Characterization of newly isolated photosynthetic marine pico green algae (Picochlorum, Trebouxiophyceae) from the Adriatic Sea

Photosynthetic picoeukaryotes represent smallest eukaryotic phytoplankton fraction of the world's seas and oceans, greatly contributing in carbon flluxes, biogeochemical cycles and overall primary production. During BIOTA (Bio-tracing Adriatic Water Masses) cruise in February 2015, strain of unknown photosynthetic pico-green algae PMFPPE4 was isolated in monoclonal culture and subsequent analyses were perfomed in order for correct taxonomical identification and biotechnology potential characterization. Newly isolated strain of pico-green algae proved to belong to genus Picochlorum (Henly) through morphology (LM and TEM), phylogeny (16S rRNA and 18S rRNA genes) and physiology (growth rate, pigment and lipid composition) characterization. Morphology of the strain correlated with other species within Picochloum genus, having round to oval cells from 2 to 3 µm in diameter, U-shaped chloroplast filling approximately two thirds of the cells and autospotulation in dividing cells. Additionally, simple ultrastructural features revealed visible in TEM (nucleus, mitochondria, U-shaped chloroplas with several starch inclusions). Pigment composition of the strain was congruent with class Trebouxiophyceae, having both chlorophyll a and b, lutein, β-carotene, violaxanthin and neoxanthin, while lipid composition revealed dominant lipid classes: sulfoquinovosyldiacylglycerols, digalactosyldiacylglycerols, monogalactosyldiacylglycerols, phosphatidylglycerols and phosphatidylcholines. During 20-days growth rate experiment, strain PMFPPE4 showed acclimatization and steady growth during first 7 days, followed by exponential phase during second week and entered stationary phase after two weeks. This study represent important finding to this underapreciated and taxonomically unresolved marine genus Piochlorum.