Photosynthetic picoeukaryotes (PPE) are recognized as major primary producers and contributors to phytoplankton biomass in oceanic and coastal environments. Molecular surveys indicate a large phylogenetic diversity in the picoeukaryotes, with members of the Prymnesiophyceae and Chrysophyseae tending to be more common in open ocean waters and Prasinophyceae dominating coastal and Arctic waters. In addition to their role as primary producers, PPE have been identified in several studies as mixotrophic and major predators of prokaryotes. Mixotrophy, the combination of photosynthesis and phagotrophy in a single organism, is well established for most photosynthetic lineages. However, green algae, including prasinophytes, were widely considered as a purely photosynthetic group. The prasinophyte Micromonas is perhaps the most common picoeukaryote in coastal and Arctic waters and is one of the relatively few cultured representatives of the picoeukaryotes available for physiological investigations. In this study, we demonstrate phagotrophy by a strain of Micromonas (CCMP2099) isolated from Arctic waters and show that environmental factors (light and nutrient concentration) affect ingestion rates in this mixotroph. In addition, we show size-selective feeding with a preference for smaller particles, and determine P vs I (photosynthesis vs irradiance) responses in different nutrient conditions. If other strains have mixotrophic abilities similar to Micromonas CCMP2099, the widespread distribution and frequently high abundances of Micromonas suggest that these green algae may have significant impact on prokaryote populations in several oceanic regimes.

Antarctic phototrophs are challenged by extreme temperatures, ice cover, nutrient limitation, and prolonged periods of darkness. Yet this environment may also provide niche opportunities for phytoplankton utilizing alternative nutritional modes. Mixotrophy, the combination of photosynthesis and particle ingestion, has been proposed as a mechanism for some phytoplankton to contend with the adverse conditions of the Antarctic. We conducted feeding experiments using fluorescent bacteria-sized tracers to compare the effects of light and nutrients on bacterivory rates in three Antarctic marine photosynthetic nanoflagellates representing two evolutionary lineages: Cryptophyceae (Geminigera cryophila) and Prasinophyceae (Pyramimonas tychotreta and Mantoniella antarctica). Only G. cryophila had previously been identified as mixotrophic. We also measured photoautotrophic abilities over a range of light intensities (P vs. I) and used dark survival experiments to assess cell population dynamics in the absence of light. Feeding behavior in these three nanoflagellates was affected by either light, nutrient levels, or a combination of both factors in a species-specific manner that was not conserved by evolutionary lineage. The different responses to environmental factors by these mixotrophs supported the idea of tradeoffs in the use of phagotrophy and phototrophy for growth.

The halotolerant alga Dunaliella salina is a model for stress tolerance and is used commercially for production of beta-carotene (=pro-vitamin A). The presented draft genome of the genuine strain CCAP19/18 will allow investigations into metabolic processes involved in regulation of stress responses, including carotenogenesis and adaptations to life in high-salinity environments.

BackgroundThe green microalga Dunaliella salina accumulates a high proportion of β-carotene during abiotic stress conditions. To better understand the intracellular flux distribution leading to carotenoid accumulation, this work aimed at reconstructing a carbon core metabolic network for D. salina CCAP 19/18 based on the recently published nuclear genome and its validation with experimental observations and literature data.ResultsThe reconstruction resulted in a network model with 221 reactions and 212 metabolites within three compartments: cytosol, chloroplast and mitochondrion. The network was implemented in the MATLAB toolbox CellNetAnalyzer and checked for feasibility. Furthermore, a flux balance analysis was carried out for different light and nutrient uptake rates. The comparison of the experimental knowledge with the model prediction revealed that the results of the stoichiometric network analysis are plausible and in good agreement with the observed behavior. Accordingly, our model provides an excellent tool for investigating the carbon core metabolism of D. salina.ConclusionsThe reconstructed metabolic network of D. salina presented in this work is able to predict the biological behavior under light and nutrient stress and will lead to an improved process understanding for the optimized production of high-value products in microalgae.