Oral Administration of Antimicrobial Peptide NZ2114 Through the Microalgal Bait Tetraselmis subcordiformis (Wille) Butcher for Improving the Immunity and Gut Health in Turbot (Scophthalmus maximus L.).

Antibiotics are widely used in aquaculture to treat the bacterial diseases. However, the improper use of antibiotics could lead to environmental pollution and development of resistance. As a safe and eco-friendly alternative, antimicrobial peptides (AMPs) are commonly explored as therapeutic agents. In this study, a mutant strain of Tetraselmis subcordiformis containing AMP NZ2114 was developed and used as an oral drug delivery system to reduce the use of antibiotics in turbot (Scophthalmus maximus) aquaculture. The gut, kidney, and liver immune-related genes and their effects on gut digestion and bacterial communities in turbot fed with NZ2114 were evaluated in an 11-day feeding experiment. The results showed that compared with the group fed with wild-type T. subcordiformis, the group fed with T. subcordiformis transformants containing NZ2114 was revealed with decreased levels of both pro-inflammatory factors (TNF-α and IL-1ß), inhibitory effect on Staphylococcus aureus, Vibrio parahaemolyticus, and Vibrio splendidus demonstrated by the in vitro simulation experiments, and increased richness and diversity of the gut microbiota of turbot. In conclusion, our study provided a novel, beneficial, and low-cost method for controlling bacteria in turbot culture through the oral drug delivery systems.

Transcriptome Analysis of the Accumulation of Astaxanthin in Haematococcus pluvialis Treated with White and Blue Lights as well as Salicylic Acid.

Haematococcus pluvialis is the most commercially valuable microalga for the production of natural astaxanthin, showing enhanced production of astaxanthin with the treatments of high-intensity light and hormones. The molecular mechanisms regulating the biosynthesis of astaxanthin in H. pluvialis treated with white light, blue light, and blue light with salicylic acid (SA) were investigated based on the transcriptome analysis. Results showed that the combined treatment with both blue light and SA generated the highest production of astaxanthin. A total of 109,443 unigenes were identified to show that the genes involved in the tricarboxylic acid (TCA) cycle, the pentose phosphate pathway (PPP), and the astaxanthin biosynthesis were significantly upregulated to increase the production of the substrates for the synthesis of astaxanthin, i.e., pyruvate and glyceraldehyde-3-phosphate generated in the TCA cycle and PPP, respectively. Results of transcriptome analysis were further verified by the quantitative real-time PCR (qRT-PCR) analysis, showing that the highest content of astaxanthin was obtained with the expression of the bkt gene significantly increased. Our study provided the novel insights into the molecular mechanisms regulating the synthesis of astaxanthin and an innovative strategy combining the exogenous hormone and physical stress to increase the commercial production of astaxanthin by H. pluvialis.

Transcriptomic and Proteomic Characterizations of the Molecular Response to Blue Light and Salicylic Acid in Haematococcus pluvialis.

Haematococcus pluvialis accumulates a large amount of astaxanthin under various stresses, e.g., blue light and salicylic acid (SA). However, the metabolic response of H. pluvialis to blue light and SA is still unclear. We investigate the effects of blue light and SA on the metabolic response in H. pluvialis using both transcriptomic and proteomic sequencing analyses. The largest numbers of differentially expressed proteins (DEPs; 324) and differentially expressed genes (DEGs; 13,555) were identified on day 2 and day 7 of the treatment with blue light irradiation (150 µmol photons m-2s-1), respectively. With the addition of SA (2.5 mg/L), a total of 63 DEPs and 11,638 DEGs were revealed on day 2 and day 7, respectively. We further analyzed the molecular response in five metabolic pathways related to astaxanthin synthesis, including the astaxanthin synthesis pathway, the fatty acid synthesis pathway, the heme synthesis pathway, the reactive oxygen species (ROS) clearance pathway, and the cell wall biosynthesis pathway. Results show that blue light causes a significant down-regulation of the expression of key genes involved in astaxanthin synthesis and significantly increases the expression of heme oxygenase, which shows decreased expression by the treatment with SA. Our study provides novel insights into the production of astaxanthin by H. pluvialis treated with blue light and SA.