GLOBAL TRANSCRIPTOME PROFILING OF MARINE FISH IN RESPONSE TO ENVIRONMENTAL CHANGE: SALINITY AND AMMONIA INTERACTIONS

Salinity fluctuation is one of the main factors affecting the performance of marine fish. In addition, water borne ammonia may occur simultaneously with salinity stress. The adaptive capacities to cope with these environmental cues alone or in combination are extensively addressed in fish at physiological, biochemical and (sub) cellular level. To date, studies revealing the global response at transcriptome level is still lacking.

The objective of the present study was to elucidate genome-wide transcriptional responses underlying reduced seawater salinity challenge and ammonia toxicity as single and combined factor in juvenile European seabass (Dicentrarchus labrax). This is a marine teleost whose juveniles migrate between open sea and estuaries or lagoons, and thus often challenged with hypo-saline environment. European seabass is one of the most preferred fish species for aquaculture; also possess high commercial and ecological value. By using RNA next generation sequencing approach we intend to identify the genes and the associated pathways that are likely to be involved  specifically as well as common to ammonia and salinity challenge. Fish were progressively acclimated to normal seawater (32 ppt) and reduced seawater salinity (10 ppt). Following acclimation to different salinities for two weeks, fish were exposed to high environmental ammonia (HEA, 20 mg/L ∼1.18 mM representing 50% of 96 h LC₅₀ value for ammonia) for 12 h, 84 h and 180 h. RNA sequencing (Illumina HiSeq platform) of brain tissue samples from the experimental conditions yielded 12490 contigs of which 2128 contigs were significantly modulated in any of the treatment (salinity, ammonia or combination). Low salinity resulted in the up-regulation of 370 and down-regulation of 832 contigs, whereas in HEA exposure 122 contigs were up-regulated and 270 contigs were down-regulated. While looking for the combinatorial effects, we found that 74 contigs were commonly up-regulated and 94 were down-regulated in both these treatments.  A total of 11,578 protein-coding contigs were identified based on sequence similarities with known proteins.  Gene enrichment analysis revealed that  genes differentially expressed in response to salt stress were highly represented in ion-regulation (typically Na⁺ transporter), cell adhesion and communication, transmembrane signalling receptor, Na⁺/K⁺-ATPase and oxidative stress. Transcripts involved in amino acid biosynthetic/metabolic process (typically glutamate synthase, glutamate receptor signalling), anti-oxidant defense system (peroxidase activity), neurotransmitter and ion-regulation were highly represented in response to ammonia toxicity. During the combined exposure of salinity stress and ammonia- ion transporters, oxidative and anti-oxidant related genes were highly expressed. Overall, our data highlighted that the compensatory responses incited at transcript level against salinity stress were improved during the combined effect of hypo-osmotic stress and ammonia. It suggests that ammonia exposure ameliorates protective responses during reduced sea water salinity (10 ppt).