Transcriptomic and isotopic data reveal central role of ammonium in facilitating the growth of the mixotrophic dinoflagellate, Dinophysis acuminata

Advanced treatment of antibiotic-polluted wastewater by a consortium composed of bacteria and mixed cyanobacteria

This study constructed a cyanobacteria-bacteria consortium using a mixture of non-toxic cyanobacteria (Synechococcus sp. and Chroococcus sp.) immobilized in calcium alginate and native bacteria in wastewater. The consortium was used for the advanced treatment of sulfamethoxazole-polluted wastewater and the production of cyanobacterial lipid. Mixed cyanobacteria increased the abundances of denitrifying bacteria and phosphorus-accumulating bacteria as well as stimulated various functional enzymes in the wastewater bacterial community, which efficiently removed 70.01-71.86% of TN, 91.45-97.04% of TP and 70.72-76.85% of COD from the wastewater. The removal efficiency of 55.29-69.90% for sulfamethoxazole was mainly attributed to the upregulation of genes encoding oxidases, reductases, oxidoreductases and transferases in two cyanobacterial species as well as the increased abundances of Stenotrophomonas, Sediminibacterium, Arenimonas, Novosphingobium, Flavobacterium and Hydrogenophaga in wastewater bacterial community. Transcriptomic responses proved that mixed cyanobacteria presented an elevated lipid productivity of 33.90 mg/L/day as an adaptive stress response to sulfamethoxazole. Sediminibacterium, Flavobacterium and Exiguobacterium in the wastewater bacterial community may also promote cyanobacterial lipid synthesis through symbiosis. Results of this study proved that the mixed cyanobacteria-bacteria consortium was a promising approach for advanced wastewater treatment coupled to cyanobacterial lipid production.

Bunkering for change: Knowledge preparedness on the environmental aspect of ammonia as a marine fuel

Nitrogen cycling is essential to ecosystem functioning and the overall health of our planet. Ammonia, a nitrogen-containing product, as well as a nutrient, is promoted as a low-carbon fuel for the maritime sector, with spectacular production increase in plan. Similar to any other widespread fuels in the past, it is paramount to be prepared for the potential environmental impact of ammonia fuel. Here, through our preliminary calculations using literature data, we suggest that the amount of ammonia to be produced to fulfil the maritime energy need by 2050 may entail large alterations in global nitrogen cycling. Currently, the literature based on limited known cases of ammonia excess is insufficient to quantify the environmental impacts caused by the probable increase in bunkering ammonia release at global scale. With a few knowledge gaps identified, we call on the marine science community to investigate the potential environmental impact related to substantial ammonia excess, contributing new knowledge to a more environmentally sustainable future.

Horizontally Acquired Nitrate Reductase Realized Kleptoplastic Photoautotrophy of Rapaza viridis

While photoautotrophic organisms utilize inorganic nitrogen as the nitrogen source, heterotrophic organisms utilize organic nitrogen and thus do not generally have an inorganic nitrogen assimilation pathway. Here, we focused on the nitrogen metabolism of Rapaza viridis, a unicellular eukaryote exhibiting kleptoplasty. Although belonging to the lineage of essentially heterotrophic flagellates, R. viridis exploits the photosynthetic products of the kleptoplasts and was therefore suspected to potentially utilize inorganic nitrogen. From the transcriptome data of R. viridis, we identified gene RvNaRL, which had sequence similarity to genes encoding nitrate reductases in plants. Phylogenetic analysis revealed that RvNaRL was acquired by a horizontal gene transfer event. To verify the function of the protein product RvNaRL, we established RNAi-mediated knock-down and CRISPR-Cas9-mediated knock-out experiments for the first time in R. viridis and applied them to this gene. The RvNaRL knock-down and knock-out cells exhibited significant growth only when ammonium was supplied. However, in contrast to the wild-type cells, no substantial growth was observed when nitrate was supplied. Such arrested growth in the absence of ammonium was attributed to impaired amino acid synthesis due to the deficiency of nitrogen supply from the nitrate assimilation pathway; this in turn resulted in the accumulation of excess photosynthetic products in the form of cytosolic polysaccharide grains, as observed. These results indicate that RvNaRL is certainly involved in nitrate assimilation by R. viridis. Thus, we inferred that R. viridis achieved its advanced kleptoplasty for photoautotrophy, owing to the acquisition of nitrate assimilation via horizontal gene transfer.

