UVA and UVB induce reactive oxygen species and damage sperm quality in the sea urchin anthocidaris crassispina

Abstract

In the last two decades, ozone depletion caused by anthropogenic activities has increased the penetration of ultraviolet irradiation (UVR) to earth. Previous studies have showed that UVR can impair reproduction of aquatic organisms, but the underlying mechanisms remain unclear. In vitro studies of mammalian somatic cells or cell cultures showed that production of reactive oxygen species (ROS) appeared to play a central role in the toxicity of UVR. Conceivably, the deleterious effect of UVR effects on reproduction of aquatic organisms is also mediated through enhanced ROS formation and subsequent oxidative damage, but this has not been demonstrated. In this thesis, sperm of the sea urchin Anthocidaris crassispina were exposed to environmentally relevant doses of UVA and UVB (0, 5.4, 10.8, 16.2, 27 kJm-2). ROS levels in sperm, sperm motility, sperm viability, mitochondrial function, lipid peroxidation and fertilization success were measured and correlated. We hypothesize that: 1) ROS production of urchin sperm is enhanced upon exposure to UVA and UVB; 2) sperm quality including sperm motility, sperm viability, mitochondrial function and subsequently % fertilization are reduced by both UVA and UVB, as mediated by the enhanced ROS production. Sperm motility (measured by curvilinear velocity, VCL and average path velocity, VAP) was reduced when urchin sperm were exposed to both UVA (by 27 to 60%) and UVB (by 17 to 88%) for 30 min.. Subsequent fertilization success was also reduced (UVA: by 50 to 80%; UVB: by 91 to 98%). A good dose-response evidently was observed between UV dose and sperm motility as well as fertilization success. Sperm viability was only affected by UVB but not UVA. Mitochondrial function of UV-treated sperm showed linear reduction with increasing doses, and was positively correlated with VCL, VAP and % fertilization (r = 0.63 to 0.89). Concomitantly, ROS production was enhanced in a dose-dependent manner when sperm were exposed to both UVA and UVB, and the increase in ROS was accompanied with an increase in lipid peroxidation. Correlation analysis showed that levels of ROS production and LPO were inversely related to VCL, VAP, % fertilization, sperm viability and mitochondrial function (r = -0.53 to -0.88; P < 0.05). For the same dose, UVB was more potent than UVA in affecting all tested parameters except LPO. Overall, we conclude that environmentally relevant UVR (UVA and UVB) could induce ROS production and subsequent lipid peroxidation in urchin sperm, thereby impairing their viability and mitochondrial functions. Such functional impairments may ultimately reduce sperm motility and hence fertilization success in natural populations.