Photoprotection in sequestered plastids of sea slugs and respective algal sources

Food shaped photosynthesis: Photophysiology of the sea slug Elysia viridis fed with two alternative chloroplast donors

Background

Some Sacoglossa sea slugs steal and integrate chloroplasts derived from the algae they feed on into their cells where they continue to function photosynthetically, a process termed kleptoplasty. The stolen chloroplasts - kleptoplasts - can maintain their functionality up to several months and support animal metabolism. However, chloroplast longevity can vary depending on sea slug species and algal donor. In this study, we focused on Elysia viridis, a polyphagous species that is mostly found associated with the macroalga Codium tomentosum, but that was reported to eat other macroalgae, including Chaetomorpha sp.

Methods

We have investigated the changes in E. viridis physiology when provided with the two different food sources to evaluate to which extent the photosynthetic and photoprotective mechanisms of the algae chloroplasts matched those of the plastids once in the animal cells. To perform the study, we rely on the evaluation of chlorophyll a variable fluorescence to study the photophysiological state of the integrated kleptoplasts and high-performance liquid chromatography (HPLC) to study variations in the photosynthetic pigments.

Results

We observed that the photosynthetic efficiency of E. viridis is lower when fed with Chaetomorpha. Also, significant differences were observed in the non-photochemical quenching (NPQ) abilities of the sea slugs. While sea slugs fed with C. tomentosum react similarly to high-light stress as the alga, E. viridis hosting Chaetomorpha chloroplasts were unable to properly recover from photoinhibition or perform a functional xanthophyll cycle (XC).

Conclusions

Our results showed that, even if the sea slugs fed with the two algae show photosynthetic activities like the respective algal donors, not all the photoprotective mechanisms present in Chaetomorpha can be maintained in E. viridis. This indicates that the functionality of the kleptoplasts does not depend solely on their origin but also on the degree of compatibility with the animal species integrating them.

Photoprotective mechanisms in Elysia species hosting Acetabularia chloroplasts shed light on host-donor compatibility in photosynthetic sea slugs

Sacoglossa sea slugs have garnered attention due to their ability to retain intracellular functional chloroplasts from algae, while degrading other algal cell components. While protective mechanisms that limit oxidative damage under excessive light are well documented in plants and algae, the photoprotective strategies employed by these photosynthetic sea slugs remain unresolved. Species within the genus Elysia are known to retain chloroplasts from various algal sources, but the extent to which the metabolic processes from the donor algae can be sustained by the sea slugs is unclear. By comparing responses to high-light conditions through kinetic analyses, molecular techniques, and biochemical assays, this study shows significant differences between two photosynthetic Elysia species with chloroplasts derived from the green alga Acetabularia acetabulum. Notably, Elysia timida displayed remarkable tolerance to high-light stress and sophisticated photoprotective mechanisms such as an active xanthophyll cycle, efficient D1 protein recycling, accumulation of heat-shock proteins and α-tocopherol. In contrast, Elysia crispata exhibited absence or limitations in these photoprotective strategies. Our findings emphasize the intricate relationship between the host animal and the stolen chloroplasts, highlighting different capacities to protect the photosynthetic organelle from oxidative damage.

Shared ancestry of algal symbiosis and chloroplast sequestration in foraminifera

Foraminifera are unicellular organisms that established the most diverse algal symbioses in the marine realm. Endosymbiosis repeatedly evolved in several lineages, while some engaged in the sequestration of chloroplasts, known as kleptoplasty. So far, kleptoplasty has been documented exclusively in the rotaliid clade. Here, we report the discovery of kleptoplasty in the species Hauerina diversa that belongs to the miliolid clade. The existence of kleptoplasty in the two main clades suggests that it is more widespread than previously documented. We observed chloroplasts in clustered structures within the foraminiferal cytoplasm and confirmed their functionality. Phylogenetic analysis of 18S ribosomal RNA gene sequences showed that H. diversa branches next to symbiont-bearing Alveolinidae. This finding represents evidence of of a relationship between kleptoplastic and symbiotic foraminifera.. Analysis of ribosomal genes and metagenomics revealed that alveolinid symbionts and kleptoplasts belong to the same clade, which suggests a common ancestry.