De novo transcriptome assembly and gene annotation for the toxic dinoflagellate Dinophysis

Species within the dinoflagellate genus Dinophysis can produce okadiac acid and dinophysistoxins leading to diarrhetic shellfish poisoning. Since the first report of D. ovum from the Gulf of Mexico in 2008, reports of other Dinophysis species across US have increased. Members of the D. cf. acuminata complex (D. acuminata, D. acuta, D. ovum, D. sacculus) are difficult to differentiate due to their morphological similarities. Dinophysis feeds on and steals the chloroplasts from the ciliate, Mesodinium rubrum, which in turn has fed on and captured the chloroplasts of its prey, the cryptophyte Teleaulax amphioxeia. The objective of this study was to generate de novo transcriptomes for new isolates of these mixotrophic organisms. The transcriptomes obtained will serve as a reference for future experiments to assess the effect of different abiotic and biotic conditions and will also provide a useful resource for screening potential marker genes to differentiate among the closely related species within the D. cf. acuminata-complex. The complete comprehensive detailed workflow and links to obtain the transcriptome data are provided.

ROS-dependent cell death of Heterosigma akashiwo induced by algicidal bacterium Hahella sp. KA22

Marine algicidal bacteria and their metabolites are considered to be one of the most effective strategies to mitigate the harmful algal blooms (HABs). The bacterium Hahella sp. KA22 has previously been confirmed to have strong algicidal activity against the HABs causing microalgae, Heterosigma akashiwo. In this study, the molecular mechanism of microalgae cell death was detected. The results showed that the cell growth rate and photosynthetic efficiency were inhibited with addition of algicidal strain KA22, while the accumulation of reactive oxygen species (ROS) and oxidative damage in H. akashiwo cells increased. A total of 2056 unigenes were recognized to be differentially expressed in transcriptome sequences. In particular, the transcriptional levels of light-harvesting pigments and structural proteins in the oxygen-evolving-complex were continuously down-regulated, corresponding to the significant reduction of photosynthetic efficiency and the accumulation of ROS. Furthermore, glutamate dehydrogenase was significantly up-regulated in abundance. Meanwhile, calcium-dependent protein kinases were also detected with significant changes. Collectively, algicidal stress caused the suppressed electron transfer in chloroplast and impaired detoxification of intracellular oxidants by glutathione, which may subsequently result in multiple cell regulation and metabolic responses and ultimately lead to the ROS-dependent cell death of H. akashiwo.

Co-occurrence of pectenotoxins and Dinophysis miles in an Indonesian semi-enclosed bay

The study aims to unravel the variability of Dinophysis spp. and their alleged toxins in conjunction with environmental drivers in Ambon Bay. Phytoplankton samples, lipophilic toxins and physiochemical water properties were analysed during a 1.5-year period. Three Dinophysis species (D. miles, D. caudata, and D. acuminata) were found in plankton samples, of which D. miles was the most abundant and persistently occurring species. Pectenotoxin-2 (PTX2) and its secoacid (PTX2sa) were detected throughout, and PTX2sa levels strongly correlated with D. miles cell abundance. The toxin showed a positive correlation with temperature, which may suggest that D. miles cells contain rather constant PTX2sa during warmer months. Dissolved nitrate concentrations were found to play a major role in regulating cell abundances and toxin levels. This study adds adequate information regarding marine biotoxins and potentially toxic species for future Harmful Algal Bloom management in Ambon and Indonesia at large.