Fates of Evolutionarily Distinct, Plastid-type Glyceraldehyde 3-phosphate Dehydrogenase Genes in Kareniacean Dinoflagellates

Expression of nuclear-encoded, haptophyte-derived ftsH genes support extremely rapid PSII repair and high-light photoacclimation in Karenia brevis (Dinophyceae)

Karenia brevis, a neurotoxic dinoflagellate that produces brevetoxins, is endemic to the Gulf of Mexico and can grow at high irradiances typical of surface waters found there. To build upon a growing number of studies addressing high-light tolerance in K. brevis, specific photobiology and molecular mechanisms underlying this capacity were evaluated in culture. Since photosystem II (PSII) repair cycle activity can be crucial to high light tolerance in plants and algae, the present study assessed this capacity in K. brevis and characterized the ftsH-like genes which are fundamental to this process. Compared with cultures grown in low-light, cultures grown in high-light showed a 65-fold increase in PSII photoinactivation, a ∼50-fold increase in PSII repair, enhanced nonphotochemical quenching (NPQ), and depressed F_v/F_m. Repair rates were among the fastest reported in phytoplankton. Publicly available K. brevis transcriptomes (MMETSP) were queried for ftsH-like sequences and refined with additional sequencing from two K. brevis strains. The genes were phylogenetically related to haptophyte orthologs, implicating acquisition during tertiary endosymbiosis. RT-qPCR of three of the four ftsH-like homologs revealed that poly-A tails predominated in all homologs, and that the most highly expressed homolog had a 5' splice leader and amino-acid motifs characteristic of chloroplast targeting, indicating nuclear encoding for this plastid-targeted gene. High-light cultures showed a ∼1.5-fold upregulation in mRNA expression of the thylakoid-associated genes. Overall, in conjunction with NPQ mechanisms, rapid PSII repair mediated by a haptophyte-derived ftsH prevents chronic photoinhibition in K. brevis. Our findings continue to build the case that high-light photobiology-supported by the acquisition and maintenance of tertiary endosymbiotic genes-is critical to the success of K. brevis in the Gulf of Mexico.

The iron-sulfur scaffold protein HCF101 unveils the complexity of organellar evolution in SAR, Haptista and Cryptista

Background

Nbp35-like proteins (Nbp35, Cfd1, HCF101, Ind1, and AbpC) are P-loop NTPases that serve as components of iron-sulfur cluster (FeS) assembly machineries. In eukaryotes, Ind1 is present in mitochondria, and its function is associated with the assembly of FeS clusters in subunits of respiratory Complex I, Nbp35 and Cfd1 are the components of the cytosolic FeS assembly (CIA) pathway, and HCF101 is involved in FeS assembly of photosystem I in plastids of plants (chHCF101). The AbpC protein operates in Bacteria and Archaea. To date, the cellular distribution of these proteins is considered to be highly conserved with only a few exceptions.

Results

We searched for the genes of all members of the Nbp35-like protein family and analyzed their targeting sequences. Nbp35 and Cfd1 were predicted to reside in the cytoplasm with some exceptions of Nbp35 localization to the mitochondria; Ind1was found in the mitochondria, and HCF101 was predicted to reside in plastids (chHCF101) of all photosynthetically active eukaryotes. Surprisingly, we found a second HCF101 paralog in all members of Cryptista, Haptista, and SAR that was predicted to predominantly target mitochondria (mHCF101), whereas Ind1 appeared to be absent in these organisms. We also identified a few exceptions, as apicomplexans possess mHCF101 predicted to localize in the cytosol and Nbp35 in the mitochondria. Our predictions were experimentally confirmed in selected representatives of Apicomplexa (Toxoplasma gondii), Stramenopila (Phaeodactylum tricornutum, Thalassiosira pseudonana), and Ciliophora (Tetrahymena thermophila) by tagging proteins with a transgenic reporter. Phylogenetic analysis suggested that chHCF101 and mHCF101 evolved from a common ancestral HCF101 independently of the Nbp35/Cfd1 and Ind1 proteins. Interestingly, phylogenetic analysis supports rather a lateral gene transfer of ancestral HCF101 from bacteria than its acquisition being associated with either α-proteobacterial or cyanobacterial endosymbionts.

Conclusion

Our searches for Nbp35-like proteins across eukaryotic lineages revealed that SAR, Haptista, and Cryptista possess mitochondrial HCF101. Because plastid localization of HCF101 was only known thus far, the discovery of its mitochondrial paralog explains confusion regarding the presence of HCF101 in organisms that possibly lost secondary plastids (e.g., ciliates, Cryptosporidium) or possess reduced nonphotosynthetic plastids (apicomplexans).

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01777-x.

Origin and diversification of the cardiolipin biosynthetic pathway in the Eukarya domain

Cardiolipin (CL) and its precursor phosphatidylglycerol (PG) are important anionic phospholipids widely distributed throughout all domains of life. They have key roles in several cellular processes by shaping membranes and modulating the activity of the proteins inserted into those membranes. They are synthesized by two main pathways, the so-called eukaryotic pathway, exclusively found in mitochondria, and the prokaryotic pathway, present in most bacteria and archaea. In the prokaryotic pathway, the first and the third reactions are catalyzed by phosphatidylglycerol phosphate synthase (Pgps) belonging to the transferase family and cardiolipin synthase (Cls) belonging to the hydrolase family, while in the eukaryotic pathway, those same reactions are catalyzed by unrelated homonymous enzymes: Pgps of the hydrolase family and Cls of the transferase family. Because of the enzymatic arrangement found in both pathways, it seems that the eukaryotic pathway evolved by convergence to the prokaryotic pathway. However, since mitochondria evolved from a bacterial endosymbiont, it would suggest that the eukaryotic pathway arose from the prokaryotic pathway. In this review, it is proposed that the eukaryote pathway evolved directly from a prokaryotic pathway by the neofunctionalization of the bacterial enzymes. Moreover, after the eukaryotic radiation, this pathway was reshaped by horizontal gene transfers or subsequent endosymbiotic processes.

A review of karenia mikimotoi: Bloom events, physiology, toxicity and toxic mechanism

Karenia mikimotoi is a worldwide bloom-forming dinoflagellate in the genus Karenia. Blooms of this alga have been observed since the 1930s and have caused mass mortalities of fish, shellfish, and other invertebrates in the coastal waters of many countries, including Japan, Norway, Ireland, and New Zealand. This species has frequently bloomed in China, causing great financial losses (more than 2 billion yuan, Fujian Province, 2012). K. mikimotoi can adapt to various light, temperature, salinity, and nutrient conditions, which together with its complex life history, strong motility, and density-dependent allelopathy, allows it to form blooms that are lethal to almost all marine organisms. However, its toxicity differs between subspecies and some target-species-specific toxicity has also been recorded. Significant gill disorder is observed in affected fish, to which the massive fish kills are attributed, rather than to the hypoxia that occurs in the fading stage of a bloom. However, although this species is haemolytic and cytotoxic, and generates reactive oxygen species, none of the isolated toxins or lipophilic extracts have toxic effects as extreme as those of the intact algal cells. The toxic effects of K. mikimotoi are strongly related to contact with intact cells. Several reasonable hypotheses of how and why this species blooms and causes mass mortalities have been proposed, but further research is required.