Divinyl chlorophyll a in the marine eukaryotic protist Alexandrium ostenfeldii (Dinophyceae)

Response of the toxic dinoflagellate Alexandrium minutum to exudates of the eelgrass Zostera marina

Biotic interactions are a key factor in the development of harmful algal blooms. Recently, a lower abundance of planktonic dinoflagellates has been reported in areas dominated by seagrass beds, suggesting a negative interaction between both groups of organisms. The interaction between planktonic dinoflagellates and marine phanerogams, as well as the way in which bacteria can affect this interaction, was studied in two experiments using a non-axenic culture of the toxic dinoflagellate Alexandrium minutum exposed to increasing additions of eelgrass (Zostera marina) exudates from old and young leaves and to the presence or absence of antibiotics. In these experiments, A. minutum abundance, growth rate and photosynthetic efficiency (Fv/Fm), as well as bacterial abundance, were measured every 48 h. Toxin concentration per cell was determined at the end of both experiments. Our results demonstrated that Z. marina exudates reduced A. minutum growth rate and, in one of the experiments, also the photosynthetic efficiency. These results are not an indirect effect mediated by the bacteria in the culture, although their growth modify the magnitude of the negative impact on the dinoflagellate growth rate. No clear pattern was observed in the variation of toxin production with the treatments.

Short-Term Interactions of Noctiluca scintillans with the Toxic Dinoflagellates Dinophysis acuminata and Alexandrium minutum: Growth, Toxins and Allelopathic Effects

The Galician Rías (NW Iberian Peninsula) are an important shellfish aquaculture area periodically affected by toxic episodes often caused by dinoflagellates such as Dinophysis acuminata and Alexandrium minutum, among others. In turn, water discolorations are mostly associated with non-toxic organisms such as the heterotrophic dinoflagellate Noctiluca scintillans, a voracious non-selective predator. The objective of this work was to study the biological interactions among these dinoflagellates and their outcome in terms of survival, growth and toxins content. To that aim, short experiments (4 days) were carried out on mixed cultures with N. scintillans (20 cells mL^-1) and (i) one strain of D. acuminata (50, 100 and 500 cells mL^-1) and (ii) two strains of A. minutum (100, 500 and 1000 cells mL^-1). Cultures of N. scintillans with two A. minutum collapsed by the end of the assays. Both D. acuminata and A. minutum exposed to N. scintillans arrested its growth, though feeding vacuoles in the latter rarely contained any prey. Toxin analyses at the end of the experiment showed an increase in intracellular OA levels in D. acuminata and a significant reduction in PSTs in both A. minutum strains. Neither OA nor PSTs were detected in N. scintillans. Overall, the present study indicated that the interactions among them were ruled by negative allelopathic effects.

Comparative transcriptome analysis identified ChlH and POLGAMMA2 in regulating yellow-leaf coloration in Forsythia

Leaf color is one of the most important features for plants used for landscape and ornamental purposes. However, the regulatory mechanism of yellow leaf coloration still remains elusive in many plant species. To understand the complex genetic mechanism of yellow-leaf Forsythia, we first compared the pigment content and leaf anatomical structure of yellow-leaf and green-leaf accessions derived from a hybrid population. The physiological and cytological analyses demonstrated that yellow-leaf progenies were chlorophyll deficient with defected chloroplast structure. With comparative transcriptome analysis, we identified a number of candidate genes differentially expressed between yellow-leaf and green-leaf Forsythia plants. Among these genes, we further screened out two candidates, ChlH (magnesium chelatase Subunit H) and POLGAMMA2 (POLYMERASE GAMMA 2), with consistent relative-expression pattern between different colored plants. To verify the gene function, we performed virus-induced gene silencing assays and observed yellow-leaf phenotype with total chlorophyll content reduced by approximately 66 and 83% in ChlH-silenced and POLGAMMA2-silenced plants, respectively. We also observed defected chloroplast structure in both ChlH-silenced and POLGAMMA2-silenced Forsythia. Transient over-expression of ChlH and POLGAMMA2 led to increased chlorophyll content and restored thylakoid architecture in yellow-leaf Forsythia. With transcriptome sequencing, we detected a number of genes related to chlorophyll biosynthesis and chloroplast development that were responsive to the silencing of ChlH and POLGAMMA2. To summarize, ChlH and POLGAMMA2 are two key genes that possibly related to yellow-leaf coloration in Forsythia through modulating chlorophyll synthesis and chloroplast ultrastructure. Our study provided insights into the molecular aspects of yellow-leaf Forsythia and expanded the knowledge of foliage color regulation in woody ornamental plants.

