When the lights go out: the evolutionary fate of free-living colorless green algae

Complex Endosymbioses I: From Primary to Complex Plastids, Serial Endosymbiotic Events

A considerable part of the diversity of eukaryotic phototrophs consists of algae with plastids that evolved from endosymbioses between two eukaryotes. These complex plastids are characterized by a high number of envelope membranes (more than two) and some of them contain a residual nucleus of the endosymbiotic alga called a nucleomorph. Complex plastid-bearing algae are thus chimeric cell assemblies, eukaryotic symbionts living in a eukaryotic host. In contrast, the primary plastids of the Archaeplastida (plants, green algae, red algae, and glaucophytes) possibly evolved from a single endosymbiosis with a cyanobacterium and are surrounded by two membranes. Complex plastids have been acquired several times by unrelated groups of eukaryotic heterotrophic hosts, suggesting that complex plastids are somewhat easier to obtain than primary plastids. Evidence suggests that complex plastids arose twice independently in the green lineage (euglenophytes and chlorarachniophytes) through secondary endosymbiosis, and four times in the red lineage, first through secondary endosymbiosis in cryptophytes, then by higher-order events in stramenopiles, alveolates, and haptophytes. Engulfment of primary and complex plastid-containing algae by eukaryotic hosts (secondary, tertiary, and higher-order endosymbioses) is also responsible for numerous plastid replacements in dinoflagellates. Plastid endosymbiosis is accompanied by massive gene transfer from the endosymbiont to the host nucleus and cell adaptation of both endosymbiotic partners, which is related to the trophic switch to phototrophy and loss of autonomy of the endosymbiont. Such a process is essential for the metabolic integration and division control of the endosymbiont in the host. Although photosynthesis is the main advantage of acquiring plastids, loss of photosynthesis often occurs in algae with complex plastids. This chapter summarizes the essential knowledge of the acquisition, evolution, and function of complex plastids.

Deep learning-based classification of microalgae using light and scanning electron microscopy images

Microalgae possess diverse applications, such as food production, animal feed, cosmetics, plastics manufacturing, and renewable energy sources. However, uncontrolled proliferation, known as algal bloom, can detrimentally impact ecosystems. Therefore, the accurate detection, monitoring, identification, and tracking of algae are imperative, albeit demanding considerable time, effort, and expertise, as well as financial resources. Deep learning, employing image pattern recognition, emerges as a practical and promising approach for rapid and precise microalgae cell counting and identification. In this study, we processed light microscopy (LM) and scanning electron microscopy (SEM) images of two Cyanobacteria species and three Chlorophyta species to classify them, utilizing state-of-the-art Convolutional Neural Network (CNN) models, including VGG16, MobileNet V2, Xception, NasnetMobile, and EfficientNetV2. In contrast to prior deep learning based identification studies limited to LM images, we, for the first time, incorporated SEM images of microalgae in our analysis. Both LM and SEM microalgae images achieved an exceptional classification accuracy of 99%, representing the highest accuracy attained by the VGG16 and EfficientNetV2 models to date. While NasnetMobile exhibited the lowest accuracy of 87% with SEM images, the remaining models achieved classification accuracies surpassing 93%. Notably, the VGG16 and EfficientNetV2 models achieved the highest accuracy of 99%. Intriguingly, our findings indicate that algal identification using optical microscopes, which are more cost-effective, outperformed electron microscopy techniques.

Comparative genomic analysis illustrates evolutionary dynamics of multisubunit tethering complexes across green algal diversity

The chlorophyte algae are a dominant group of photosynthetic eukaryotes. Although many are photoautotrophs, there are also mixotrophs, heterotrophs, and even parasites. The physical characteristics of green algae are also highly diverse, varying greatly in size, shape, and habitat. Given this morphological and trophic diversity, we postulated that diversity may also exist in the protein components controlling intracellular movement of material by vesicular transport. One such set is the multisubunit tethering complexes (MTCs)-components regulating cargo delivery. As they span endomembrane organelles and are well-conserved across eukaryotes, MTCs should be a good proxy for assessing the evolutionary dynamics across the diversity of Chlorophyta. Our results reveal that while green algae carry a generally conserved and unduplicated complement of MTCs, some intriguing variation exists. Notably, we identified incomplete sets of TRAPPII, exocyst, and HOPS/CORVET components in all Mamiellophyceae, and what is more, not a single subunit of Dsl1 was found in Cymbomonas tetramitiformis. As the absence of Dsl1 has been correlated with having unusual peroxisomes, we searched for peroxisome biogenesis machinery, finding very few components in Cymbomonas, suggestive of peroxisome degeneration. Overall, we demonstrate conservation of MTCs across green algae, but with notable taxon-specific losses suggestive of unusual endomembrane systems.

