Histopathological alterations, physiological limits, and molecular changes of juvenile Sparus aurata in response to thermal stress

Current concerns about climate change have led to a great body of literature on thermal tolerance. However, it is still difficult to understand and relate biological changes at different organizational levels. This is especially important in commercial species. We aimed to test the effects of temperature stress in juvenile gilthead seabream Sparus aurata in order to contribute to the understanding of the vulnerability of this species to temperature changes, heat waves, and potentially climate warming. Here we applied an integrative approach, considering indicators at the biochemical, cellular, and physiological levels. Firstly, the upper thermal limit was estimated via the critical thermal maximum (CTMax); secondly, levels of Hsp70 (reversible protein damage) and total ubiquitin (irreversible protein damage) were quantified in several tissues via ELISAs; and thirdly, histological analyses were performed to identify cellular structural damage due to temperature and how it correlates to biochemical alterations. Results showed that mean (+/-SD) CTMax was 35.5 +/- 0.5 degrees C. Absolute amounts of both Hsp70 and total ubiquitin varied significantly among organs, with gills having the highest amounts of both. Biomarker and histopathological results indicated that S. aurata might be particularly sensitive to water temperatures >= 28 degrees C. At 30 degrees C, S. aurata juveniles showed severe signs of stress, with increased biomarker levels in almost every organ tested and significant cellular damage (atrophy, inflammation, micro-hemorrhage, hyperemia, hyperplasia). Therefore, S. aurata may be vulnerable to heat wave events that currently make water temperature attain 28 degrees C for several weeks (30 degrees C by 2100 under a climate change scenario). Thus, this species might be vulnerable to a rise in sea temperature, and our research may be important from a management perspective, as S. aurata is a major commercial species.