Latitudinal Variation in the Toxicity and Sexual Compatibility of Alexandrium catenella Strains from Southern Chile

The bloom-forming toxic dinoflagellate Alexandrium catenella was first detected in southern Chile (39.5–55° S) 50 years ago and is responsible for most of the area’s cases of paralytic shellfish poisoning (PSP). Given the complex life history of A. catenella, which includes benthic sexual cysts, in this study, we examined the potential link between latitude, toxicity, and sexual compatibility. Nine clones isolated from Chilean Patagonia were used in self- and out-crosses in all possible combinations (n = 45). The effect of latitude on toxicity, reproductive success indexes, and cyst production was also determined. Using the toxin profiles for all strains, consisting of C1, C2, GTX4, GTX1, GTX3, and NeoSTX, a latitudinal gradient was determined for their proportions (%) and content per cell (pg cell⁻¹), with the more toxic strains occurring in the north (−40.6° S). Reproductive success also showed a latitudinal tendency and was lower in the north. None of the self-crosses yielded resting cysts. Rather, the production of resting cysts was highest in pairings of clones separated by distances of 1000–1650 km. Our results contribute to a better understanding of PSP outbreaks in the region and demonstrate the importance of resting cysts in fueling new toxic events. They also provide additional evidence that the introduction of strains from neighboring regions is a cause for concern.

Paralytic and Amnesic Shellfish Toxins Impacts on Seabirds, Analyses and Management

Marine biotoxins have been frequently implicated in morbidity and mortality events in numerous species of birds worldwide. Nevertheless, their effects on seabirds have often been overlooked and the associated ecological impact has not been extensively studied. On top of that, the number of published studies confirming by analyses the presence of marine biotoxins from harmful algal blooms (HABs) in seabirds, although having increased in recent years, is still quite low. This review compiles information on studies evidencing the impact of HAB toxins on marine birds, with a special focus on the effects of paralytic and amnesic shellfish toxins (PSTs and ASTs). It is mainly centered on studies in which the presence of PSTs and/or ASTs in seabird samples was demonstrated through analyses. The analytical techniques commonly employed, the tissues selected and the adjustments done in protocols for processing seabird matrixes are summarized. Other topics covered include the role of different vectors in the seabird intoxications, information on clinical signs in birds affected by PSTs and ASTs, and multifactorial causes which could aggravate the syndromes. Close collaboration between seabird experts and marine biotoxins researchers is needed to identify and report the potential involvement of HABs and their toxins in the mortality events. Future studies on the PSTs and ASTs pharmacodynamics, together with the establishment of lethal doses in various seabird species, are also necessary. These studies would aid in the selection of the target organs for toxins analyses and in the postmortem intoxication diagnoses.

First Report of Paralytic Shellfish Toxins in Marine Invertebrates and Fish in Spain

A paralytic shellfish poisoning (PSP) episode developed in summer 2018 in the Rías Baixas (Galicia, NW Spain). The outbreak was associated with an unprecedentedly intense and long-lasting harmful algal bloom (HAB) (~one month) caused by the dinoflagellate Alexandrium minutum. Paralytic shellfish toxins (PSTs) were analyzed in extracts of 45 A. minutum strains isolated from the bloom by high-performance liquid chromatography with post-column oxidation and fluorescence detection (HPLC-PCOX-FLD). PSTs were also evaluated in tissues from marine fauna (invertebrates and fish) collected during the episode and in dolphin samples. The analysis of 45 A. minutum strains revealed a toxic profile including GTX1, GTX2, GTX3 and GTX4 toxins. With regard to the marine fauna samples, the highest PSTs levels were quantified in bivalve mollusks, but the toxins were also found in mullets, mackerels, starfish, squids and ascidians. This study reveals the potential accumulation of PSTs in marine invertebrates other than shellfish that could act as vectors in the trophic chain or pose a risk for human consumption. To our knowledge, this is the first time that PSTs are reported in ascidians and starfish from Spain. Moreover, it is the first time that evidence of PSTs in squids is described in Europe.

Comparative ecophysiology of Dinophysis acuminata and D. acuta (DINOPHYCEAE, DINOPHYSIALES): effect of light intensity and quality on growth, cellular toxin content, and photosynthesis