The plastomes of Hyalomonas oviformis and Hyalogonium fusiforme evolved dissimilar architectures after the loss of photosynthesis

The loss of photosynthesis in land plants and algae is typically associated with parasitism but can also occur in free-living species, including chlamydomonadalean green algae. The plastid genomes (ptDNAs) of colorless chlamydomonadaleans are surprisingly diverse in architecture, including highly expanded forms (Polytoma uvella and Leontynka pallida) as well as outright genome loss (Polytomella species). Here, we explore the ptDNAs of Hyalomonas (Hm.) oviformis (SAG 62-27; formerly known as Polytoma oviforme) and Hyalogonium (Hg.) fusiforme (SAG 62-1c), each representing independent losses of photosynthesis within the Chlamydophyceae. The Hm. oviformis ptDNA is moderately sized (132 kb) with a reduced gene complement (but still encoding the ATPase subunits) and is in fact smaller than that of its photosynthetic relative Hyalomonas chlamydogama SAG 11-48b (198.3 kb). The Hg. fusiforme plastome, however, is the largest yet observed in nonphotosynthetic plants or algae (~463 kb) and has a coding repertoire that is almost identical to that of its photosynthetic relatives in the genus Chlorogonium. Furthermore, the ptDNA of Hg. fusiforme shows no clear evidence of pseudogenization, which is consistent with our analyses showing that Hg. fusiforme is the nonphotosynthetic lineage of most recent origin among known colorless Chlamydophyceae. Together, these new ptDNAs clearly show that, in contrast to parasitic algae, plastid genome compaction is not an obligatory route following the loss of photosynthesis in free-living algae, and that certain chlamydomonadalean algae have a remarkable propensity for genomic expansion, which can persist regardless of the trophic strategy.

A new lineage of non-photosynthetic green algae with extreme organellar genomes

Background

The plastid genomes of the green algal order Chlamydomonadales tend to expand their non-coding regions, but this phenomenon is poorly understood. Here we shed new light on organellar genome evolution in Chlamydomonadales by studying a previously unknown non-photosynthetic lineage. We established cultures of two new Polytoma-like flagellates, defined their basic characteristics and phylogenetic position, and obtained complete organellar genome sequences and a transcriptome assembly for one of them.

Results

We discovered a novel deeply diverged chlamydomonadalean lineage that has no close photosynthetic relatives and represents an independent case of photosynthesis loss. To accommodate these organisms, we establish the new genus Leontynka, with two species (L. pallida and L. elongata) distinguishable through both their morphological and molecular characteristics. Notable features of the colourless plastid of L. pallida deduced from the plastid genome (plastome) sequence and transcriptome assembly include the retention of ATP synthase, thylakoid-associated proteins, the carotenoid biosynthesis pathway, and a plastoquinone-based electron transport chain, the latter two modules having an obvious functional link to the eyespot present in Leontynka. Most strikingly, the ~362 kbp plastome of L. pallida is by far the largest among the non-photosynthetic eukaryotes investigated to date due to an extreme proliferation of sequence repeats. These repeats are also present in coding sequences, with one repeat type found in the exons of 11 out of 34 protein-coding genes, with up to 36 copies per gene, thus affecting the encoded proteins. The mitochondrial genome of L. pallida is likewise exceptionally large, with its >104 kbp surpassed only by the mitogenome of Haematococcus lacustris among all members of Chlamydomonadales hitherto studied. It is also bloated with repeats, though entirely different from those in the L. pallida plastome, which contrasts with the situation in H. lacustris where both the organellar genomes have accumulated related repeats. Furthermore, the L. pallida mitogenome exhibits an extremely high GC content in both coding and non-coding regions and, strikingly, a high number of predicted G-quadruplexes.