Kleptoplasty: Getting away with stolen chloroplasts

Kleptoplasty, the process by which a host organism sequesters and retains algal chloroplasts, is relatively common in protists. The origin of the plastid varies, as do the length of time it is retained in the host and the functionality of the association. In metazoa, the capacity for long-term (several weeks to months) maintenance of photosynthetically active chloroplasts is a unique characteristic of a handful of sacoglossan sea slugs. This capability has earned these slugs the epithets "crawling leaves" and "solar-powered sea slugs." This Unsolved Mystery explores the basis of chloroplast maintenance and function and attempts to clarify contradictory results in the published literature. We address some of the mysteries of this remarkable association. Why are functional chloroplasts retained? And how is the function of stolen chloroplasts maintained without the support of the algal nucleus?

Ultraviolet screening by slug tissue and tight packing of plastids protect photosynthetic sea slugs from photoinhibition

One of the main mysteries regarding photosynthetic sea slugs is how the slug plastids handle photoinhibition, the constant light-induced damage to Photosystem II of photosynthesis. Recovery from photoinhibition involves proteins encoded by both the nuclear and plastid genomes, and slugs with plastids isolated from the algal nucleus are therefore expected to be incapable of constantly repairing the damage as the plastids inside the slugs grow old. We studied photoinhibition-related properties of the sea slug Elysia timida that ingests its plastids from the green alga Acetabularia acetabulum. Spectral analysis of both the slugs and the algae revealed that there are two ways the slugs use to avoid major photoinhibition of their plastids. Firstly, highly photoinhibitory UV radiation is screened by the slug tissue or mucus before it reaches the plastids. Secondly, the slugs pack the plastids tightly in their thick bodies, and therefore plastids in the outer layers protect the inner ones from photoinhibition. Both properties are expected to greatly improve the longevity of the plastids inside the slugs, as the plastids do not need to repair excessive amounts of damage.

Discovery of a novel siphonaxanthin biosynthetic precursor in Codium fragile that accumulates only by exposure to blue-green light

Photosynthetic organisms adapt to a variety of light conditions. Codium fragile, a macrosiphonous green alga, binds a unique carbonyl carotenoid, siphonaxanthin, to its major photosynthetic light-harvesting complexes, allowing it to utilize dim blue-green light for photosynthesis. Here, we describe the absolute chemical structure of a novel siphonaxanthin biosynthetic precursor, 19-deoxysiphonaxanthin, that accumulates specifically in the photosynthetic antenna only when cultivated under blue-green light. The action spectra of pigment accumulation suggest that siphonaxanthin biosynthesis is regulated by a specific wavelength profile. The results provide clues to a new acclimation mechanism to withstand hours of intense light at low tide and why siphonous algae have been growing invasively on the world's coasts for more than a century.

Pigment and Fatty Acid Heterogeneity in the Sea Slug Elysia crispata Is Not Shaped by Habitat Depth

Simple Summary

Some species of sacoglossan sea slugs are able to steal chloroplasts from the algae they feed on and maintain them functional for several months, a process termed “kleptoplasty”. One of these photosynthetic slugs is Elysia crispata, found in coral reefs of the Gulf of Mexico. This sacoglossan inhabits different depths (0–25 m), being exposed to different food sources and contrasting light conditions. In this work, we characterized the pigment and fatty acid (FA) profiles, and quantified the total lipid, glycolipid and phospholipid contents of E. crispata from shallow (0–4 m) and deeper (8–12 m) waters, after a month of starvation to determine the longest and more stable retention of chloroplasts and its relation to habitat depth. Biochemical analyses allowed the identification of 12 photosynthetic pigments and 27 FAs. Heterogeneity in the composition of pigments confirmed the long-term retention of functional chloroplasts ingested from different algae. However, the differences found in pigment profile, total lipid content, and FA composition on individuals of E. crispata were not related to habitat depth. High amounts of glycolipids, exclusive chloroplast lipids, suggest a good condition of these photosynthetic organelles in animal cells. These results contribute baseline physiological data that may help explain evolutionary associations such as endosymbiosis.

