Lysosomal responses to heat-shock of seasonal temperature extremes in Cd-exposed mussels

• Heat-shock induced remarkable lysosomal enlargement and membrane destabilisation.
• Mussels seem to be better prepared against heat shock in summer than in winter.
• Lysosomal responses were more marked in summer than in winter, but recovery was faster as well.
• Cd-exposure attenuated inter-season differences in lysosomal biomarkers.
• Lysosomal responses to heat-shock were compromised in Cd-exposed mussels.

The present study was aimed at determining the effect of temperature extremes on lysosomal biomarkers in mussels exposed to a model toxic pollutant (Cd) at different seasons. For this purpose, temperature was elevated 10 °C (from 12 °C to 22 °C in winter and from 18 °C to 28 °C in summer) for a period of 6 h (heat-shock) in control and Cd-exposed mussels, and then returned back to initial one. Lysosomal membrane stability and lysosomal structural changes in digestive gland were investigated. In winter, heat-shock reduced the labilisation period (LP) of the lysosomal membrane, especially in Cd-exposed mussels, and provoked transient lysosomal enlargement. LP values recovered after the heat-shock cessation but lysosomal enlargement prevailed in both experimental groups. In summer, heat-shock induced remarkable reduction in LP and lysosomal enlargement (more markedly in Cd-exposed mussels), which recovered within 3 days. Besides, whilst heat-shock effects on LP were practically identical for Cd-exposed mussels in winter and summer, the effects were longer-lasting in summer than in winter for control mussels. Thus, lysosomal responsiveness after heat-shock was higher in summer than in winter but recovery was faster as well, and therefore the consequences of the heat shock seem to be more decisive in winter. In contrast, inter-season differences were attenuated in the presence of Cd. Consequently, mussels seem to be better prepared in summer than in winter to stand short periods of abrupt temperature change; this is, however, compromised when mussels are exposed to pollutants such as Cd.