Dinoflagellates of the genus Dinophysis are the most persistent producers of lipophilic shellfish toxins in Western Europe. Their mixotrophic nutrition requires a food chain of cryptophytes and plastid-bearing ciliates for sustained growth and photosynthesis. In this study, cultures of D. acuminata and D. acuta, their ciliate prey Mesodinium rubrum and the cryptophyte, Teleaulax amphioxeia, were subject to three experimental settings to study their physiological response to different combinations of light intensity and quality. Growth rates, pigment analyses (HPLC), photosynthetic parameters (PAM-fluorometry), and cellular toxin content (LC-MS) were determined. Specific differences in photosynthetic parameters were observed in Dinophysis exposed to different photon fluxes (10-650 μmol photons · m^-2  · s^-1 ), light quality (white, blue and green), and shifts in light regime. Dinophysis acuta was more susceptible to photodamage under high light intensities (370-650 μmol photons · m^-2  · s^-1 ) than D. acuminata but survived better with low light (10 μmol photons · m^-2  · s^-1 ) and to a prolonged period (28 d) of darkness. Mesodinium rubrum and T. amphioxeia showed their maximal growth rate and yield under white and high light whereas Dinophysis seemed better adapted to grow under green and blue light. Toxin analyses in Dinophysis showed maximal toxin per cell under high light after prey depletion at the late exponential-plateau phase. Changes observed in photosynthetic light curves of D. acuminata cultures after shifting light conditions from low intensity-blue light to high intensity-white light seemed compatible with photoacclimation in this species. Results obtained here are discussed in relation to different spatiotemporal distributions observed in field populations of D. acuminata and D. acuta in northwestern Iberia.

Yellow-Leaf 1 encodes a magnesium-protoporphyrin IX monomethyl ester cyclase, involved in chlorophyll biosynthesis in rice (Oryza sativa L.)

Magnesium-protoporphyrin IX monomethyl ester cyclase (MPEC) catalyzes the conversion of MPME to divinyl protochlorophyllide (DVpchlide). This is an essential enzyme during chlorophyll (Chl) biosynthesis but details of its function in rice are still lacking. Here, we identified a novel rice mutant yellow-leaf 1 (yl-1), which showed decreased Chl accumulation, abnormal chloroplast ultrastructure and attenuated photosynthetic activity. Map-based cloning and over-expression analysis suggested that YL-1 encodes a subunit of MPEC. The YL-1 protein localizes in chloroplasts, and it is mainly expressed in green tissues, with greatest abundance in leaves and young panicles. Results of qRT-PCR showed that Chl biosynthesis upstream genes were highly expressed in the yl-1 mutant, while downstream genes were compromised, indicating that YL-1 plays a pivotal role in the Chl biosynthesis. Furthermore, the expression levels of photosynthesis and chloroplast development genes were also affected. RNA-seq results futher proved that numerous membrane-associated genes, including many plastid membrane-associated genes, have altered expression pattern in the yl-1 mutant, implying that YL-1 is required for plastid membrane stability. Thus, our study confirms a putative MPME cyclase as a novel key enzyme essential for Chl biosynthesis and chloroplast membrane stability in rice.

Two Unrelated 8-Vinyl Reductases Ensure Production of Mature Chlorophylls in Acaryochloris marina

The major photopigment of the cyanobacterium Acaryochloris marina is chlorophyll d, while its direct biosynthetic precursor, chlorophyll a, is also present in the cell. These pigments, along with the majority of chlorophylls utilized by oxygenic phototrophs, carry an ethyl group at the C-8 position of the molecule, having undergone reduction of a vinyl group during biosynthesis. Two unrelated classes of 8-vinyl reductase involved in the biosynthesis of chlorophylls are known to exist, BciA and BciB. The genome of Acaryochloris marina contains open reading frames (ORFs) encoding proteins displaying high sequence similarity to BciA or BciB, although they are annotated as genes involved in transcriptional control (nmrA) and methanogenesis (frhB), respectively. These genes were introduced into an 8-vinyl chlorophyll a-producing ΔbciB strain of Synechocystis sp. strain PCC 6803, and both were shown to restore synthesis of the pigment with an ethyl group at C-8, demonstrating their activities as 8-vinyl reductases. We propose that nmrA and frhB be reassigned as bciA and bciB, respectively; transcript and proteomic analysis of Acaryochloris marina reveal that both bciA and bciB are expressed and their encoded proteins are present in the cell, possibly in order to ensure that all synthesized chlorophyll pigment carries an ethyl group at C-8. Potential reasons for the presence of two 8-vinyl reductases in this strain, which is unique for cyanobacteria, are discussed.

IMPORTANCE The cyanobacterium Acaryochloris marina is the best-studied phototrophic organism that uses chlorophyll d for photosynthesis. Unique among cyanobacteria sequenced to date, its genome contains ORFs encoding two unrelated enzymes that catalyze the reduction of the C-8 vinyl group of a precursor molecule to an ethyl group. Carrying a reduced C-8 group may be of particular importance to organisms containing chlorophyll d. Plant genomes also contain orthologs of both of these genes; thus, the bacterial progenitor of the chloroplast may also have contained both bciA and bciB.