Conclusions

With its unprecedented combination of plastid and mitochondrial genome characteristics, Leontynka pushes the frontiers of organellar genome diversity and is an interesting model for studying organellar genome evolution.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01263-w.

Experimental identification and in silico prediction of bacterivory in green algae

While algal phago-mixotrophs play a major role in aquatic microbial food webs, their diversity remains poorly understood. Recent studies have indicated several species of prasinophytes, early diverging green algae, to be able to consume bacteria for nutrition. To further explore the occurrence of phago-mixotrophy in green algae, we conducted feeding experiments with live fluorescently labeled bacteria stained with CellTracker Green CMFDA, heat-killed bacteria stained with 5-(4,6-dichlorotriazin-2-yl) aminofluorescein (DTAF), and magnetic beads. Feeding was detected via microscopy and/or flow cytometry in five strains of prasinophytes when provided with live bacteria: Pterosperma cristatum NIES626, Pyramimonas parkeae CCMP726, Pyramimonas parkeae NIES254, Nephroselmis pyriformis RCC618, and Dolichomastix tenuilepis CCMP3274. No feeding was detected when heat-killed bacteria or magnetic beads were provided, suggesting a strong preference for live prey in the strains tested. In parallel to experimental assays, green algal bacterivory was investigated using a gene-based prediction model. The predictions agreed with the experimental results and suggested bacterivory potential in additional green algae. Our observations underline the likelihood of widespread occurrence of phago-mixotrophy among green algae, while additionally highlighting potential biases introduced when using prey proxy to evaluate bacterial ingestion by algal cells.

The Cryptic Plastid of Euglena longa Defines a New Type of Nonphotosynthetic Plastid Organelle

Most secondary nonphotosynthetic eukaryotes have retained residual plastids whose physiological role is often still unknown. One such example is Euglena longa, a close nonphotosynthetic relative of Euglena gracilis harboring a plastid organelle of enigmatic function. By mining transcriptome data from E. longa, we finally provide an overview of metabolic processes localized to its elusive plastid. The organelle plays no role in the biosynthesis of isoprenoid precursors and fatty acids and has a very limited repertoire of pathways concerning nitrogen-containing metabolites. In contrast, the synthesis of phospholipids and glycolipids has been preserved, curiously with the last step of sulfoquinovosyldiacylglycerol synthesis being catalyzed by the SqdX form of an enzyme so far known only from bacteria. Notably, we show that the E. longa plastid synthesizes tocopherols and a phylloquinone derivative, the first such report for nonphotosynthetic plastids studied so far. The most striking attribute of the organelle could be the presence of a linearized Calvin-Benson (CB) pathway, including RuBisCO yet lacking the gluconeogenetic part of the standard cycle, together with ferredoxin-NADP⁺ reductase (FNR) and the ferredoxin/thioredoxin system. We hypothesize that the ferredoxin/thioredoxin system activates the linear CB pathway in response to the redox status of the E. longa cell and speculate on the role of the pathway in keeping the redox balance of the cell. Altogether, the E. longa plastid defines a new class of relic plastids that is drastically different from the best-studied organelle of this category, the apicoplast.IMPORTANCE Colorless plastids incapable of photosynthesis evolved in many plant and algal groups, but what functions they perform is still unknown in many cases. Here, we study the elusive plastid of Euglena longa, a nonphotosynthetic cousin of the familiar green flagellate Euglena gracilis We document an unprecedented combination of metabolic functions that the E. longa plastid exhibits in comparison with previously characterized nonphotosynthetic plastids. For example, and truly surprisingly, it has retained the synthesis of tocopherols (vitamin E) and a phylloquinone (vitamin K) derivative. In addition, we offer a possible solution of the long-standing conundrum of the presence of the CO₂-fixing enzyme RuBisCO in E. longa Our work provides a detailed account on a unique variant of relic plastids, the first among nonphotosynthetic plastids that evolved by secondary endosymbiosis from a green algal ancestor, and suggests that it has persisted for reasons not previously considered in relation to nonphotosynthetic plastids.

