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

Abstract

Elysia chlorotica is a kleptoplastic sea slug that preys on Vaucheria litorea, stealing its plastids which continue to photosynthesize for months inside the animal cells. We investigated the native 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 up to seven 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 ROS formation in V. litorea. In comparison to other tested genes, the transcripts of ftsH and translation elongation factor EF-Tu (tufA) decreased slowly during incubation of isolated V. litorea plastids. Higher level of FtsH was 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 ( 1O2), and the plastids also contain ROS-protective compounds. Our results support the view that the genetic characteristics of the plastids themselves are crucial in creating a photosynthetic sea slug. The plastid’s autonomous repair machinery is likely enhanced by low 1O2 production and by elevated expression of FtsH in the plastids.

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

“…The results of these two studies contradict each other, as Christa et al did not find any significant differences in the recovery of PSII in the presence or absence of lincomycin, whereas our data showed that lincomycin efficiently blocks the recovery process in photoinhibited plastids of E. timida. Our results would imply that freshly fed slugs do retain some capacity for PSII repair even without the help of the algal nucleus that encodes many of the proteins that are known to be related to the repair cycle at least in plants (Järvi et al 2015;Havurinne et al 2021). However, additional support for the existence of deficiencies in the PSII repair cycle in the slug plastids is available in the literature.…”

“…S1 in Online Resource). Effectiveness of 10 mg/ml lincomycin in stopping PSII repair cycle in the slug plastids has been illustrated previously (Havurinne et al 2021).…”

“…Acetabularia samples selected for photoinhibition treatments were treated in an identical manner to the slugs in f/2 medium in the absence or presence of 10 mg/ml lincomycin. Lincomycin concentrations that have been used to block the PSII repair cycle in the plastids of photosynthetic sea slugs are high, for example 8 mg/ml (Christa et al 2018) or 10 mg/ml (Havurinne et al 2021). We opted to use the same concentration of lincomycin for the slugs and the algae because preliminary experiments showed that even in the algae a high lincomycin concentration is required to block PSII recovery after photoinhibition (Fig.…”

“…It is also uncertain just how important are the native properties of the plastids themselves in terms of facilitating their survival for weeks and months inside animal cytosol in isolation from the algal nucleus. The slugs are only able to retain plastids that come from specific algae species (Christa et al 2013;de Vries et al 2013), but to what extent this is due to the general robustness of plastids of these algae (Giles and Sarafis 1972;Trench et al 1973a;1973b;Green et al 2005) or their specific genetic and photosynthetic properties (de Vries et al 2013;Christa et al 2018;Havurinne et al 2021) remains to be fully tested.…”

“…However, the effectiveness of these NPQ mechanisms in preventing net photoinhibition in E. timida in the absence of lincomycin remains controversial (Christa et al 2018;Cartaxana et al 2019). Photoinhibition and subsequent recovery of PSII in photosynthetic sea slugs in the presence of lincomycin has only been evaluated twice; once by Christa et al (2018) in E. timida and E. viridis, and recently by us in E. timida (Havurinne et al 2021). The results of these two studies contradict each other, as Christa et al did not find any significant differences in the recovery of PSII in the presence or absence of lincomycin, whereas our data showed that lincomycin efficiently blocks the recovery process in photoinhibited plastids of E. timida.…”