Relationship between carbon and nitrogen metabolism in the marine photosynthetic picoeukaryote Ostreococcus tauri.

G.F. CALÓ; H.G. PONTIS; G.L. SALERNO

San Miguel de Tucumán

Congreso; VII Congreso Argentino de Microbiología General; 2011

Universidad Nacional de Tucumán

  Picoeukaryotes are a taxonomically diverse group of microorganisms with less than 2 micrometers in diameter. In this group photosynthetic picoeukaryotes (PPEs) are widely distributed from temperate to polar marine habitats, being significant contributors to primary production. PPEs, Prasinophytes are marine planktonic green algae with a phylogenetic position branching near the base of the green lineage that evolved after the endosymbiosis event that gave rise to photosynthetic eukaryotes. Marine PPEs of the genus Ostreococcus thrive in ecosystems ranging from oligotrophic to eutrophic environments and could be key organisms for biogeochemical fluxes of modern oceans during climate change. In the green lineage the most important soluble disaccharide is Sucrose (Suc), which mainly occurs in oxygenic photosynthetic organisms, including land plants, unicellular algae and cyanobacteria. Trehalose (Tre) is another important non-reductant disaccharide distributed in nature, is accumulated mainly in bacteria, fungi, and arthropods, but not in land plants. In addition to its role as energy and carbon source, accumulation of Tre has been associated with improved stress tolerance towards both biotic and abiotic stress situations. Its role in the green lineage is still controversial. The aim of this study was to investigate which are the important soluble carbohydrates in O. tauri and its relationship with nitrogen assimilation. Preliminary experiments showed that Tre but not Suc is the main soluble sugar in this Prasinophyte. Tre content increases in O. tauri cells grown under nitrogen deficiency in a time-dependent manner. Total reducing sugars and starch levels were measured in parallel. An opposite pattern was observed with total sugars reducing content. On the other hand, starch content also showed a high increase under nitrogen deficiency. Reversion of that condition restored carbohydrates to basal levels. The enzyme activity responsible of Tre synthesis (TPS, Tre-Phosphate Synthase) and expression at the mRNA level paralleled the accumulation of the disaccharide. We conclude that Tre plays an important physiological role as principal soluble sugar interconnecting carbon and nitrogen metabolism in O. tauri.