The growth and nitrogen utilization strategies in two dominant Symbiodiniaceae species facing nitrogen deficiency and enrichment

Symbiodiniaceae, a diverse group of unicellular dinoflagellates, are well known as endosymbionts of marine invertebrates (e.g., corals, giant clams and foraminiferans). Currently, how in vitro cultured Symbiodiniaceae cope with nitrogen (N) deficiency and enhancement remain largely unexplored. To address this knowledge gap, this study investigated the dynamics of growth, photosynthesis and crystalline guanine formation in response to differential N supplies in two dominant Symbiodiniaceae species: Durusdinium trenchii and Cladocopium goreaui. The results indicated growth rate and photosynthesis were closely related to N sources and concentrations. Under N deficiency, cell growth was inhibited. As N concentration increased, both D. trenchii and C. goreaui exhibited flexible strategies for utilizing different N sources. When provided with nitrate (NO₃⁻), C. goreaui and D. trenchii showed an enhancement in the algal growth and photosynthetic efficiency (Fv/Fm). When ammonium (NH₄⁺) was supplied, a moderate increase of NH₄⁺ can benefit cell growth and photosynthesis, but excessive enrichment adversely affected algal growth. Additionally, Raman microscopy demonstrated that cellular crystalline guanine was formed by C. goreaui when exposed to N supply, but gradually decreased as N was consumed in medium. A similar phenomenon was observed in D. trenchii. We proposed that crystalline guanine may serve as an important N storage and utilization strategy. This study delved into the growth strategies and adaptability of Symbiodiniaceae to varying N nutritional environments, which contributes to understanding the symbiotic relationship between Symbiodiniaceae and corals.