Abstract

Long-term retention of functional chloroplasts in animal cells occurs only in sacoglossan sea slugs. Analysis of molecules related to the maintenance of these organelles can provide valuable information on this trait (kleptoplasty). The goal of our research was to characterize the pigment and fatty acid (FA) composition of the sea slug Elysia crispata and their associated chloroplasts that are kept functional for a long time, and to quantify total lipid, glycolipid and phospholipid contents, identifying differences between habitats: shallow (0–4 m) and deeper (8–12 m) waters. Specimens were sampled and analyzed after a month of food deprivation, through HPLC, GC-MS and colorimetric methods, to ensure an assessment of long-term kleptoplasty in relation to depth. Pigment signatures indicate that individuals retain chloroplasts from different macroalgal sources. FA classes, phospholipid and glycolipid contents displayed dissimilarities between depths. However, heterogeneities in pigment and FA profiles, as well as total lipid, glycolipid and phospholipid amounts in E. crispata were not related to habitat depth. The high content of chloroplast origin molecules, such as Chl a and glycolipids after a month of starvation, confirms that E. crispata retains chloroplasts in good biochemical condition. This characterization fills a knowledge gap of an animal model commonly employed to study kleptoplasty.

Genetic autonomy and low singlet oxygen yield support kleptoplast functionality in photosynthetic sea slugs

The kleptoplastic sea slug Elysia chlorotica consumes Vaucheria litorea, stealing its plastids, which then photosynthesize inside the animal cells for months. We investigated the properties of V. litorea plastids to understand how they withstand the rigors of photosynthesis in isolation. Transcription of specific genes in laboratory-isolated V. litorea plastids was monitored for 7 days. The involvement of plastid-encoded FtsH, a key plastid maintenance protease, in recovery from photoinhibition in V. litorea was estimated in cycloheximide-treated cells. In vitro comparison of V. litorea and spinach thylakoids was applied to investigate reactive oxygen species formation in V. litorea. In comparison to other tested genes, the transcripts of ftsH and translation elongation factor EF-Tu (tufA) decreased slowly in isolated V. litorea plastids. Higher levels of FtsH were also evident in cycloheximide-treated cells during recovery from photoinhibition. Charge recombination in PSII of V. litorea was found to be fine-tuned to produce only small quantities of singlet oxygen, and the plastids also contained reactive oxygen species-protective compounds. Our results support the view that the genetic characteristics of the plastids are crucial in creating a photosynthetic sea slug. The plastid's autonomous repair machinery is likely enhanced by low singlet oxygen production and elevated expression of FtsH.

Genetic autonomy and low singlet oxygen yield support kleptoplast functionality in photosynthetic sea slugs

The kleptoplastic sea slug Elysia chlorotica consumes Vaucheria litorea, stealing its plastids, which then photosynthesize inside the animal cells for months. We investigated the properties of V. litorea plastids to understand how they withstand the rigors of photosynthesis in isolation. Transcription of specific genes in laboratory-isolated V. litorea plastids was monitored for 7 days. The involvement of plastid-encoded FtsH, a key plastid maintenance protease, in recovery from photoinhibition in V. litorea was estimated in cycloheximide-treated cells. In vitro comparison of V. litorea and spinach thylakoids was applied to investigate reactive oxygen species formation in V. litorea. In comparison to other tested genes, the transcripts of ftsH and translation elongation factor EF-Tu (tufA) decreased slowly in isolated V. litorea plastids. Higher levels of FtsH were also evident in cycloheximide-treated cells during recovery from photoinhibition. Charge recombination in PSII of V. litorea was found to be fine-tuned to produce only small quantities of singlet oxygen, and the plastids also contained reactive oxygen species-protective compounds. Our results support the view that the genetic characteristics of the plastids are crucial in creating a photosynthetic sea slug. The plastid's autonomous repair machinery is likely enhanced by low singlet oxygen production and elevated expression of FtsH.

Prevalence and Photobiology of Photosynthetic Dinoflagellate Endosymbionts in the Nudibranch Berghia stephanieae

Simple Summary

Some sea slugs have evolved highly specialized feeding habits and solely prey upon a reduced number of species. This is the case of Berghia stephanieae, a sea slug that feeds exclusively on glass anemones, such as Exaiptasia diaphana. Glass anemones host photosynthetic microalgae that B. stephanieae ingest when preying upon E. diaphana. The association between these photosynthetic microalgae and sea slugs appears to be limited in time, particularly if B. stephanieae is deprived of prey hosting these microalgae. In the present study, we validate the use of a non-invasive and non-destructive approach that allows monitoring the persistence of this association in live sea slugs by measuring chlorophyll fluorescence. A complete loss of photosynthetic microalgae was observed within 8 days when animals were deprived of food or fed glass anemones with no microalgae (bleached anemones). As such, the association between B. stephanieae and photosynthetic microalgae acquired when preying glass anemones is not a true symbiosis. Future studies may use the technique here described to monitor the prevalence of the association between sea slugs and photosynthetic microalgae, particularly under bleaching events that will impair sea slugs to acquire microalgae by preying upon their invertebrate hosts.

