ultraviolet radiation (UVR): the UAM813 strain, originally isolated from a cryptoendolithic microhabitat within halite (NaCl), and UAM816 strain originally isolated from a chasmoendolithic microhabitat within calcite (CaCO₃). The oxidative stress induced by exposure to PAR or UVR + PAR was determined to observe their short-term response, as were the long-term scytonemin production, changes in metabolic activity and ultrastructural damage induced. Both strains showed oxidative stress to both types of light radiation. The UAM813 strain showed a lower acclimation capacity than the UAM816 strain, showing an ever-increasing accumulation of reactive oxygen species (ROS) and a smaller accumulation of scytonemin. This would appear to reflect differences in the adaptation strategies followed to meet the demands of their different microhabitats.

The Atacama Desert is known to be the place on Earth with one of the highest solar radiation limiting the presence of life to endolithic microhabitats and soil microbial ecosystems. Endolithic microbial communities are supported by photosynthetic primary producers, mainly cyanobacteria, which can be injured by UVR. Nevertheless, cyanobacteria exposed to high solar radiation and its harmful effects have developed a series of defense mechanisms: avoidance, antioxidant systems or production of photoprotective compounds such as scytonemin among others. Scytonemin is a liposoluble pigment whose absorption maxima are located in UVA and UVC range and highly absorbing in the UVB range. In order to elucidate the protection capacity of endolithic cyanobacteria against harmful radiation, two cyanobacterial strains from Chroococcidiopsis genus were isolated from different endolithic microhabitats in the Atacama Desert: UAM813 strain, originally from the cryptoendolithic microhabitat of halite (NaCl), and UAM816 strain from chasmoendolithic microhabitat of calcite (CaCO ₃ ). Both were exposed to PAR and UVR+PAR conditions studying their short-term response, as oxidative stress and long-term response, as scytonemin production, metabolic activity and ultrastructural damage. The observed response of both strains reveals a high sensitivity to direct light exposure, even to PAR. The differences in their acclimation suggest specific adaptation strategies related to their original microhabitat, revealing their protective potential and the strain specific environmental pressure selection to inhabit different microhabitats and exposed to different light conditions. Importance: Cyanobacteria are photosynthetic prokaryotes that inhabit most types of illuminated environments, even the endolithic microhabitats in cold and hot deserts. The environmental pressure caused by the extreme solar irradiation in the Atacama Desert involve that only those cyanobacterial strains able to cope with it can be found in these endolithic communities, usually dominated by members belonging to the extremotolerant Chroococcidiopsis genus. Here, a comprehensive analysis of multiple lines of defense against harmful sun radiation was conducted to diagnose the response of two Chroococcidiopsis strains isolated from different endolithic microhabitats and lithic substrates, and identify its relation with the original microenvironmental conditions of each strain. Our results contribute to a better understanding of the acclimation strategies developed by these cyanobacterial strains and its potential protective role for the whole endolithic microbial community.

Endolithic microhabitats have been described as the last refuge for life in arid and hyper-arid deserts where life has to deal with harsh environmental conditions. A number of rock substrates from the hyper-arid Atacama Desert, colonized by endolithic microbial communities, such as halite, gypsum crusts, gypcrete, calcite, granite and ignimbrite, have been characterized and compared using different approaches. In this work, three different endolithic microhabitats are described, each one with a particular origin and architecture, found within a lithic substrate known as gypcrete. Gypcrete, an evaporitic rock mainly composed of gypsum (CaSO₄ ⋅ 2H₂O) and collected in the Cordón de Lila area of the desert (Preandean Atacama Desert), was found to harbour cryptoendolithic (within pore spaces in the rock), chasmoendolithic (within cracks and fissures) and hypoendolithic (within microcave-like pores in rock-bottom layer) microhabitats. A combination of microscopy investigations strategies and high-throughput sequencing approaches were used to characterize the endolithic communities at the microscale in these microhabitats within the same piece of lithic substrate. Microscopy techniques revealed differences in the architecture of the endolithic microhabitats and in the distribution of the microorganisms within those microhabitats. Cyanobacteria and Proteobacteria were dominant in the endolithic communities, of which the hypoendolithic community was the least diverse and hosted unique taxa. These results show, for the first time, that the differences in the architecture of a microhabitat, even within the same piece of lithic substrate, might be an essential factor in shaping the diversity and composition of endolithic microbial communities.