Growth-promoting and low-salt adaptation responses boosted by spermidine in Strombidium parasulcatum, a marine bacteriovorous ciliate potentially applied to live feeds for marine larviculture

Ciliates have been suggested as potential live food organisms for the first-feeding stage of various aquaculture animal larvae because of, among others, their wide range of body size and relatively good availability in the surrounding environment. Unfortunately, studies on the nutritional contents or large-scale cultivation methods of ciliates are relatively sparse. Here we report a novel approach to efficiently separate the spermidine-enriched bacterial feeds from marine planktonic ciliate Strombidium parasulcatum. We also report the role of spermidine (Spd) in the standard mass culture of this ciliate. Our method was able to concentrate the ciliates up to 1.22 × 10⁵ cells/mL (approximately 12-fold higher than the normal cultivation) with a recovery rate of 63.89 ± 2.254%. Regarding the role of Spd, we have allowed the bacterial feeds to accumulate a high amount of intracellular Spd from the culture medium (4.10-fold that of the control group). The ciliates receiving Spd-enrich feed exhibited a significant increase in the growth rate (increased by 10.5% to 56.4%), cellular Spd (increased by 1.23-fold to 2.81-fold), and clearance rate (∼20% within 4 h). In terms of stress vulnerability, we observed that S. parasulcatum was still able to accumulate Spd under low-salt stress (10‰ salinity). Besides, the addition of Spd could also assist ciliates in overcoming oxidative stress. Taken together, we concluded that Spd has an important function in the cultivation of the ciliate S. parasulcatum and our novel method facilitated its mass production. Therefore, the applied concepts and findings provided in this study may serve as a baseline for the future development of S. parasulcatum as a commercial live feed.