Abstract

Berghia stephanieae is a stenophagous sea slug that preys upon glass anemones, such as Exaiptasia diaphana. Glass anemones host photosynthetic dinoflagellate endosymbionts that sea slugs ingest when consuming E. diaphana. However, the prevalence of these photosynthetic dinoflagellate endosymbionts in sea slugs appears to be short-lived, particularly if B.stephanieae is deprived of prey that host these microalgae (e.g., during bleaching events impacting glass anemones). In the present study, we investigated this scenario, along with food deprivation, and validated the use of a non-invasive and non-destructive approach employing chlorophyll fluorescence as a proxy to monitor the persistence of the association between sea slugs and endosymbiotic photosynthetic dinoflagellates acquired through the consumption of glass anemones. Berghia stephanieae deprived of a trophic source hosting photosynthetic dinoflagellate endosymbionts (e.g., through food deprivation or by feeding on bleached E. diaphana) showed a rapid decrease in minimum fluorescence (F_o) and photosynthetic efficiency (F_v/F_m) when compared to sea slugs fed with symbiotic anemones. A complete loss of endosymbionts was observed within 8 days, confirming that no true symbiotic association was established. The present work opens a new window of opportunity to rapidly monitor in vivo and over time the prevalence of associations between sea slugs and photosynthetic dinoflagellate endosymbionts, particularly during bleaching events that prevent sea slugs from incorporating new microalgae through trophic interactions.

Chloroplast acquisition without the gene transfer in kleptoplastic sea slugs, Plakobranchus ocellatus

Some sea slugs sequester chloroplasts from algal food in their intestinal cells and photosynthesize for months. This phenomenon, kleptoplasty, poses a question of how the chloroplast retains its activity without the algal nucleus. There have been debates on the horizontal transfer of algal genes to the animal nucleus. To settle the arguments, this study reported the genome of a kleptoplastic sea slug, Plakobranchus ocellatus, and found no evidence of photosynthetic genes encoded on the nucleus. Nevertheless, it was confirmed that light illumination prolongs the life of mollusk under starvation. These data presented a paradigm that a complex adaptive trait, as typified by photosynthesis, can be transferred between eukaryotic kingdoms by a unique organelle transmission without nuclear gene transfer. Our phylogenomic analysis showed that genes for proteolysis and immunity undergo gene expansion and are up-regulated in chloroplast-enriched tissue, suggesting that these molluskan genes are involved in the phenotype acquisition without horizontal gene transfer.

Chlorophyll fluorescence - A tool to assess photosynthetic performance and stress photophysiology in symbiotic marine invertebrates and seaplants

Chlorophyll a fluorescence is increasingly being used as a rapid, non-invasive, sensitive and convenient indicator of photosynthetic performance in marine autotrophs. This review presents the methodology, applications and limitations of chlorophyll fluorescence in marine studies. The various chlorophyll fluorescence tools such as Pulse-Amplitude-Modulated (PAM) and Fast Repetition Rate (FRR) fluorometry used in marine scientific studies are discussed. Various commonly employed chlorophyll fluorescence parameters are elaborated. The application of chlorophyll fluorescence in measuring natural variations, stress, stress tolerance and acclimation/adaptation to changing environment in primary producers such as microalgae, macroalgae, seagrasses and mangroves, and marine symbiotic invertebrates, namely symbiotic sponges, hard corals and sea anemones, kleptoplastic sea slugs and giant clams is critically assessed. Stressors include environmental, biological, physical and chemical ones. The strengths, limitations and future perspectives of the use of chlorophyll fluorescence technique as an assessment tool in symbiotic marine organisms and seaplants are discussed.

Photosynthetic sea slugs induce protective changes to the light reactions of the chloroplasts they steal from algae

Sacoglossan sea slugs are able to maintain functional chloroplasts inside their own cells, and mechanisms that allow preservation of the chloroplasts are unknown. We found that the slug Elysia timida induces changes to the photosynthetic light reactions of the chloroplasts it steals from the alga Acetabularia acetabulum. Working with a large continuous laboratory culture of both the slugs (>500 individuals) and their prey algae, we show that the plastoquinone pool of slug chloroplasts remains oxidized, which can suppress reactive oxygen species formation. Slug chloroplasts also rapidly build up a strong proton-motive force upon a dark-to-light transition, which helps them to rapidly switch on photoprotective non-photochemical quenching of excitation energy. Finally, our results suggest that chloroplasts inside E. timida rely on oxygen-dependent electron sinks during rapid changes in light intensity. These photoprotective mechanisms are expected to contribute to the long-term functionality of the chloroplasts inside the slugs.

