Giulia Cheloni

Effects of organic contaminants (OCs) on phytoplankton physiology were extensively studied in the last years while a knowledge gap exists regarding the ability of phytoplankton to transform OCs. Knowledge about biotransformation pathways in these organisms lag far behind that of other microorganisms. A better understanding of biotransformation pathways would help identify biomarkers of contaminants exposure, improve microalgae-based water remediation strategies and help better assess contaminants persistence and trophic transfer in natural aquatic environments. The present study investigated diclofenac (DCF) physiological effects, transcriptional responses and metabolism in the marine diatom Phaeodactylum tricornutum with the aim of getting an insight on detoxifications pathways. P. tricornutum did not result in significant removal capacity of DCF from the exposure medium. Bioconcentration factors varied depending on the exposure concentration (3.9 and 2.7 for 1.5 mg L ^-1 and 10 mg L ^-1 DCF respectively) but remained relatively low. DCF resulted in mild physiological effects on P. tricornutum but gene expression analysis indicated that multiple molecular functions and biological processes were altered by DCF exposure. Transcriptomic analysis suggested increased nutrients and energy requirements possibly associated with the contaminant stress and detoxification metabolism. CYP gene expression was not significantly regulated upon DCF exposure but 4’-Hydroxy Diclofenac (OH-DCF), a metabolite generally associated with CYP enzymatic activity, was detected. However, CYP gene expression was not significantly regulated upon DCF exposure. Five additional DCF metabolites with high molecular weight were detected. These metabolites were not previously described in the literature and were suggested to be generated via amino acid (or peptides) conjugation. Gene ontology analysis indicated that amino acid and peptide biosynthetic pathways were regulated upon DCF exposure supporting a possible correlation between organic contaminants detoxification responses and amino acid and protein content in phytoplankton cells. Our findings contribute to highlighting the diversity of biotransformation pathways in phytoplankton and provide mechanistic information about contaminants detoxification.

Phytoplankton are characterized by a great phenotypic plasticity and amazing morphological variability, both playing a primary role in the acclimation to changing environments. However, there is a knowledge gap concerning the role of algal morphological plasticity in stress responses and acclimation to micropollutants. The present study aims examining the palmelloid colony formation of the green alga Chlamydomonas reinhardtii upon micropollutants exposure. Cells were exposed to four micropollutants (MPs) with different modes of action (copper, cadmium, PFOS and paraquat) for a duration of 72h. Effects of MPs on palmelloid formation, growth and physiological traits (chlorophyll fluorescence, membrane integrity and oxidative stress) were monitored via flow cytometry and fluorescence microscopy. Palmelloid formation was observed upon treatment with the four micropollutants. Number of palmelloid colonies and their size were dependent on MP concentration and exposure duration. Cells reverted to their unicellular lifestyle when colonies were harvested and inoculated in fresh medium indicating that palmelloid formation is a plastic response to micropollutants. No physiological effects of these compounds were observed in cells forming palmelloids and palmelloid colonies accumulated lower Cd concentration than unicellular C. reinhardtii suggesting that colony formation protects the cells form MPs exposure. The results show that colony formation in Chlamydomonas reinhardtii is a stress response strategy activated to face sub-lethal micropollutant concentrations. HIGHLIGHTS: Sub-lethal concentrations of micropollutants (MPs) induce palmelloid formation in C. reinhardtii Morphological changes are not associated to adverse effects on algal cells Palmelloid formation is transitory, cells revert to unicellular lifestyle in the absence of MPs Cells within large colonies experience lower Cd exposure than unicellular C. reinhardtii Palmelloid formation is a morphological stress response that plays a role in cells acclimation to MPs

Trace metals (TMs) have a central role in the functioning of aquatic and terrestrial ecosystems.

In the present review, we critically examine the state-of-the-art of the research on combined effects of trace metals and light on photosynthetic microorganisms in aquatic environment. Light of different intensity and spectral composition affects the interactions between trace metals and photosynthetic microorganisms directly, by affecting vital cellular functions and metal toxicokinetics and toxicodynamics, and indirectly, by changing ambient medium characteristics. Light radiation and in particular, the ultraviolet radiation component (UVR) alters the structure and reactivity of dissolved organic matter in natural water, which in most of the cases decreases its metal binding capacity and enhances metal bioavailability. The increase of cellular metal concentrations is generally associated with increasing light intensity, however further studies are necessary to better understand the underlying mechanisms. Studies on the combined exposures of photosynthetic microorganisms to metals and UVR reveal antagonistic, additive or synergistic interactions depending on light intensity, spectral composition or light pre-exposure history. Among the light spectrum components, most of the research was performed with UVR, while the knowledge on the role of high-intensity visible light and environmentally relevant solar light radiation is still limited. The extent of combined effects also depends on the exposure sequence and duration, as well as the species-specific sensitivity of the tested microorganisms and the activation of stress defense responses.

High intensity visible light and ultraviolet radiation (UVR) could trigger oxidative stress (OS) and pigment bleaching in photosynthetic organisms and thus might represent a serious threat for cell survival. In the present study, green alga Chlamydomonas reinhardtii and cyanobacterium Synechocystis sp. were exposed to solar simulated light with different spectral composition: (i) visible light, (ii) visible light with environmental level of UVR, (iii) visible light with enhanced ultraviolet B (UVB) radiation. Photo-induced increase of the cellular reactive oxygen species (ROS), OS, and damage, as well as the alterations of the cell autofluorescence were thoroughly examined using flow cytometry. A small percentage of the cell population was affected by visible light and light with environmental levels of UVR. Exposure to light with enhanced UVB radiation resulted in severe OS, lipid peroxidation, and membrane damage. Light with enhanced UVB also induced a shift of the photosynthetic pigment fluorescence towards low values. C. reinhardtii was more sensitive to UVB as compared with Synechocystissp.The obtained results highlight the considerable difference in the levels of OS induced by visible, UVA, and UVB radiation in both species, as well as the species-specific sensitivity to UVB radiation, which should be taken into account when assessing the impact of enhanced UVB radiation on natural phytoplankton communities.

Summary: Algal cultures are usually co‐cultures of algae and bacteria, especially when considering outdoor mass cultivation. The influence of associated bacteria on algal culture performance has been poorly investigated, although bacteria may strongly affect biomass (or derived product) yield and quality. In this work, the influence on growth and productivity of Tetraselmis suecica F&M‐M33 of bacterial communities and single bacterial isolates from the algal phycosphere was investigated. Xenic laboratory and outdoor cultures were compared with an axenic culture in batch. The presence of the bacterial community significantly promoted culture growth. Single bacterial isolates previously found to be strictly associated with T. suecica F&M‐M33 also increased growth compared with the axenic culture, whereas loosely associated and common seawater bacteria induced variable growth responses, from positive to detrimental. The increased growth was mainly evidenced as increased algal biomass production and cell size, and occurred after exhaustion of nutrients. This finding is of interest for biofuel production from microalgae, often attained through nutrient starvation processes leading to oil or carbohydrate accumulation. As axenic T. suecica F&M‐M33 showed a similar growth with or without vitamins, the most probable mechanism behind bacterial positive influence on algal growth seems nutrient recycling.