A non-photosynthetic green alga illuminates the reductive evolution of plastid electron transport systems

Background

Plastid electron transport systems are essential not only for photosynthesis but also for dissipating excess reducing power and sinking excess electrons generated by various redox reactions. Although numerous organisms with plastids have lost their photoautotrophic lifestyles, there is a spectrum of known functions of remnant plastids in non-photosynthetic algal/plant lineages; some of non-photosynthetic plastids still retain diverse metabolic pathways involving redox reactions while others, such as apicoplasts of apicomplexan parasites, possess highly reduced sets of functions. However, little is known about underlying mechanisms for redox homeostasis in functionally versatile non-photosynthetic plastids and thus about the reductive evolution of plastid electron transport systems.

Results

Here we demonstrated that the central component for plastid electron transport systems, plastoquinone/plastoquinol pool, is still retained in a novel strain of an obligate heterotrophic green alga lacking the photosynthesis-related thylakoid membrane complexes. Microscopic and genome analyses revealed that the Volvocales green alga, chlamydomonad sp. strain NrCl902, has non-photosynthetic plastids and a plastid DNA that carries no genes for the photosynthetic electron transport system. Transcriptome-based in silico prediction of the metabolic map followed by liquid chromatography analyses demonstrated carotenoid and plastoquinol synthesis, but no trace of chlorophyll pigments in the non-photosynthetic green alga. Transient RNA interference knockdown leads to suppression of plastoquinone/plastoquinol synthesis. The alga appears to possess genes for an electron sink system mediated by plastid terminal oxidase, plastoquinone/plastoquinol, and type II NADH dehydrogenase. Other non-photosynthetic algae/land plants also possess key genes for this system, suggesting a broad distribution of an electron sink system in non-photosynthetic plastids.

Conclusion

The plastoquinone/plastoquinol pool and thus the involved electron transport systems reported herein might be retained for redox homeostasis and might represent an intermediate step towards a more reduced set of the electron transport system in many non-photosynthetic plastids. Our findings illuminate a broadly distributed but previously hidden step of reductive evolution of plastid electron transport systems after the loss of photosynthesis.

Highly Reduced Plastid Genomes of the Non-photosynthetic Dictyochophyceans Pteridomonas spp. (Ochrophyta, SAR) Are Retained for tRNA-Glu-Based Organellar Heme Biosynthesis

Organisms that have lost their photosynthetic capabilities are present in a variety of eukaryotic lineages, such as plants and disparate algal groups. Most of such non-photosynthetic eukaryotes still carry plastids, as these organelles retain essential biological functions. Most non-photosynthetic plastids possess genomes with varied protein-coding contents. Such remnant plastids are known to be present in the non-photosynthetic, bacteriovorous alga Pteridomonas danica (Dictyochophyceae, Ochrophyta), which, regardless of its obligatory heterotrophic lifestyle, has been reported to retain the typically plastid-encoded gene for ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) large subunit (rbcL). The presence of rbcL without photosynthetic activity suggests that investigating the function of plastids in Pteridomonas spp. would likely bring unique insights into understanding the reductive evolution of plastids, their genomes, and plastid functions retained after the loss of photosynthesis. In this study, we demonstrate that two newly established strains of the non-photosynthetic genus Pteridomonas possess highly reduced plastid genomes lacking rbcL gene, in contrast to the previous report. Interestingly, we discovered that all plastid-encoded proteins in Pteridomonas spp. are involved only in housekeeping processes (e.g., transcription, translation and protein degradation), indicating that all metabolite synthesis pathways in their plastids are supported fully by nuclear genome-encoded proteins. Moreover, through an in-depth survey of the available transcriptomic data of another strain of the genus, we detected no candidate sequences for nuclear-encoded, plastid-directed Fe-S cluster assembly pathway proteins, suggesting complete loss of this pathway in the organelle, despite its widespread conservation in non-photosynthetic plastids. Instead, the transcriptome contains plastid-targeted components of heme biosynthesis, glycolysis, and pentose phosphate pathways. The retention of the plastid genomes in Pteridomonas spp. is not explained by the Suf-mediated constraint against loss of plastid genomes, previously proposed for Alveolates, as they lack Suf genes. Bearing all these findings in mind, we propose the hypothesis that plastid DNA is retained in Pteridomonas spp. for the purpose of providing glutamyl-tRNA, encoded by trnE gene, as a substrate for the heme biosynthesis pathway.