Plants, algae and a few other organisms rely on a process known as photosynthesis to fuel themselves, as they can harness cellular structures called chloroplasts to convert light into usable energy. Animals typically lack chloroplasts, making them unable to use photosynthesis to power themselves. The sea slug Elysia timida, however, can steal whole chloroplasts from the cells of the algae it consumes: the stolen structures then become part of the cells in the gut of the slug, allowing the animal to gain energy from sunlight.

Once they are in the digestive system of the slug, the chloroplasts survive and keep working for longer than expected. Indeed, these structures are often harmed as a side effect of photosynthesis, but the sea slug does not have the right genes to help repair this damage. In addition, conditions inside animal cells are widely different to the ones found inside algae and plants. It is not clear then how the sea slug extends the lifespan of its chloroplasts by preventing damage caused by sunlight.

To investigate this question, Havurinne and Tyystjärvi compared photosynthesis in sea slugs and the algae they eat. A range of methods, including measuring fluorescence from the chloroplasts, was used: this revealed that the slug changes the inside of the stolen chloroplasts, making them more resistant to damage.

First, when exposed to light the stolen chloroplasts can quickly switch on a mechanism that dissipates light energy to heat, which is less damaging. Second, a molecule that serves as an intermediate during photosynthesis is kept in a ‘safe’ state which prevents it from creating harmful compounds. And finally, additional safeguard molecules ‘deactivate’ compounds that could otherwise mediate damaging reactions. Overall, these measures may reduce the efficiency of the chloroplasts but allow them to keep working for much longer.

Early chloroplasts were probably independent bacteria that were captured and ‘domesticated’ by other cells for their ability to extract energy from the sun. Photosynthesizing sea slugs therefore provide an interesting way to understand some of the challenges of early life. The work by Havurinne and Tyystjärvi may also reveal new ways to harness biological processes such as photosynthesis for energy production in other contexts.

Photoprotective Role of Neoxanthin in Plants and Algae

Light is a paramount parameter driving photosynthesis. However, excessive irradiance leads to the formation of reactive oxygen species that cause cell damage and hamper the growth of photosynthetic organisms. Xanthophylls are key pigments involved in the photoprotective response of plants and algae to excessive light. Of particular relevance is the operation of xanthophyll cycles (XC) leading to the formation of de-epoxidized molecules with energy dissipating capacities. Neoxanthin, found in plants and algae in two different isomeric forms, is involved in the light stress response at different levels. This xanthophyll is not directly involved in XCs and the molecular mechanisms behind its photoprotective activity are yet to be fully resolved. This review comprehensively addresses the photoprotective role of 9′-cis-neoxanthin, the most abundant neoxanthin isomer, and one of the major xanthophyll components in plants’ photosystems. The light-dependent accumulation of all-trans-neoxanthin in photosynthetic cells was identified exclusively in algae of the order Bryopsidales (Chlorophyta), that lack a functional XC. A putative photoprotective model involving all-trans-neoxanthin is discussed.

Response of a kleptoplastidic foraminifer to heterotrophic starvation: photosynthesis and lipid droplet biogenesis

The aim of this work is to document the complex nutritional strategy developed by kleptoplastic intertidal foraminifera. We study the mixotrophic ability of a common intertidal foraminifer, Elphidium williamsoni, by (i) investigating the phylogenetic identity of the foraminiferal kleptoplasts, (ii) following their oxygenic photosynthetic capacity and (iii) observing the modification in cellular ultrastructural features in response to photoautotrophic conditions. This was achieved by coupling molecular phylogenetic analyses and TEM observations with non-destructive measurements of kleptoplast O2 production over a 15-day experimental study. Results show that the studied E. williamsoni actively selected kleptoplasts mainly from pennate diatoms and had the ability to produce oxygen, up to 13.4 nmol O2 cell-1 d-1, from low to relatively high irradiance over at least 15 days. Ultrastructural features and photophysiological data showed significant differences over time, the number of lipid droplets, residual bodies and the dark respiration increased; whereas, the number of kleptoplasts decreased accompanied by a minor decrease of the photosynthetic rate. These observations suggest that in E. williamsoni kleptoplasts might provide extra carbon storage through lipid droplets synthesis and highlight the complexity of E. williamsoni feeding strategy and the necessity of further dedicated studies regarding mechanisms developed by kleptoplastidic foraminifera for carbon partitioning and storage.

