Baumgartner Finn A, Pavia Henrik, Toth Gunilla B
University of Gothenburg, Department of Biological and Environmental Sciences-Tjärnö, Strömstad, Sweden.
PLoS One. 2015 Apr 1;10(4):e0120874. doi: 10.1371/journal.pone.0120874. eCollection 2015.
Photosynthesis is a fundamental process sustaining heterotrophic organisms at all trophic levels. Some mixotrophs can retain functional chloroplasts from food (kleptoplasty), and it is hypothesized that carbon acquired through kleptoplasty may enhance trophic energy transfer through increased host growth efficiency. Sacoglossan sea slugs are the only known metazoans capable of kleptoplasty, but the relative fitness contributions of heterotrophy through grazing, and phototrophy via kleptoplasts, are not well understood. Fitness benefits (i.e. increased survival or growth) of kleptoplasty in sacoglossans are commonly studied in ecologically unrealistic conditions under extended periods of complete darkness and/or starvation. We compared the growth efficiency of the sacoglossan Elysia viridis with access to algal diets providing kleptoplasts of differing functionality under ecologically relevant light conditions. Individuals fed Codium fragile, which provide highly functional kleptoplasts, nearly doubled their growth efficiency under high compared to low light. In contrast, individuals fed Cladophora rupestris, which provided kleptoplasts of limited functionality, showed no difference in growth efficiency between light treatments. Slugs feeding on Codium, but not on Cladophora, showed higher relative electron transport rates (rETR) in high compared to low light. Furthermore, there were no differences in the consumption rates of the slugs between different light treatments, and only small differences in nutritional traits of algal diets, indicating that the increased growth efficiency of E. viridis feeding on Codium was due to retention of functional kleptoplasts. Our results show that functional kleptoplasts from Codium can provide sacoglossan sea slugs with fitness advantages through photosynthesis.
光合作用是维持所有营养级异养生物的基本过程。一些混合营养生物可以从食物中保留功能性叶绿体（盗食质体现象），据推测，通过盗食质体获得的碳可能通过提高宿主生长效率来增强营养能量传递。绿叶海蛞蝓是已知唯一能够进行盗食质体现象的后生动物，但通过摄食进行的异养以及通过盗食质体进行的光合作用对其相对适合度的贡献尚未得到充分了解。在完全黑暗和/或饥饿的延长时期等生态上不现实的条件下，通常会研究绿叶海蛞蝓盗食质体现象的适合度益处（即增加的生存或生长）。我们比较了在生态相关光照条件下，绿叶海蛞蝓（Elysia viridis）在获取提供不同功能盗食质体的藻类食物时的生长效率。喂食脆江蓠（Codium fragile）的个体，脆江蓠能提供高功能性的盗食质体，与低光照相比，在高光照下其生长效率几乎翻倍。相比之下，喂食石莼（Cladophora rupestris）的个体，石莼提供的盗食质体功能有限，不同光照处理下其生长效率没有差异。以江蓠为食而非石莼的海蛞蝓，与低光照相比，在高光照下显示出更高的相对电子传递速率（rETR）。此外，不同光照处理下海蛞蝓的摄食率没有差异，藻类食物的营养特性也只有很小的差异，这表明以江蓠为食的绿叶海蛞蝓生长效率的提高是由于功能性盗食质体的保留。我们的结果表明，来自江蓠的功能性盗食质体可以通过光合作用为绿叶海蛞蝓提供适合度优势。
