Differential acclimation kinetics of the two forms of type IV chromatic acclimaters occurring in marine Synechococcus cyanobacteria

Synechococcus, the second most abundant marine phytoplanktonic organism, displays the widest variety of pigment content of all marine oxyphototrophs, explaining its ability to colonize all spectral niches occurring in the upper lit layer of oceans. Seven Synechococcus pigment types (PTs) have been described so far based on the phycobiliprotein composition and chromophorylation of their light-harvesting complexes, called phycobilisomes. The most elaborate and abundant PT (3d) in the open ocean consists of cells capable of type IV chromatic acclimation (CA4), i.e., to reversibly modify the ratio of the blue light-absorbing phycourobilin (PUB) to the green light-absorbing phycoerythrobilin (PEB) in phycobilisome rods to match the ambient light color. Two genetically distinct types of chromatic acclimaters, so-called PTs 3dA and 3dB, occur at similar global abundance in the ocean, but the precise physiological differences between these two types and the reasons for their complementary niche partitioning in the field remain obscure. Here, photoacclimation experiments in different mixes of blue and green light of representatives of these two PTs demonstrated that they differ by the ratio of blue-to-green light required to trigger the CA4 process. Furthermore, shift experiments between 100% blue and 100% green light, and vice-versa, revealed significant discrepancies between the acclimation pace of the two types of chromatic acclimaters. This study provides novel insights into the finely tuned adaptation mechanisms used by Synechococcus cells to colonize the whole underwater light field

Comparative thermophysiology of marine synechococcus CRD1 strains isolated from different thermal niches in iron-depleted areas

Marine Synechococcus cyanobacteria are ubiquitous in the ocean, a feature likely related to their extensive genetic diversity. Amongst the major lineages, clades I and IV preferentially thrive in temperate and cold, nutrient-rich waters, whilst clades II and III prefer warm, nitrogen or phosphorus-depleted waters. The existence of such cold (I/IV) and warm (II/III) thermotypes is corroborated by physiological characterization of representative strains. A fifth clade, CRD1, was recently shown to dominate the Synechococcus community in iron-depleted areas of the world ocean and to encompass three distinct ecologically significant taxonomic units (ESTUs CRD1A-C) occupying different thermal niches, suggesting that distinct thermotypes could also occur within this clade. Here, using comparative thermophysiology of strains representative of these three CRD1 ESTUs we show that the CRD1A strain MITS9220 is a warm thermotype, the CRD1B strain BIOS-U3-1 a cold temperate thermotype, and the CRD1C strain BIOS-E4-1 a warm temperate stenotherm. Curiously, the CRD1B thermotype lacks traits and/or genomic features typical of cold thermotypes. In contrast, we found specific physiological traits of the CRD1 strains compared to their clade I, II, III, and IV counterparts, including a lower growth rate and photosystem II maximal quantum yield at most temperatures and a higher turnover rate of the D1 protein. Together, our data suggests that the CRD1 clade prioritizes adaptation to low-iron conditions over temperature adaptation, even though the occurrence of several CRD1 thermotypes likely explains why the CRD1 clade as a whole occupies most iron-limited waters

Phytoplancton et couleur de l'eau : analyse multi-échelles de l'avantage adaptatif conféré par l'acclimatation chromatique chez les cyanobactéries marines