The photon menace: kleptoplast protection in the photosynthetic sea slug Elysia timida

Absorption of excessive light by photosymbiotic organisms leads to the production of reactive oxygen species that can damage both symbiont and host. This is highly relevant in sacoglossan sea slugs that host functional chloroplasts 'stolen' from their algal foods (kleptoplasts), because of limited repair capacities resulting from the absence of algal nuclear genes. Here, we experimentally demonstrate (i) a host-mediated photoprotection mechanism in the photosynthetic sea slug Elysia timida, characterized by the closure of the parapodia under high irradiance and the reduction of kleptoplast light exposure; and (ii) the activation of a reversible xanthophyll cycle in kleptoplasts, which allows excessive energy to be dissipated. The described mechanisms reduce photoinactivation under high irradiance. We conclude that both host-mediated behavioural and plastid-based physiological photoprotective mechanisms can mitigate oxidative stress induced by high light in E. timida These mechanisms may play an important role in the establishment of long-term photosynthetically active kleptoplasts.

Light harvesting complexes in chlorophyll c-containing algae

Besides the so-called 'green lineage' of eukaryotic photosynthetic organisms that include vascular plants, a huge variety of different algal groups exist that also harvest light by means of membrane intrinsic light harvesting proteins (Lhc). The main taxa of these algae are the Cryptophytes, Haptophytes, Dinophytes, Chromeridae and the Heterokonts, the latter including diatoms, brown algae, Xanthophyceae and Eustigmatophyceae amongst others. Despite the similarity in Lhc proteins between vascular plants and these algae, pigmentation is significantly different since no Chl b is bound, but often replaced by Chl c, and a large diversity in carotenoids functioning in light harvesting and/or photoprotection is present. Due to the presence of Chl c in most of the taxa the name 'Chl c-containing organisms' has become common, however, Chl b-less is more precise since some harbour Lhc proteins that only bind one type of Chl, Chl a. In recent years huge progress has been made about the occurrence and function of Lhc in diatoms, so-called fucoxanthin chlorophyll proteins (FCP), where also the first molecular structure became available recently. In addition, especially energy transfer amongst the unusual pigments bound was intensively studied in many of these groups. This review summarises the present knowledge about the molecular structure, the arrangement of the different Lhc in complexes, the excitation energy transfer abilities and the involvement in photoprotection of the different Lhc systems in the so-called Chl c-containing organisms. This article is part of a Special Issue entitled Light harvesting, edited by Dr. Roberta Croce.

Kleptoplast photoacclimation state modulates the photobehaviour of the solar-powered sea slug Elysia viridis

Some sacoglossan sea slugs incorporate intracellular functional algal chloroplasts (kleptoplasty) for periods ranging from a few days to several months. Whether this association modulates the photobehaviour of solar-powered sea slugs is unknown. In this study, the long-term kleptoplast retention species Elysia viridis showed avoidance of dark independently of light acclimation state. In contrast, Placida dendritica, which shows non-functional retention of kleptoplasts, showed no preference over dark, low or high light. High light-acclimated (HL_ac) E. viridis showed a higher preference for high light than low light-acclimated (LL_ac) conspecifics. The position of the lateral folds (parapodia) was modulated by irradiance, with increasing light levels leading to a closure of parapodia and protection of kleptoplasts from high light exposure. Furthermore, closure of parapodia occurred at higher irradiance in HL_acE. viridis Our results strongly indicate that kleptoplast photoacclimation state modulates the photobehaviour of the solar-powered sea slug E. viridis.

Lapsyn controls branch extension and positioning of astrocyte-like glia in the Drosophila optic lobe

Astrocytes have diverse, remarkably complex shapes in different brain regions. Their branches closely associate with neurons. Despite the importance of this heterogeneous glial cell type for brain development and function, the molecular cues controlling astrocyte branch morphogenesis and positioning during neural circuit assembly remain largely unknown. We found that in the Drosophila visual system, astrocyte-like medulla neuropil glia (mng) variants acquire stereotypic morphologies with columnar and layered branching patterns in a stepwise fashion from mid-metamorphosis onwards. Using knockdown and loss-of-function analyses, we uncovered a previously unrecognized role for the transmembrane leucine-rich repeat protein Lapsyn in regulating mng development. lapsyn is expressed in mng and cell-autonomously required for branch extension into the synaptic neuropil and anchoring of cell bodies at the neuropil border. Lapsyn works in concert with the fibroblast growth factor (FGF) pathway to promote branch morphogenesis, while correct positioning is essential for mng survival mediated by gliotrophic FGF signaling.

