Oxygen supersaturation mitigates the impact of the regime of contaminated sediment reworking on sea urchin fertilization process

Increased sensitivity of sea urchin larvae to metal toxicity as a consequence of the past two decades of Climate Change and Ocean Acidification in the Mediterranean Sea

The Mediterranean Sea represents a natural laboratory to infer the possible impacts of climate change and ocean acidification. In this article, we report the deteriorating ability of sea urchin larvae (Paracentrotus lividus) to cope with toxicity of a reference contaminant (Cu EC50) over the past 20 years and assessed the influence of 5 environmental factors from satellite measurements. This timeframe was divided in before and after January 2016 (46.57 μg/L vs 28.56 μg/L respectively, p < 0.001). In the second subset of data, correlation of the biological variable with CO2 and pH strengthened compared to the first part (rCO2-EC50: -0.21 vs -0.83 and rpH-EC50: 0.25 vs 0.87 respectively), with a causal link starting from one year and ending 4 months prior to EC50 measurements. Considering the continuous increase in CO2 concentrations recorded recently, this study could reveal a rapid deterioration of the health condition of this population of sea urchins in a coastal ecosystem.

Ocean Acidification, but Not Environmental Contaminants, Affects Fertilization Success and Sperm Motility in the Sea Urchin Paracentrotus lividus

Ocean acidification poses an increasing concern for broadcast spawning species that release gametes in the water column where fertilization occurs. Indeed, the functionality of gametes and their interactions may be negatively affected by reduced pH. Susceptibility to other environmental stressors, such as pollutants, may be also altered under acidified conditions, resulting in more detrimental effects. To verify this hypothesis, combined exposures to CO2-driven acidification and environmentally relevant concentrations (0.5 µg/L) of three contaminants (caffeine, diclofenac, and PFOS, all singularly or in mixture) were carried out to highlight potential negative effects on fertilization success and motility of sperm in the sea urchin Paracentrotus lividus. Our results showed a significant reduction in the percentage of fertilized eggs when sperm were pre-exposed to reduced pH (ambient pH minus 0.4 units) compared to that of controls (ambient, pH = 8.1). Sperm speed and motility also decreased when sperm were activated and then exposed at reduced pH. Conversely, at both pH values tested, no significant effect due to the contaminants, nor of their interaction with pH, was found on any of the biological endpoints considered.

Effects of Dithiothreitol on Fertilization and Early Development in Sea Urchin

The vitelline layer (VL) of a sea urchin egg is an intricate meshwork of glycoproteins that intimately ensheathes the plasma membrane. During fertilization, the VL plays important roles. Firstly, the receptors for sperm reside on the VL. Secondly, following cortical granule exocytosis, the VL is elevated and transformed into the fertilization envelope (FE), owing to the assembly and crosslinking of the extruded materials. As these two crucial stages involve the VL, its alteration was expected to affect the fertilization process. In the present study, we addressed this question by mildly treating the eggs with a reducing agent, dithiothreitol (DTT). A brief pretreatment with DTT resulted in partial disruption of the VL, as judged by electron microscopy and by a novel fluorescent polyamine probe that selectively labelled the VL. The DTT-pretreated eggs did not elevate the FE but were mostly monospermic at fertilization. These eggs also manifested certain anomalies at fertilization: (i) compromised Ca2+ signaling, (ii) blocked translocation of cortical actin filaments, and (iii) impaired cleavage. Some of these phenotypic changes were reversed by restoring the DTT-exposed eggs in normal seawater prior to fertilization. Our findings suggest that the FE is not the decisive factor preventing polyspermy and that the integrity of the VL is nonetheless crucial to the egg's fertilization response.

Diel oxygen fluctuation drives the thermal response and metabolic performance of coastal marine ectotherms

Coastal marine systems are characterized by high levels of primary production that result in diel oxygen fluctuations from undersaturation to supersaturation. Constant normoxia, or 100% oxygen saturation, is therefore rare. Since the thermal sensitivity of invertebrates is directly linked to oxygen availability, we hypothesized that (i) the metabolic response of coastal marine invertebrates would be more sensitive to thermal stress when exposed to oxygen supersaturation rather than 100% oxygen saturation and (ii) natural diel fluctuation in oxygen availability rather than constant 100% oxygen saturation is a main driver of the thermal response. We tested the effects of oxygen regime on the metabolic rate, and haemocyanin and lactate levels, of velvet crabs (Necora puber) and blue mussels (Mytilus edulis), under rising temperatures (up to 24°C) in the laboratory. Oxygen supersaturation and photosynthetically induced diel oxygen fluctuation amplified animal metabolic thermal response significantly in both species, demonstrating that the natural variability of oxygen in coastal environments can provide considerable physiological benefits under ocean warming. Our study highlights the significance of integrating ecologically relevant oxygen variability into experimental assessments of animal physiology and thermal response, and predictions of metabolic performance under climate warming. Given the escalating intensity and frequency of climate anomalies, oxygen variation caused by coastal vegetation will likely become increasingly important in mitigating the effects of higher temperatures on coastal fauna.

Antioxidant and immune response of the sea urchin Paracentrotus lividus to different re-suspension patterns of highly polluted marine sediments

Marine pollution due to disused industrial activities is a major threat to ecosystems and human health, for example through the effects of re-suspension of toxic substances that are present in contaminated sediments. Here, we examined the effects of different re-suspension patterns of polluted sediments from the site of national interest Bagnoli-Coroglio, on the immune system of the sea urchin Paracentrotus lividus. An indoor experiment was set up exposing sea urchins for 34 days to such sediments and evaluating the effects of two patterns of water turbulence, mimicking natural storms at sea. One group of animals experienced an "aggregated" pattern of turbulence, consisting in two events, each lasting 2 days, separated by only 3 calm days, while a second group experienced two events of turbulence separated by 17 calm days (spaced pattern). At different times from the beginning of the experiment, coelomic fluid was collected from the animals and immune cells were examined for cell count and morphology, oxidative stress variables, and expression of genes involved in metal detoxification, stress response and inflammation. Our results highlighted that the aggregated pattern of turbulence was more noxious for sea urchins. Indeed, their immune system was altered, over the exposure time, as indicated by the increase of red amoebocytes number. Moreover, despite of an increase of the antioxidant power, animals from this group displayed a very significant ROS over-production at the end of the experiment. Conversely, animals in the spaced condition activated a different immune response, mainly having phagocytes as actors, and were able to partially recover from the received stress at the end of the experiment. No changes in the expression of genes related to antioxidant and anti-inflammatory responses were observed in both groups. By contrast, a down-regulation of various metallothioneins (4, 6, 7 and 8) in the group subjected to aggregated pattern was observed, while metallothionein 8 was up-regulated in the animals from the group exposed to the spaced pattern of turbulence. This work provides the first evidence of how sea urchins can respond to different re-suspension patterns of polluted sediments by modulating their immune system functions. The present data are relevant in relation to the possible environmental restoration of the study site, whose priorities include the assessment of the effects of marine pollution on local organisms, among which P. lividus represents a key benthic species.