Global change is predicted to have numerous impacts on the physico-chemical properties of the ocean. This includes a rapid expansion of warm, nutrient-poor areas that in turn alters the underwater light field, and subsequently impacts the competition between phytoplankton species that have an obligate requirement for light to produce energy. As the second organism in the world’s ocean, and the most diversified with regard to its pigmentation, Synechococcus constitutes an excellent model to assess the consequences of ongoing changes in water optical properties on the distribution, dynamics and composition of marine phytoplanktonic communities. To date, seven Synechococcus pigment types (PTs) have been identified based on the composition and chromophorylation of their light-harvesting complexes, called phycobilisomes. While PTs 3a and 3c have a fixed pigmentation that enable them to efficiently collect either green (GL) or blue light (BL), respectively, PT 3d are capable of Type IV chromatic acclimation (CA4). In other words, they can reversibly modify their phycobilisomes pigmentation in order to capture either green or blue photons, according to the ambient light color. Two genetically distinct types of CA4 strains (PT 3dA and 3dB) have been evidenced and found to be equally abundant in the ocean. Together, they constitute the most prevalent Synechococcus PT at the global scale. However, the reasons for their prevalence in natural populations remain unclear. Both PTs 3dA and 3dB possess one genomic island involved in CA4, with partially distinct gene content (CA4-A and CA4-B islands respectively). While the molecular bases of CA4-A begin to be well understood, CA4-B is less well known. One objective of my thesis was therefore to characterize genes of the CA4-B island using CRISPR-Cas12 technology. Partial characterization of one of these genes (fciB) suggested that the regulation of CA4 process in PT 3dB cells may be reversed compared to that of PT 3dA strains, since FciB appears to act as an activator or a repressor of the process. To investigate phenotypic differences between the two types of CA4 strains, I also examined their response to various conditions of temperature, light quantity and quality. Interestingly, I found that both differ in the ratio of blue to green light required to trigger chromatic acclimation. The second objective consisted in better characterize the advantage conferred by chromatic acclimation over fixed pigmentation. For this purpose, I conducted mono- and co-cultures of PTs 3a, 3c and 3dB representatives in various light qualities and quantities, and demonstrated that blue (PT 3c) and green (PT 3a) light specialists where the best competitors in low BL and GL, respectively. At high light, the CA4 strain wined the competition in BL and the green light specialist in GL, while in a mixture of both lights, co-existence of the CA4 strain and the green light specialist was achieved. Finally, the last objective was to refine our knowledge of the temporal and spatial dynamics of Synechococcus. To do so, I participated in a two-year time series at two oceanographically distinct sites (the SOMLIT-Astan station in the English Channel and the BOUSSOLE station in the Mediterranean Sea). Analysis of the picocyanobacteria cell concentration and ancillary parameters (nutrients availability, pigments, seawater optical characteristics) strongly suggests a seasonal succession of PT 3 subtypes at both sites, with variations in timing and relative proportion of PT 3a, 3c and 3d cells. Upcoming metagenomic analyses should provide further insights into the relative abundance of the different Synechococcus PTs.Les propriétés physico-chimiques des océans sont particulièrement affectées par le changement climatique. Une expansion rapide des zones chaudes et pauvres en nutriments a notamment été remarquée, modifiant la couleur de l’eau et, par conséquent, la compétition entre les diverses espèces du phytoplancton qui ont besoin de lumière pour produire de l’énergie. En tant que deuxième organisme le plus abondant des océans, Synechococcus constitue un excellent modèle pour évaluer les conséquences de l’altération des propriétés optiques de l’eau sur la distribution, la dynamique et la composition des communautés phytoplanctoniques marines. A ce jour, sept types pigmentaires (TPs) ont été identifiés chez Synechococcus sur la base de la composition et de la chromophorylation de leurs complexes collecteurs de lumière, appelés phycobilisomes. Alors que les TPs 3a et 3c ont une pigmentation fixe qui leur permet de collecter efficacement la lumière verte ou bleue, respectivement, le TP 3d est capable d’acclimatation chromatique de Type IV (CA4). En d’autres termes, ce sous-type peut modifier de manière réversible la pigmentation de ses phycobilismes afin de capturer les photons verts ou bleus, en fonction de la couleur de la lumière ambiante. Deux types génétiquement distincts d’acclimateurs chromatiques également abondants dans l’océan (TPs 3dA et 3dB) ont été mis en évidence. Ensemble, ils constituent le TP de Synechococcus le plus répandu à l’échelle globale, bien que les raisons de leur prévalence dans les populations naturelles ne soient pas claires. Les TPs 3dA et 3dB possèdent chacun un îlot génomique impliqué dans la CA4 (îlots CA4-A et CA4-B). Alors que les bases moléculaires de la CA4-A commencent à être bien comprises, celles de la CA4-B sont moins connues. Un des objectifs de ma thèse a donc été de caractériser des gènes de l’îlot CA4-B en utilisant la technologie CRISPR-Cas12. La caractérisation partielle de l’un des gènes (fciB) a suggéré que la régulation de la CA4 chez les TPs 3dB pourrait être inversée par rapport à celle rencontrée chez les TPs 3dA. En effet, FciB semble agir comme un activateur chez l’un, et comme un répresseur chez l’autre. Pour étudier les différences phénotypiques entre les deux types d’acclimateurs chromatiques, j’ai également examiné leur réponse dans diverses conditions de température, quantité et qualité de la lumière. De toute évidence, les TPs 3dA et 3dB diffèrent dans le rapport de lumière bleue et verte nécessaire pour déclencher la CA4. Le deuxième objectif a consisté à mieux caractériser l’avantage conféré par l’acclimatation chromatique par rapport à la pigmentation fixe. Pour cela, j’ai réalisé des mono-cultures et co-cultures de représentants des TPs 3a, 3c et 3dB dans différentes conditions de qualité et de quantité de lumière. J’ai démontré que les spécialistes de la lumière bleue (PT 3c) et verte (PT 3a) sont les meilleurs compétiteurs dans des conditions de faible lumière bleue et verte, respectivement. A plus forte intensité lumineuse, le spécialiste de la lumière verte a remporté la compétition en vert, et l’acclimateur chromatique en bleu. Dans un mélange des deux lumières, la coexistence des deux mêmes souches a été observée. Enfin, le dernier objectif a été d’affiner nos connaissances concernant la dynamique temporelle et spatiale des Synechococcus. Pour ce faire, j’ai participé à une série temporelle de deux ans au niveau de deux sites océanographiquement distincts : la station SOMLIT-Astan en Manche et la station BOUSSOLE en Méditerranée. L’analyse de l’abondance de picocyanobactéries, couplée à divers autres paramètres auxiliaires (disponibilité des nutriments, pigments, caractéristiques optiques de l'eau de mer), suggère fortement une succession saisonnière des TPs 3a, 3c et 3d au niveau des deux sites. Les prochaines analyses métagénomiques devraient permettre de mieux décrire l'abondance relative des différents TPs de Synechococcus, ainsi que leurs cycle saisonniers

