Looking at the benefits of marine macroalgae dietary supplementation in gilthead seabream (Sparus aurata) by blocking oxidative stress and neurotransmission disorders inflicted by formalin in eyes and brain

• ¹ University of Aveiro, Biology and CESAM, Portugal

Many health benefits have been associated with bioactive compounds of green, brown and red macroalgae (Pangestuti and Kim, 2011), including protection against oxidative stress and synaptic loss that are typical of neurodegenerative disorders (Gammone et al., 2015). While the benefits of macroalgae have been largely explored with a focus on human health related with neurological conditions (among others), advantages to fish health and, subsequently, to aquaculture remain elusive. Based on similarities of neurological pathways and neurosensory structures (sensory organs and brain) between fish and mammals, beneficial effects of macroalgae-enriched feeds are expected for fish. These benefits are probably more evident when fish are exposed to conditions that challenge their neurological status, as can be the case of formalin, a frequently used anti-parasitic agent in aquaculture (Ispir et al., 2017), previously associated with neurotoxic effects (Pandey, 2000). This study was designed to address, for the first time, the antioxidant and neurotransmission protection afforded by macroalgae-enriched diet to the eyes and brain of the gilthead seabream (Sparus aurata) exposed to formalin. For this purpose, fish were fed during 2 months with a macroalgae-enriched feed [total incorporation of 5%, with the species Ulva sp., Fucus vesiculosus and Gracilaria sp., equitably represented - algae supplementation (A)], while non-supplemented fish were fed with a standard diet (S) (feed without macroalgae). Then, both dietary background groups were subjected to a formalin (F) bath (150 μl L-1) for 1 hour (groups AF and SF). This formalin treatment was repeated two days later. Control groups, unexposed to formalin, were maintained over the experiment (A and S). Four and 18 days after the 1st exposure to formalin, fish of the different groups (A, S, AF, SF) were sacrificed and eyes and brain were collected for total glutathione quantification (GSHt), lipid peroxidation (LPO) and protein oxidation (PO; only in the eyes) assessment, as well as for an evaluation of acetylcholinesterase (AChE) activity. During the whole experiment, fish were fed twice a day, while water quality was daily monitored. After 2 months of macroalgae supplementation, no significant alterations on the eye and brain parameters could be associated to the distinct of S. aurata dietary backgrounds. However, upon formalin exposure, the effects of macroalgae supplementation on the pro-oxidant status and neurotransmission of those organs were noticeable. Concerning the eye responses, 4 days after the beginning of the formalin treatment, fish fed with a macroalgae-supplemented diet (AF) displayed an improved non-enzymatic antioxidant defense against the formalin challenge, depicted on the GSHt increase. In the same direction, macroalgae-enriched diet impaired the occurrence of protein oxidation and AChE induction promoted by formalin in the eyes of fish fed with a standard diet (SF). After 18 days, it was evident a persistence of the macroalgae protection, depicted on the capacity to avoid formalin-induced GSHt depletion, oxidative stress (as PO and LPO increases) and neurotoxicity (as AChE inhibition) observed in the eyes of non-supplemented group. Surprisingly, protein oxidation data in the eyes bring some doubts on the benefits of the macroalgae-enriched diet in the absence of a pro-oxidant challenge. A delayed impact of formalin was perceived in the brain in comparison to the eyes, since formalin effects were detected only 18 days after the exposure. At this time, formalin-induced lipid peroxidation was prevented in fish supplemented with macroalgae (AF), which couldn’t be dissociated from a notorious increase on GSHt content (that also occurred in fish fed with standard diet - SF). Regarding AChE activity, formalin induced a late activatory effect (day 18), suggesting an imbalance in cholinergic homeostasis, that was not barred by macroalgae enrichment. Overall, it was demonstrated a higher impact of formalin on S. aurata eyes, comparing to the brain, probably related with the direct contact, translating a pathophysiological challenge that brought into light the benefits of macroalgae supplementation to the fish neurosensory function. Such shielding properties deserve further research under the aquaculture context, mitigating the impact of the use of formalin and other chemotherapeutic agents.