Should I stay or should I go? Retention and loss of components in vestigial endosymbiotic organelles

Our knowledge on the variability of the reduced forms of endosymbiotic organelles - mitochondria and plastids - is expanding rapidly, thanks to growing interest in peculiar microbial eukaryotes, along with the availability of the methods used in modern genomics and transcriptomics. The aim of this work is to highlight the most recent advances in understanding these organelles' diversity, physiology and evolution. We also outline the known mechanisms behind the convergence of traits between organelles which have undergone reduction independently, the importance of the earliest evolutionary events in determining the vestigial organelles' eventual fate, and a proposed classification of nonphotosynthetic plastids.

Dynamic evolution of mitochondrial genomes in Trebouxiophyceae, including the first completely assembled mtDNA from a lichen-symbiont microalga (Trebouxia sp. TR9)

Trebouxiophyceae (Chlorophyta) is a species-rich class of green algae with a remarkable morphological and ecological diversity. Currently, there are a few completely sequenced mitochondrial genomes (mtDNA) from diverse Trebouxiophyceae but none from lichen symbionts. Here, we report the mitochondrial genome sequence of Trebouxia sp. TR9 as the first complete mtDNA sequence available for a lichen-symbiont microalga. A comparative study of the mitochondrial genome of Trebouxia sp. TR9 with other chlorophytes showed important organizational changes, even between closely related taxa. The most remarkable change is the enlargement of the genome in certain Trebouxiophyceae, which is principally due to larger intergenic spacers and seems to be related to a high number of large tandem repeats. Another noticeable change is the presence of a relatively large number of group II introns interrupting a variety of tRNA genes in a single group of Trebouxiophyceae, which includes Trebouxiales and Prasiolales. In addition, a fairly well-resolved phylogeny of Trebouxiophyceae, along with other Chlorophyta lineages, was obtained based on a set of seven well-conserved mitochondrial genes.

A single loss of photosynthesis in the diatom order Bacillariales (Bacillariophyta)

PREMISE OF THE STUDY

Loss of photosynthesis is a common and often repeated trajectory in nearly all major groups of photosynthetic eukaryotes. One small subset of "apochloritic" diatoms in the genus Nitzschia have lost their ability to photosynthesize and require extracellular carbon for growth. Similar to other secondarily nonphotosynthetic taxa, apochloritic diatoms maintain colorless plastids with highly reduced plastid genomes. Although the narrow taxonomic breadth of apochloritic Nitzschia suggests a single loss of photosynthesis in their common ancestor, previous phylogenetic analyses suggested that photosynthesis was lost multiple times.

METHODS

We analyzed genes from the nuclear, plastid, and mitochondrial genomes for a broad set of taxa to test whether photosynthesis was lost one or multiple times in Bacillariales. We also sequenced and characterized the plastid genome of a nonphotosynthetic Nitzschia species.

KEY RESULTS

Phylogenetic analyses showed that genes from all three genetic compartments either supported or failed to reject monophyly of apochloritic Nitzschia species, consistent with a single loss of photosynthesis in this group. The plastid genomes of two apochloritic Nitzschia are highly similar in all respects, indicating streamlining of the plastid genome before the split of these two species.

CONCLUSIONS

A better understanding of the phylogeny and ecology of apochloritic Nitzschia, together with emerging genomic resources, will help identify the factors that have driven and maintained the loss of photosynthesis in this group of diatoms. Finally, some habitats host diverse communities of co-occurring nonphotosynthetic diatoms, reflecting resource abundance or resource partitioning in ecologically favorable habitats.

Genomics-Informed Insights into Endosymbiotic Organelle Evolution in Photosynthetic Eukaryotes

The conversion of free-living cyanobacteria to photosynthetic organelles of eukaryotic cells through endosymbiosis transformed the biosphere and eventually provided the basis for life on land. Despite the presumable advantage conferred by the acquisition of photoautotrophy through endosymbiosis, only two independent cases of primary endosymbiosis have been documented: one that gave rise to the Archaeplastida, and the other to photosynthetic species of the thecate, filose amoeba Paulinella. Here, we review recent genomics-informed insights into the primary endosymbiotic origins of cyanobacteria-derived organelles. Furthermore, we discuss the preconditions for the evolution of nitrogen-fixing organelles. Recent genomic data on previously undersampled cyanobacterial and protist taxa provide new clues to the origins of the host cell and endosymbiont, and proteomic approaches allow insights into the rearrangement of the endosymbiont proteome during organellogenesis. We conclude that in addition to endosymbiotic gene transfers, horizontal gene acquisitions from a broad variety of prokaryotic taxa were crucial to organelle evolution.