Phytoplancton et couleur de l'eau : analyse multi-échelles de l'avantage adaptatif conféré par l'acclimatation chromatique chez les cyanobactéries marines

Les propriétés physico-chimiques des océans sont particulièrement affectées par le changement climatique. Une expansion rapide des zones chaudes et pauvres en nutriments a notamment été remarquée, modifiant la couleur de l’eau et, par conséquent, la compétition entre les diverses espèces du phytoplancton qui ont besoin de lumière pour produire de l’énergie. En tant que deuxième organisme le plus abondant des océans, Synechococcus constitue un excellent modèle pour évaluer les conséquences de l’altération des propriétés optiques de l’eau sur la distribution, la dynamique et la composition des communautés phytoplanctoniques marines. A ce jour, sept types pigmentaires (TPs) ont été identifiés chez Synechococcus sur la base de la composition et de la chromophorylation de leurs complexes collecteurs de lumière, appelés phycobilisomes. Alors que les TPs 3a et 3c ont une pigmentation fixe qui leur permet de collecter efficacement la lumière verte ou bleue, respectivement, le TP 3d est capable d’acclimatation chromatique de Type IV (CA4). En d’autres termes, ce sous-type peut modifier de manière réversible la pigmentation de ses phycobilismes afin de capturer les photons verts ou bleus, en fonction de la couleur de la lumière ambiante. Deux types génétiquement distincts d’acclimateurs chromatiques également abondants dans l’océan (TPs 3dA et 3dB) ont été mis en évidence. Ensemble, ils constituent le TP de Synechococcus le plus répandu à l’échelle globale, bien que les raisons de leur prévalence dans les populations naturelles ne soient pas claires. Les TPs 3dA et 3dB possèdent chacun un îlot génomique impliqué dans la CA4 (îlots CA4-A et CA4-B). Alors que les bases moléculaires de la CA4-A commencent à être bien comprises, celles de la CA4-B sont moins connues. Un des objectifs de ma thèse a donc été de caractériser des gènes de l’îlot CA4-B en utilisant la technologie CRISPR-Cas12. La caractérisation partielle de l’un des gènes (fciB) a suggéré que la régulation de la CA4 chez les TPs 3dB pourrait être inversée par rapport à celle rencontrée chez les TPs 3dA. En effet, FciB semble agir comme un activateur chez l’un, et comme un répresseur chez l’autre. Pour étudier les différences phénotypiques entre les deux types d’acclimateurs chromatiques, j’ai également examiné leur réponse dans diverses conditions de température, quantité et qualité de la lumière. De toute évidence, les TPs 3dA et 3dB diffèrent dans le rapport de lumière bleue et verte nécessaire pour déclencher la CA4. Le deuxième objectif a consisté à mieux caractériser l’avantage conféré par l’acclimatation chromatique par rapport à la pigmentation fixe. Pour cela, j’ai réalisé des mono-cultures et co-cultures de représentants des TPs 3a, 3c et 3dB dans différentes conditions de qualité et de quantité de lumière. J’ai démontré que les spécialistes de la lumière bleue (PT 3c) et verte (PT 3a) sont les meilleurs compétiteurs dans des conditions de faible lumière bleue et verte, respectivement. A plus forte intensité lumineuse, le spécialiste de la lumière verte a remporté la compétition en vert, et l’acclimateur chromatique en bleu. Dans un mélange des deux lumières, la coexistence des deux mêmes souches a été observée. Enfin, le dernier objectif a été d’affiner nos connaissances concernant la dynamique temporelle et spatiale des Synechococcus. Pour ce faire, j’ai participé à une série temporelle de deux ans au niveau de deux sites océanographiquement distincts : la station SOMLIT-Astan en Manche et la station BOUSSOLE en Méditerranée. L’analyse de l’abondance de picocyanobactéries, couplée à divers autres paramètres auxiliaires (disponibilité des nutriments, pigments, caractéristiques optiques de l'eau de mer), suggère fortement une succession