How glial cells, such as astrocytes, acquire their characteristic morphology during development is poorly understood. Here the authors describe the morphogenesis of astrocyte-like glia in the Drosophila optic lobe, and through a RNAi screen, they identify a transmembrane LRR protein–Lapsyn–that plays a critical role in this process.

On Being the Right Size as an Animal with Plastids

Plastids typically reside in plant or algal cells-with one notable exception. There is one group of multicellular animals, sea slugs in the order Sacoglossa, members of which feed on siphonaceous algae. The slugs sequester the ingested plastids in the cytosol of cells in their digestive gland, giving the animals the color of leaves. In a few species of slugs, including members of the genus Elysia, the stolen plastids (kleptoplasts) can remain morphologically intact for weeks and months, surrounded by the animal cytosol, which is separated from the plastid stroma by only the inner and outer plastid membranes. The kleptoplasts of the Sacoglossa are the only case described so far in nature where plastids interface directly with the metazoan cytosol. That makes them interesting in their own right, but it has also led to the idea that it might someday be possible to engineer photosynthetic animals. Is that really possible? And if so, how big would the photosynthetic organs of such animals need to be? Here we provide two sets of calculations: one based on a best case scenario assuming that animals with kleptoplasts can be, on a per cm2 basis, as efficient at CO2 fixation as maize leaves, and one based on 14CO2 fixation rates measured in plastid-bearing sea slugs. We also tabulate an overview of the literature going back to 1970 reporting direct measurements or indirect estimates of the CO2 fixing capabilities of Sacoglossan slugs with plastids.

Kleptoplast photosynthesis is nutritionally relevant in the sea slug Elysia viridis

Several sacoglossan sea slug species feed on macroalgae and incorporate chloroplasts into tubular cells of their digestive diverticula. We investigated the role of the “stolen” chloroplasts (kleptoplasts) in the nutrition of the sea slug Elysia viridis and assessed how their abundance, distribution and photosynthetic activity were affected by light and starvation. Elysia viridis individuals feeding on the macroalga Codium tomentosum were compared with starved specimens kept in dark and low light conditions. A combination of variable Chl a fluorescence and hyperspectral imaging, and HPLC pigment analysis was used to evaluate the spatial and temporal variability of photopigments and of the photosynthetic capacity of kleptoplasts. We show increased loss of weight and body length in dark-starved E. viridis as compared to low light-starved sea slugs. A more pronounced decrease in kleptoplast abundance and lower photosynthetic electron transport rates were observed in dark-starved sea slugs than in low light-starved animals. This study presents strong evidence of the importance of kleptoplast photosynthesis for the nutrition of E. viridis in periods of food scarcity. Deprived of photosynthates, E. viridis could accelerate the breakdown of kleptoplasts in the dark to satisfy its’ energy requirements.

Photoprotection in a monophyletic branch of chlorophyte algae is independent of energy-dependent quenching (qE)

Phototrophic organisms need to ensure high photosynthetic performance whilst suppressing reactive oxygen species (ROS)-induced stress occurring under excess light conditions. The xanthophyll cycle (XC), related to the high-energy quenching component (qE) of the nonphotochemical quenching (NPQ) of excitation energy, is considered to be an obligatory component of photoprotective mechanisms. The pigment composition of at least one representative of each major clade of Ulvophyceae (Chlorophyta) was investigated. We searched for a light-dependent conversion of pigments and investigated the NPQ capacity with regard to the contribution of XC and the qE component when grown under different light conditions. A XC was found to be absent in a monophyletic group of Ulvophyceae, the Bryopsidales, when cultivated under low light, but was triggered in one of the 10 investigated bryopsidalean species, Caulerpa cf. taxifolia, when cultivated under high light. Although Bryopsidales accumulate zeaxanthin (Zea) under high-light (HL) conditions, NPQ formation is independent of a XC and not related to qE. qE- and XC-independent NPQ in the Bryopsidales contradicts the common perception regarding its ubiquitous occurrence in Chloroplastida. Zea accumulation in HL-acclimated Bryopsidales most probably represents a remnant of a functional XC. The existence of a monophyletic algal taxon that lacks qE highlights the need for broad biodiversity studies on photoprotective mechanisms.