Complex Endosymbioses I: From Primary to Complex Plastids, Multiple Independent Events

A substantial portion of eukaryote diversity consists of algae with complex plastids, i.e., plastids originating from eukaryote-to-eukaryote endosymbioses. These plastids are characteristic by a deviating number of envelope membranes (higher than two), and sometimes a remnant nucleus of the endosymbiont alga, termed the nucleomorph, is present. Complex plastid-bearing algae are therefore much like living matryoshka dolls, eukaryotes within eukaryotes. In comparison, primary plastids of Archaeplastida (plants, green algae, red algae, and glaucophytes) arose upon a single endosymbiosis event with a cyanobacterium and are surrounded by two membranes. Complex plastids were acquired several times by unrelated groups nested within eukaryotic heterotrophs, suggesting complex plastids are somewhat easier to obtain than primary plastids. This is consistent with the existence of higher-order and serial endosymbioses, i.e., engulfment of complex plastid-bearing algae by (tertiary) eukaryotic hosts and functional plastid replacements, respectively. Plastid endosymbiosis is typical by a massive transfer of genetic material from the endosymbiont to the host nucleus and metabolic rearrangements related to the trophic switch to phototrophy; this is necessary to establish metabolic integration of the plastid and control over its division. Although photosynthesis is the main advantage of plastid acquisition, algae that lost photosynthesis often maintain complex plastids, suggesting their roles beyond photosynthesis. This chapter summarizes basic knowledge on acquisition and functions of complex plastid.

Complete chloroplast genomes of the Chlamydomonas reinhardtii nonphotosynthetic mutants CC-1375, CC-373, CC-4199, CC-2359 and CC-1051

The chloroplast genomes (cpDNA) of five Chlamydomonas reinhardtii nonphotosynthetic mutants were sequenced. The architecture, gene content, and synteny of the cpDNAs from the five mutants are identical to the C. reinhardtii 'wild-type' plastome. A small number of differences at sequence level between coding regions of the reference genome and the cpDNAs of the mutants were detected. The vast majority of the sequence differences were synonymous and likely due to nucleotide substitutions preceding the generation of the mutant strains, but not caused by the erosion of the cpDNA following the loss of photosynthesis.

Effect of light and prey availability on gene expression of the mixotrophic chrysophyte, Ochromonas sp

Background

Ochromonas is a genus of mixotrophic chrysophytes that is found ubiquitously in many aquatic environments. Species in this genus can be important consumers of bacteria but vary in their ability to perform photosynthesis. We studied the effect of light and bacteria on growth and gene expression of a predominantly phagotrophic Ochromonas species. Axenic cultures of Ochromonas sp. were fed with heat-killed bacteria (HKB) and grown in constant light or darkness. RNA was extracted from cultures in the light or in the dark with HKB present (Light + HKB; Dark + HKB), and in the light after HKB were depleted (Light + depleted HKB).

Results

There were no significant differences in the growth or bacterial ingestion rates between algae grown in light or dark conditions. The availability of light led to a differential expression of only 8% of genes in the transcriptome. A number of genes associated with photosynthesis, phagotrophy, and tetrapyrrole synthesis was upregulated in the Light + HKB treatment compared to Dark + HKB. Conversely, the comparison between the Light + HKB and Light + depleted HKB treatments revealed that the presence of HKB led to differential expression of 59% of genes, including the majority of genes involved in major carbon and nitrogen metabolic pathways. Genes coding for unidirectional enzymes for the utilization of glucose were upregulated in the presence of HKB, implying increased glycolytic activities during phagotrophy. Algae without HKB upregulated their expression of genes coding for ammonium transporters, implying uptake of inorganic nitrogen from the culture medium when prey were unavailable.

Conclusions

Transcriptomic results agreed with previous observations that light had minimal effect on the population growth of Ochromonas sp. However, light led to the upregulation of a number of phototrophy- and phagotrophy-related genes, while the availability of bacterial prey led to prominent changes in major carbon and nitrogen metabolic pathways. Our study demonstrated the potential of transcriptomic approaches to improve our understanding of the trophic physiologies of complex mixotrophs, and revealed responses in Ochromonas sp. not apparent from traditional culture studies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3549-1) contains supplementary material, which is available to authorized users.