saisonnière des TPs 3a, 3c et 3d au niveau des deux sites. Les prochaines analyses métagénomiques devraient permettre de mieux décrire l'abondance relative des différents TPs de Synechococcus, ainsi que leurs cycle saisonniers.Global change is predicted to have numerous impacts on the physico-chemical properties of the ocean. This includes a rapid expansion of warm, nutrient-poor areas that in turn alters the underwater light field, and subsequently impacts the competition between phytoplankton species that have an obligate requirement for light to produce energy. As the second organism in the world’s ocean, and the most diversified with regard to its pigmentation, Synechococcus constitutes an excellent model to assess the consequences of ongoing changes in water optical properties on the distribution, dynamics and composition of marine phytoplanktonic communities. To date, seven Synechococcus pigment types (PTs) have been identified based on the composition and chromophorylation of their light-harvesting complexes, called phycobilisomes. While PTs 3a and 3c have a fixed pigmentation that enable them to efficiently collect either green (GL) or blue light (BL), respectively, PT 3d are capable of Type IV chromatic acclimation (CA4). In other words, they can reversibly modify their phycobilisomes pigmentation in order to capture either green or blue photons, according to the ambient light color. Two genetically distinct types of CA4 strains (PT 3dA and 3dB) have been evidenced and found to be equally abundant in the ocean. Together, they constitute the most prevalent Synechococcus PT at the global scale. However, the reasons for their prevalence in natural populations remain unclear. Both PTs 3dA and 3dB possess one genomic island involved in CA4, with partially distinct gene content (CA4-A and CA4-B islands respectively). While the molecular bases of CA4-A begin to be well understood, CA4-B is less well known. One objective of my thesis was therefore to characterize genes of the CA4-B island using CRISPR-Cas12 technology. Partial characterization of one of these genes (fciB) suggested that the regulation of CA4 process in PT 3dB cells may be reversed compared to that of PT 3dA strains, since FciB appears to act as an activator or a repressor of the process. To investigate phenotypic differences between the two types of CA4 strains, I also examined their response to various conditions of temperature, light quantity and quality. Interestingly, I found that both differ in the ratio of blue to green light required to trigger chromatic acclimation. The second objective consisted in better characterize the advantage conferred by chromatic acclimation over fixed pigmentation. For this purpose, I conducted mono- and co-cultures of PTs 3a, 3c and 3dB representatives in various light qualities and quantities, and demonstrated that blue (PT 3c) and green (PT 3a) light specialists where the best competitors in low BL and GL, respectively. At high light, the CA4 strain wined the competition in BL and the green light specialist in GL, while in a mixture of both lights, co-existence of the CA4 strain and the green light specialist was achieved. Finally, the last objective was to refine our knowledge of the temporal and spatial dynamics of Synechococcus. To do so, I participated in a two-year time series at two oceanographically distinct sites (the SOMLIT-Astan station in the English Channel and the BOUSSOLE station in the Mediterranean Sea). Analysis of the picocyanobacteria cell concentration and ancillary parameters (nutrients availability, pigments, seawater optical characteristics) strongly suggests a seasonal succession of PT 3 subtypes at both sites, with variations in timing and relative proportion of PT 3a, 3c and 3d cells. Upcoming metagenomic analyses should provide further insights into the relative abundance of the different Synechococcus PTs

