Abstract
Zooplankton from clear alpine lakes is exposed to stressful levels of solar UV radiation (UVR). As these pelagic organisms experience high UVR and large changes in solar radiation conditions between ice‐free and ice‐cover periods, they have evolved various strategies to minimize UVR exposure and damage. Here, we studied the relation between photoprotection levels (mycosporine‐like amino acids, carotenoids), antioxidant capacities, and gene expression of heat shock proteins (hsps) as indicator of stress in the copepod Cyclops abyssorum tatricus during the course of a year. Expression of hsp60, hsp70, and hsp90 was measured in the field (baseline expression [BE]) and after UVR exposure in the laboratory. The BE differed among genes and seasons (hsp60: high during summer, hsp70 and hsp90: high during the ice‐cover period). The gene expression of hsp70 was upregulated after exposure to UVR (up to 5.2‐fold change), while hsp60 and hsp90 were only constitutively expressed. A strong seasonal pattern was found in the photoprotective compounds and antioxidant capacities, with highest levels during the ice‐free period. The extent of upregulation of hsp70 gene expression increased with decreasing photoprotection levels and peaked 24 h post UVR exposure (9.6‐fold change) at the time of lowest photoprotection (February). Our data suggest that hsp70 gene expression is modulated by seasonal plasticity in photoprotection. This ability of adequate stress response is essential for survival in highly variable ecosystems such as alpine lakes.
Highlights
The exposure of zooplankton to solar UV radiation (UVR) depends largely on the transparency of the lakes which they inhabit (Rautio and Tartarotti 2010)
Hsp60, Hsp70, and Hsp90 gene expression BLAST searches of the C. abyssorum tatricus deduced hsp60, hsp70, and hsp90 amino acid sequences resulted in high similarity (80–90%) to corresponding sequences from other copepods such as P. nana, T. japonicus, and Eurytemora pacifica
During the ice-free period when photoprotection levels are high in these organisms, hsp70 gene expression was upregulated after UVR exposure, but the response is relatively weak (Fig. 2)
Summary
The exposure of zooplankton to solar UV radiation (UVR) depends largely on the transparency of the lakes which they inhabit (Rautio and Tartarotti 2010). The dominant copepod species in lakes of the Eastern Alps, revealed high UVR tolerance when exposed to potentially lethal surface UVR levels during summer. Copepods avoid UVR exposure by staying deep in the water column during the day (Tartarotti et al 1999, 2017; Fischer et al 2015) and they accumulate high levels of photoprotective compounds such as mycosporine-like amino acids (MAAs) (Tartarotti et al 2001, 2004, 2017; Persaud et al 2007) or carotenoids (Hairston 1978; Tartarotti et al 1999; Persaud et al 2007; Sommaruga 2010). In C. abyssorum tatricus, the concentrations of MAAs are three times higher during summer than during the period with ice-cover (Tartarotti and Sommaruga 2006), indicating the importance of these compounds in protecting aquatic organisms against high solar radiation levels. In two congeneric Tigriopus species, the Antarctic T. kingsejongensis showed upregulation of the expression of small, but not large hsp genes, while in the temperate T. japonicus the gene expression of all hsps increased in response to UVR, resulting in better survival and greater adaptiveness (i.e., upregulation of antioxidant defense genes) in the latter species
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