The plastid genomes of nonphotosynthetic algae are not so small after all

The thing about plastid genomes in nonphotosynthetic plants and algae is that they are usually very small and highly compact. This is not surprising: a heterotrophic existence means that genes for photosynthesis can be easily discarded. But the loss of photosynthesis cannot explain why the plastomes of heterotrophs are so often depauperate in noncoding DNA. If plastid genomes from photosynthetic taxa can span the gamut of compactness, why can't those of nonphotosynthetic species? Well, recently we showed that they can. The free-living, heterotrophic green alga Polytoma uvella has a plastid genome boasting more than 165 kilobases of noncoding DNA, making it the most bloated plastome yet found in a heterotroph. In this addendum to the primary study, we elaborate on why the P. uvella plastome is so inflated, discussing the potential impact of a free-living vs. parasitic lifestyle on plastid genome expansion in nonphotosynthetic lineages.

The Plastid Genome of Polytoma uvella Is the Largest Known among Colorless Algae and Plants and Reflects Contrasting Evolutionary Paths to Nonphotosynthetic Lifestyles

The loss of photosynthesis is frequently associated with parasitic or pathogenic lifestyles, but it also can occur in free-living, plastid-bearing lineages. A common consequence of becoming nonphotosynthetic is the reduction in size and gene content of the plastid genome. In exceptional circumstances, it can even result in the complete loss of the plastid DNA (ptDNA) and its associated gene expression system, as reported recently in several lineages, including the nonphotosynthetic green algal genus Polytomella Closely related to Polytomella is the polyphyletic genus Polytoma, the members of which lost photosynthesis independently of Polytomella Species from both genera are free-living organisms that contain nonphotosynthetic plastids, but unlike Polytomella, Polytoma members have retained a genome in their colorless plastid. Here, we present the plastid genome of Polytoma uvella: to our knowledge, the first report of ptDNA from a nonphotosynthetic chlamydomonadalean alga. The P. uvella ptDNA contains 25 protein-coding genes, most of which are related to gene expression and none are connected to photosynthesis. However, despite its reduced coding capacity, the P. uvella ptDNA is inflated with short repeats and is tens of kilobases larger than the ptDNAs of its closest known photosynthetic relatives, Chlamydomonas leiostraca and Chlamydomonas applanata In fact, at approximately 230 kb, the ptDNA of P. uvella represents the largest plastid genome currently reported from a nonphotosynthetic alga or plant. Overall, the P. uvella and Polytomella plastid genomes reveal two very different evolutionary paths following the loss of photosynthesis: expansion and complete deletion, respectively. We hypothesize that recombination-based DNA-repair mechanisms are at least partially responsible for the different evolutionary outcomes observed in such closely related nonphotosynthetic algae.

An intact plastid genome is essential for the survival of colorless Euglena longa but not Euglena gracilis

Euglena gracilis growth with antibacterial agents leads to bleaching, permanent plastid gene loss. Colorless Euglena (Astasia) longa resembles a bleached E. gracilis. To evaluate the role of bleaching in E. longa evolution, the effect of streptomycin, a plastid protein synthesis inhibitor, and ofloxacin, a plastid DNA gyrase inhibitor, on E. gracilis and E. longa growth and plastid DNA content were compared. E. gracilis growth was unaffected by streptomycin and ofloxacin. Quantitative PCR analyses revealed a time dependent loss of plastid genes in E. gracilis demonstrating that bleaching agents produce plastid gene deletions without affecting cell growth. Streptomycin and ofloxacin inhibited E. longa growth indicating that it requires plastid genes to survive. This suggests that evolutionary divergence of E. longa from E. gracilis was triggered by the loss of a cytoplasmic metabolic activity also occurring in the plastid. Plastid metabolism has become obligatory for E. longa cell growth. A process termed "intermittent bleaching", short term exposure to subsaturating concentrations of reversible bleaching agents followed by growth in the absence of a bleaching agent, is proposed as the molecular mechanism for E. longa plastid genome reduction. Various non-photosynthetic lineages could have independently arisen from their photosynthetic ancestors via a similar process.