Efficient, fast and inexpensive bioassay to monitor benthic microalgae toxicity: Application to Ostreopsis species

Even though HPLC-MS is commonly used to quantify the toxin content of Ostreopsis spp. cells, there is a need to develop easy-to-use toxicological tests to set thresholds during Ostreopsis spp. blooms. The crustacean Artemia has been widely used to evaluate the presence and toxicity of chemicals and biological contaminants and we anticipated that it could also be useful to test Ostreopsis spp. toxicity. Its relevance was first assessed by investigating the variability of the toxic effects among Ostreopsis spp. strains and throughout the dinoflagellate life cycle in combination with chemical analyses of the toxinic content by UHPLC-HRMS. After testing the toxicity of fractions prepared from Ostreopsis spp. cells, the known ova- and paly-toxins were not the only toxic metabolites to Artemia franciscana, indicating that other toxic compounds synthesized by Ostreopsis spp. still remain to be identified. To extend the bioassay to in situ monitoring, the toxicity of the benthic microalgal consortium was tested during a natural bloom of Ostreopsis cf. ovata in the NW Mediterranean Sea. The results highlight the accuracy and sensitivity of the ecotoxicological assay with Artemia franciscana to assess the toxicity of Ostreopsis spp. blooms

The phycoerythrobilin isomerization activity of MpeV in Synechococcus sp. WH8020 is prevented by the presence of a histidine at position 141 within its phycoerythrin-I β-subunit substrate

International audienceMarine Synechococcus efficiently harvest available light for photosynthesis using complex antenna systems, called phycobilisomes, composed of an allophycocyanin core surrounded by rods, which in the open ocean are always constituted of phycocyanin and two phycoerythrin (PE) types: PEI and PEII. These cyanobacteria display a wide pigment diversity primarily resulting from differences in the ratio of the two chromophores bound to PEs, the greenlight absorbing phycoerythrobilin and the blue-light absorbing phycourobilin. Prior to phycobiliprotein assembly, bilin lyases post-translationally catalyze the ligation of phycoerythrobilin to conserved cysteine residues on α-or β-subunits, whereas the closely related lyase-isomerases isomerize phycoerythrobilin to phycourobilin during the attachment reaction. MpeV was recently shown in Synechococcus sp. RS9916 to be a lyase-isomerase which doubly links phycourobilin to two cysteine residues (C50 and C61; hereafter C50, 61) on the β-subunit of both PEI and PEII. Here we show that Synechococcus sp. WH8020, which belongs to the same pigment type as RS9916, contains MpeV that demonstrates lyase-isomerase activity on the PEII β-subunit but only lyase activity on the PEI β-subunit. We also demonstrate that occurrence of a histidine at position 141 of the PEI β-subunit from WH8020, instead of a leucine in its counterpart from RS9916, prevents the isomerization activity by WH8020 MpeV, showing for the first time that both the substrate and the enzyme play a role in the isomerization reaction. We propose a structural-based mechanism for the role of H141 in blocking isomerization. More generally, the knowledge of the amino acid present at position 141 of the β-subunits may be used to TYP