Molecular and morphological convergence to sulfide-tolerant fishes in a new species of Jenynsia (Cyprinodontiformes: Anablepidae), the first extremophile member of the family

Freshwater sulfide springs have extreme environmental conditions that only few vertebrate species can tolerate. These species often develop a series of morphological and molecular adaptations to cope with the challenges of life under the toxic and hypoxic conditions of sulfide springs. In this paper, we described a new fish species of the genus Jenynsia, Anablepidae, from a sulfide spring in Northwestern Argentina, the first in the family known from such extreme environment. Jenynsia sulfurica n. sp. is diagnosable by the lack of scales on the pre-pelvic area or the presence of a single row of scales, continuous or not, from the isthmus to the bases of the pelvic fins. Additionally, it presents a series of morphological and molecular characteristics that appear convergent with those seen in other fish species (e.g., Poeciliids) inhabiting sulfide springs. Most notably, J. sulfurica has an enlarged head and postorbital area compared to other fish of the genus and a prognathous lower jaw with a hypertrophied lip, thought to facilitate respiration at the air-water interface. Analyses of cox1 sequence showed that J. sulfurica has two unique mutations resulting in amino acid substitutions convergent to those seen in Poeciliids from sulfide springs and known to provide a physiological mechanism related to living in sulfide environments. A phylogenetic analysis, including molecular and morphological characters, placed J. sulfurica as sister taxa to J. alternimaculata, a species found in nearby, non-sulfide habitats directly connected to the sulfide springs. Thus, it can be inferred that the selection imposed by the presence of H2S has resulted in the divergence between these two species and has potentially served as a barrier to gene flow.

Transcriptional response of the extremophile red alga Cyanidioschyzon merolae to changes in CO2 concentrations

Cyanidioschyzon merolae (C. merolae) is an acidophilic red alga growing in a naturally low carbon dioxide (CO₂) environment. Although it uses a ribulose 1,5-bisphosphate carboxylase/oxygenase with high affinity for CO₂, the survival of C. merolae relies on functional photorespiratory metabolism. In this study, we quantified the transcriptomic response of C. merolae to changes in CO₂ conditions. We found distinct changes upon shifts between CO₂ conditions, such as a concerted up-regulation of photorespiratory genes and responses to carbon starvation. We used the transcriptome data set to explore a hypothetical CO₂ concentrating mechanism in C. merolae, based on the assumption that photorespiratory genes and possible candidate genes involved in a CO₂ concentrating mechanism are co-expressed. A putative bicarbonate transport protein and two α-carbonic anhydrases were identified, which showed enhanced transcript levels under reduced CO₂ conditions. Genes encoding enzymes of a PEPCK-type C₄ pathway were co-regulated with the photorespiratory gene cluster. We propose a model of a hypothetical low CO₂ compensation mechanism in C. merolae integrating these low CO₂-inducible components.