A survey of Polytomella (Chlorophyceae, Chlorophyta) strains in public culture collections

Polytomella is a genus of colorless green algae in the Reinhardtinia clade of the Chlamydomonadales, which has proven useful for a broad range of studies particularly those exploring the evolutionary loss of photosynthesis and mitochondrial genomics/biochemistry. Although 13 Polytomella strain accessions are currently available from public culture collections, the taxonomic status and redundancy of many of these strains is not clear because of possible mix-ups, deficient historical records, and incomplete molecular data. This study therefore considers previously available and/or new cox1 and mitochondrial DNA telomere sequences from all 13 Polytomella strain accessions. Among four of these, namely P. parva SAG 63-3, P. piriformis SAG 63-10, P. capuana SAG 63-5, and P. magna SAG 63-9, cox1 and mitochondrial telomere regions are both highly divergent between strains. All of the remaining nine Polytomella strain accessions have cox1 sequences that are identical to that of P. parva SAG 63-3 and although five of these have a mitochondrial telomere haplotype that is identical to that of P. parva SAG 63-3, the remaining four have one of three different haplotypes. Among the 10 strains with identical cox1 sequences, we suggest that three of the telomere haplotypes are associated with distinct geographical isolates of Polytomella and the fourth evolved from one of these isolates during 50 years of active culture.

Streptophyte Terrestrialization in Light of Plastid Evolution

Key steps in evolution are often singularities. The emergence of land plants is one such case and it is not immediately apparent why. A recent analysis found that the zygnematophycean algae represent the closest relative to embryophytes. Intriguingly, many exaptations thought essential to conquer land are common among various streptophytes, but zygnematophycean algae share with land plants the transfer of a few plastid genes to the nucleus. Considering the contribution of the chloroplast to terrestrialization highlights potentially novel exaptations that currently remain unexplored. We discuss how the streptophyte chloroplast evolved into what we refer to as the embryoplast, and argue this was as important for terrestrialization by freshwater algae as the host cell-associated exaptations that are usually focused upon.

Auxenochlorella protothecoides and Prototheca wickerhamii plastid genome sequences give insight into the origins of non-photosynthetic algae

The forfeiting of photosynthetic capabilities has occurred independently many times throughout eukaryotic evolution. But almost all non-photosynthetic plants and algae still retain a colorless plastid and an associated genome, which performs fundamental processes apart from photosynthesis. Unfortunately, little is known about the forces leading to photosynthetic loss; this is largely because there is a lack of data from transitional species. Here, we compare the plastid genomes of two “transitional” green algae: the photosynthetic, mixotrophic Auxenochlorella protothecoides and the non-photosynthetic, obligate heterotroph Prototheca wickerhamii. Remarkably, the plastid genome of A. protothecoides is only slightly larger than that of P. wickerhamii, making it among the smallest plastid genomes yet observed from photosynthetic green algae. Even more surprising, both algae have almost identical plastid genomic architectures and gene compositions (with the exception of genes involved in photosynthesis), implying that they are closely related. This close relationship was further supported by phylogenetic and substitution rate analyses, which suggest that the lineages giving rise to A. protothecoides and P. wickerhamii diverged from one another around six million years ago.

Massive and widespread organelle genomic expansion in the green algal genus Dunaliella

The mitochondrial genomes of chlamydomonadalean green algae are renowned for their highly reduced and conserved gene repertoires, which are almost fixed at 12 genes across the entire lineage. The sizes of these genomes, however, are much more variable, with some species having small, compact mitochondrial DNAs (mtDNAs) and others having expanded ones. Earlier work demonstrated that the halophilic genus Dunaliella contains extremely inflated organelle genomes, but to date the mtDNA of only one isolate has been explored. Here, by surveying mtDNA architecture across the Chlamydomonadales, we show that various Dunaliella species have undergone massive levels of mitochondrial genomic expansion, harboring the most inflated, intron-dense mtDNAs available from chlorophyte green algae. The same also appears to be true for their plastid genomes, which are potentially among the largest of all plastid-containing eukaryotes. Genetic divergence data are used to investigate the underlying causes of such extreme organelle genomic architectures, and ultimately reveal order-of-magnitude differences in mitochondrial versus plastid mutation rates within Dunaliella.