Critical thermal maxima and oxygen uptake in Elysia viridis, a sea slug that steals chloroplasts to photosynthesize

Photosynthetic animals produce oxygen, providing an ideal lens for studying how oxygen dynamics influence thermal sensitivity. The algivorous sea slug Elysia viridis can steal and retain chloroplasts from the marine alga Bryopsis sp. for months when starved, but chloroplast retention is mere weeks when they are fed another green alga, Chaetomorpha sp. To examine plasticity in thermal tolerance and changes in net oxygen exchange when fed and starving, slugs fed each alga were acclimated to 17°C (the current maximum temperature to which they are exposed in nature) and 22°C (the increase predicted for 2100) and measured at different points during starvation. We also examined increased illumination to evaluate a potential tradeoff between increased oxygen production but faster chloroplast degradation. Following acclimation, we subjected slugs to acute thermal stress to determine their thermal tolerance. We also measured net oxygen exchange before and after acute thermal stress. Thermal tolerance improved in slugs acclimated to 22°C, indicating they can acclimate to temperatures higher than they naturally experience. All slugs exhibited net oxygen uptake, and rates were highest in recently fed slugs before exposure to acute thermal stress. Oxygen uptake was suppressed following acute thermal stress. Under brighter light, slugs exhibited improved thermal tolerance, possibly because photosynthetic oxygen production alleviated oxygen limitation. Accordingly, this advantage disappeared later in starvation when photosynthesis ceased. Thus, E. viridis can cope with heatwaves by suppressing metabolism and plastically adjusting heat tolerance; however, starvation influences a slug's thermal tolerance and oxygen uptake such that continuous access to algal food for its potential nutritive and oxygenic benefits is critical when facing thermal stress.

Rubisco and sucrose synthesis and translocation are involved in the regulation of photosynthesis in wheat with different source-sink relationships

Source-sink relationships influence photosynthesis. So far, the limiting factors for photosynthesis of wheat cultivars with different source-sink relationships have not been determined. We aimed to determine the variation patterns of photosynthetic characteristics of wheat cultivars with different source-sink relationships. In this study, two wheat cultivars with different source-sink relationships were selected for photosynthetic physiological analyses. The results showed that YM25 (source-limited cultivar) had higher photosynthetic efficiency compared to YM1 (sink-limited cultivar). This is mainly due to a stronger photochemical efficiency, electron transfer capacity, and Rubisco carboxylation capacity of YM25. YM25 accumulated less soluble carbohydrates in flag leaves than YM1. This is mainly due to the stronger sucrose synthesis and transport capacity of YM25 by presenting higher sucrose-related enzyme activities and gene expression. A PCA analysis showed that Rubisco was the main factor limiting the photosynthetic capacity of YM25. The soluble sugar accumulation in flag leaves and sink limitation decreased the photosynthetic activity of YM1. Increased N application improved source-sink relationships and increased grain yield and source leaf photosynthetic capacity in both two wheat cultivars. Taken together, our findings suggest that Rubisco and sucrose synthesis and translocation are involved in the regulation of photosynthesis of wheat cultivars with different source-sink relationships and that source and sink limitation effects should be considered in photosynthesis.

Prey species and abundance affect growth and photosynthetic performance of the polyphagous sea slug Elysia crispata

Some sacoglossan sea slugs steal functional macroalgal chloroplasts (kleptoplasts). In this study, we investigated the effects of algal prey species and abundance on the growth and photosynthetic capacity of the tropical polyphagous sea slug Elysia crispata. Recently hatched sea slugs fed and acquired chloroplasts from the macroalga Bryopsis plumosa, but not from Acetabularia acetabulum. However, adult sea slugs were able to switch diet to A. acetabulum, rapidly replacing the great majority of the original kleptoplasts. When fed with B. plumosa, higher feeding frequency resulted in significantly higher growth and kleptoplast photosynthetic yield, as well as a slower relative decrease in these parameters upon starvation. Longevity of A. acetabulum-derived chloroplasts in E. crispata was over twofold that of B. plumosa. Furthermore, significantly lower relative weight loss under starvation was observed in sea slugs previously fed on A. acetabulum than on B. plumosa. This study shows that functionality and longevity of kleptoplasts in photosynthetic sea slugs depend on the origin of the plastids. Furthermore, we have identified A. acetabulum as a donor of photosynthetically efficient chloroplasts common to highly specialized monophagous and polyphagous sea slugs capable of long-term retention, which opens new experimental routes to unravel the unsolved mysteries of kleptoplasty.

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.

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.

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.

Finding the Sweet Spot: Sub-Ambient Light Increases Fitness and Kleptoplast Survival in the Sea Slug Plakobranchus cf. ianthobaptus Gould, 1852

Sacoglossans, or "sap-sucking" sea slugs, are primarily algivorous, with many taxa exhibiting kleptoplasty, the feeding and retaining of photosynthetically active chloroplasts from algae. The Plakobranchus species complex exhibits some of the longest kleptoplast retention and survival times under starvation conditions, but the contributions of these kleptoplasts to their survival and overall fitness have been widely debated. In this study we assessed the effects of starvation and light on the fitness of Plakobranchus cf. ianthobaptus and its kleptoplasts by placing starved individuals in eight daily average light treatments, ranging from near dark (2 µmol photon m^-2 s^-1) to ambient light (470 µmol photon m^-2 s^-1). Slug weight was used as a metric of fitness, and kleptoplast photosynthetic activity was determined via maximum quantum yield (F_v/F_m) by pulse-amplitude modulated fluorometry as a proxy for kleptoplast health. Plakobranchus individuals in near-dark and high light treatments (>160 µmol photon m^-2 s^-1) experienced significantly greater weight loss than those in low light (65 µmol photon m^-2 s^-1) and moderate light treatments (95-135 µmol photon m^-2 s^-1). Additionally, individuals in high light treatments experienced a rapid decline in kleptoplast photosynthetic activity, while all other treatments experienced minimal decline. This relationship between kleptoplast degradation and weight loss suggests an important link between fitness and kleptoplasty. Given the significant negative effects of ambient conditions, regular refreshment and replenishment of kleptoplasts or physiological or behavioral adjustments are likely employed for the benefits of kleptoplasty to be maintained.

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.