Arginine Biosynthesis by a Bacterial Symbiont Enables Nitric Oxide Production and Facilitates Larval Settlement in the Marine-Sponge Host

• Published in: Current biology Vol. 31; no. 2; pp. 433 - 437.e3
• Main Authors: Song, Hao, Hewitt, Olivia H., Degnan, Sandie M.
• Format: Journal Article
• Language: English
• Published: England Elsevier Inc 25-01-2021
• Subjects: amino acid; Amphimedon; arginine; holobiont; larva; metamorphosis; Porifera; symbiosis; arginine; holobiont; Porifera; amino acid; Amphimedon; symbiosis; larva; metamorphosis
• ISSN: 0960-9822; 1879-0445
• DOI: 10.1016/j.cub.2020.10.051

Larval settlement and metamorphosis are regulated by nitric oxide (NO) signaling in a wide diversity of marine invertebrates.1–10 It is thus surprising that, in most invertebrates, the substrate for NO synthesis—arginine—cannot be biosynthesized but instead must be exogenously sourced.11 In the sponge Amphimedon queenslandica, vertically inherited proteobacterial symbionts in the larva are able to biosynthesize arginine.12,13 Here, we test the hypothesis that symbionts provide arginine to the sponge host so that nitric oxide synthase expressed in the larva can produce NO, which regulates metamorphosis,8 and the byproduct citrulline (Figure 1). First, we find support for an arginine-citrulline biosynthetic loop in this sponge larval holobiont by using stable isotope tracing. In symbionts, incorporated 13C-citrulline decreases as 13C-arginine increases, consistent with the use of exogenous citrulline for arginine synthesis. In contrast, 13C-citrulline accumulates in larvae as 13C-arginine decreases, demonstrating the uptake of exogenous arginine and its conversion to NO and citrulline. Second, we show that, although Amphimedon larvae can derive arginine directly from seawater, normal settlement and metamorphosis can occur in artificial sea water lacking arginine. Together, these results support holobiont complementation of the arginine-citrulline loop and NO biosynthesis in Amphimedon larvae, suggesting a critical role for bacterial symbionts in the development of this marine sponge. Given that NO regulates settlement and metamorphosis in diverse animal phyla1–10 and arginine is procured externally in most animals,11 we propose that symbionts might play an equally critical regulatory role in this essential life cycle transition in other metazoans.
•Bacterial symbionts can use citrulline from the sponge host to synthesize arginine•Symbionts can supply arginine back to their sponge host•Arginine is used to produce nitric oxide necessary for settlement by sponge larvae•Symbionts play a critical role in completing the sponge life cycle
Many marine invertebrates require nitric oxide (NO) for larval settlement yet cannot synthesize the arginine needed for NO production. Song et al. show that, in the marine sponge Amphimedon queenslandica, bacterial symbionts can play a critical role in animal development by providing their host with the arginine needed for larval settlement.