Functional xanthophyll cycle and pigment content of a kleptoplastic benthic foraminifer: Haynesina germanica

Some shallow water benthic foraminifera are able to retain functional chloroplasts (kleptoplasts) from their food source, i.e. diatoms. Here we assessed the functionality of the kleptoplast xanthophyll cycle (XC, i.e. the main diatom short-term photo-regulation mechanism) and we surveyed Haynesina germanica kleptoplast pigment composition over time and at different light regimes. Six common diatom lipophilic pigments were detected, two chlorophylls (Chl a, Chl c) and four carotenoids (fucoxanthin and by-products, diadinoxanthin, diatoxanthin and β-carotene), the same pigment profile as the diatom species frequently isolated at the sampling site. The xanthophyll cycle (XC) was functional with kleptoplast diatoxanthin (DT) content increase with concomitant diadinoxanthin (DD) decrease after short term light exposure. DT/(DT+DD) and DT/DD ratios increased significantly in specimens exposed to low light and high light in comparison to specimens maintained in the dark. Specimens placed in very low light after the light treatments reverted to values close to the initial ones, suggesting that H. germanica XC is functional. A functional XC is an indication of H. germanica kleptoplasts capacity for short-term photo-protection from photo-oxidative damages caused by excess of light. Furthermore, the pigment survey suggests that H. germanica preserved some chloroplasts over a longer time than others and that pigment content is influenced by previous light history. Finally, the current study highlighted seasonal differences, with higher pigment contents in winter specimens (27.35 ± 1.30 ng cell^-1) and lower in summer specimens (6.08 ± 1.21 ng cell^-1), a quantitative and qualitative composition suggesting light acclimation to low or high light availability, according to the season.

Why It Is Time to Look Beyond Algal Genes in Photosynthetic Slugs

Eukaryotic organelles depend on nuclear genes to perpetuate their biochemical integrity. This is true for mitochondria in all eukaryotes and plastids in plants and algae. Then how do kleptoplasts, plastids that are sequestered by some sacoglossan sea slugs, survive in the animals’ digestive gland cells in the absence of the algal nucleus encoding the vast majority of organellar proteins? For almost two decades, lateral gene transfer (LGT) from algae to slugs appeared to offer a solution, but RNA-seq analysis, later supported by genome sequencing of slug DNA, failed to find any evidence for such LGT events. Yet, isolated reports continue to be published and are readily discussed by the popular press and social media, making the data on LGT and its support for kleptoplast longevity appear controversial. However, when we take a sober look at the methods used, we realize that caution is warranted in how the results are interpreted. There is no evidence that the evolution of kleptoplasty in sea slugs involves LGT events. Based on what we know about photosystem maintenance in embryophyte plastids, we assume kleptoplasts depend on nuclear genes. However, studies have shown that some isolated algal plastids are, by nature, more robust than those of land plants. The evolution of kleptoplasty in green sea slugs involves many promising and unexplored phenomena, but there is no evidence that any of these require the expression of slug genes of algal origin.

Behaviour of the plathelminth Symsagittifera roscoffensis under different light conditions and the consequences for the symbiotic algae Tetraselmis convolutae

Symsagittifera roscoffensis is a plathelminth living in symbiosis with the green algae Tetraselmis convolutae. Host and symbiont are a model system for the study of endosymbiosis, which has so far mainly focused on their biochemical interactions. Symsagittifera roscoffensis is well known for its positive phototaxis that is hypothesized to optimize the symbiont's light perception for photosynthesis. In this study, we conducted a detailed analysis of phototaxis using light sources of different wavelength and brightness by videotracking. Furthermore, we compared the behavioural data with the electron transfer rate of the photosystem from cultured symbiotic cells. The symbiotic algae is adapted to low light conditions, showing a positive electron transfer rate at a photosynthetically active radiation of 0.112 µmol photons m(-2) s(-1), and S. roscoffensis showed a positive phototactic behaviour for light intensities up to 459.17 µmol photons m(-2) s(-1), which is not optimal regarding the needs of the symbiotic cells and may even harm host and symbiont. Red light cannot be detected by the animals and therefore their eyes seem not to be suitable for measuring the exact photosynthetically active radiation to the benefit of the photosymbionts.