Many cyanobacterial morphospecies with hemispherical cells have been assigned to the genus Chroococcus, but increasingly genetic information has shown that the genus is polyphyletic. Here we describe two new genera based on two new species, Inacoccus gen. nov. based on Inacoccus carmineus sp. nov. and Cryptococcum gen. nov. based on Cryptococcum brasiliense sp. nov. Both genera have morphological features typical of the genus Chroococcus and are based on strains isolated from terrestrial habitats of the Brazilian Atlantic Forest. Inacoccus differs from Chroococcus in forming nanocytes and by producing an intensely red-coloured sheath. Cryptococcum is a cryptic genus and two species, C. brasiliense and C. komarkovaum, have been recognized by a molecular approach (16 rRNA and 16S-23S ITS sequences). The 16S rRNA gene sequence phylogeny indicates that these proposed genera form monophyletic clades apart from the Chroococcus sensu stricto cluster. Additionally, the tropical species Chroococcus subviolaceus was studied by molecular methods for the first time. The results confirmed a vast phylogenetic diversity hidden under the simple morphology of Chroococcus morphotypes, and show that terrestrial environments are a promising field of research.

For decades, marine plankton have been investigated for their capacity to modulate biogeochemical cycles and provide fishery resources. Between the sunlit (epipelagic) layer and the deep dark waters, lies a vast and heterogeneous part of the ocean: the mesopelagic zone. How plankton composition is shaped by environment has been well-explored in the epipelagic but much less in the mesopelagic ocean. Here, we conducted comparative analyses of trans-kingdom community assemblages thriving in the mesopelagic oxygen minimum zone (OMZ), mesopelagic oxic, and their epipelagic counterparts. We identified nine distinct types of intermediate water masses that correlate with variation in mesopelagic community composition. Furthermore, oxygen, NO₃⁻ and particle flux together appeared as the main drivers governing these communities. Novel taxonomic signatures emerged from OMZ while a global co-occurrence network analysis showed that about 70% of the abundance of mesopelagic plankton groups is organized into three community modules. One module gathers prokaryotes, pico-eukaryotes and Nucleo-Cytoplasmic Large DNA Viruses (NCLDV) from oxic regions, and the two other modules are enriched in OMZ prokaryotes and OMZ pico-eukaryotes, respectively. We hypothesize that OMZ conditions led to a diversification of ecological niches, and thus communities, due to selective pressure from limited resources. Our study further clarifies the interplay between environmental factors in the mesopelagic oxic and OMZ, and the compositional features of communities.

For decades, marine plankton have been investigated for their capacity to modulate biogeochemical cycles and provide fishery resources. Between the sunlit (epipelagic) layer and the deep dark waters, lies a vast and heterogeneous part of the ocean: the mesopelagic zone. How plankton composition is shaped by environment has been well-explored in the epipelagic but much less in the mesopelagic ocean. Here, we conducted comparative analyses of trans-kingdom community assemblages thriving in the mesopelagic oxygen minimum zone (OMZ), mesopelagic oxic, and their epipelagic counterparts. We identified nine distinct types of intermediate water masses that correlate with variation in mesopelagic community composition. Furthermore, oxygen, NO ^- and particle flux together appeared as the main drivers governing these communities. Novel taxonomic signatures emerged from OMZ while a global co-occurrence network analysis showed that about 70% of the abundance of mesopelagic plankton groups is organized into three community modules. One module gathers prokaryotes, pico-eukaryotes and Nucleo-Cytoplasmic Large DNA Viruses (NCLDV) from oxic regions, and the two other modules are enriched in OMZ prokaryotes and OMZ pico-eukaryotes, respectively. We hypothesize that OMZ conditions led to a diversification of ecological niches, and thus communities, due to selective pressure from limited resources. Our study further clarifies the interplay between environmental factors in the mesopelagic oxic and OMZ, and the compositional features of communities.

Organismal life cycles are influenced by Earth’s rotation and orbit, generating daily and seasonal light cycles that vary with latitude, especially in temperate and polar zones. Photoperiodism relies on organisms’ ability to measure time via the circadian clock and detect light through specific photoreceptors. Molecular basis of photoperiodism is well-characterized in plants, but photoperiod adaptation in phytoplankton remain largely unexplored. Here, we investigated circadian clock components, photoreceptors, and associated effectors in eukaryote picoalga species from Ostreococcus, Bathycoccus, and Micromonas. We showed that the investigated species shared a conserved set of homologous circadian clock-related genes that appeared in the early evolution of Mamielalles order. Furthermore, gene duplication events account for the specific occurrences and uneven gene copy numbers among these genera. Through metagenomic and metatranscriptomic analyses, we assessed the gene expression profiles of candidate photoperiod-related genes across the global ocean. Our findings reveal an unexpected diversity in photoreceptors, particularly within Micromonas, and highlight the CCT domain family, a key group of transcription factors governing circadian rhythms (TOC1 family) and photoperiodism (CONSTANS family) in plants. TOC1, a central component of the circadian clock in Ostreococcus tauri, is either absent or truncated in tropical species. Functional assays further indicate that the TOC1/CCA1 oscillator is nonfunctional in the tropical strain of Ostreococcus sp. RCC809. These results imply that certain circadian mechanisms may be dispensable at low latitudes, underscoring the diversity of photoperiod adaptations in marine phytoplankton. These results provide valuable insights into the molecular evolution of cosmopolitan plankton groups, particularly their mechanisms of local adaptation.