Anoxic survival potential of bivalves: (arte)facts (2023)

The Thau lagoon is a Mediterranean coastal lagoon used for shellfish farming. It is periodically affected by anoxia events that trigger oyster mortality. To investigate the effects of an anoxia event focussed on nutrient dynamics and the responses of the microbial planktonic communities a 13-day in situ experiment was performed in September 2020. Transparent mesocosms (270L) were placed at a depth of 4m, inserted in the sediment, and kept closed throughout the experiment. The experiment comprised three treatments: i) Natural environment (N), i.e. in the natural water outside the mesocosms containing a rope of 30 oysters (Crassostrea gigas), ii) Control mesocosm (C) filled with natural water with no oysters, and iii) Oyster mesocosm (O) filled with natural water containing a rope of 30 oysters. Oyster respiration in the oyster mesocosm depleted oxygen after 54h. All the oysters from O mesocosm were dead after nine days and decomposition of their flesh combined with releases from the water-sediment interface increased dissolved inorganic nitrogen (dominated by ammonium), phosphates, and ∑H₂S up to 390, 17 and 295μmol·L⁻¹, respectively. Phytoplankton biomass consequently increased by 20 (11.8μg chlal ⁻¹) and abundance by 4.5 (186×10⁶ cells·L¹) dominated largely by green algae <5μm. During the oyster mortality period (day 6 to day 9) high abundances of heterotrophic flagellates and large ciliate specimens were observed. This shift in the community towards small phytoplankton favours the microbial loop and is detrimental to shellfish farming. In a context of global warming in which the risk of anoxia is higher, the results of the present investigation demonstrate that anoxia triggers shellfish mortality and that the change in the plankton community disrupts the normal functioning of the ecosystem, causing serious financial losses. In this context, it is crucial to predict possible hypoxia and anoxia events using high frequency measurements of dissolved oxygen, by avoiding using shallow zones for oyster production and by reducing shellfish stocks, or by mechanically lifting the oysters out of the water during the night to reduce oxygen respiration in the ecosystem.

Eutrophication-induced hypoxia is among the most widespread anthropogenically deleterious environmental issues occurring globally in coastal marine environments, contributing to a suite of major stressors on marine organisms. Changes to biogeochemical processes and loss of biodiversity and ecosystem function are symptoms of stressors on aerobic organisms, particularly in estuaries. The response of a species to a single stressor is often very different when compared to exposure in a multiple stressor environment. Using an experimental approach, we tested the sub-lethal and lethal effects of hypoxia and chronic nutrient enrichment on the critical estuarine bivalve Austrovenus stutchburyi. Findings of the present study highlight the cumulative mortality of A. stutchburyi nearly doubled when exposed to elevated ammonia when it was combined with elevated hypoxic stress. A positive feedback loop is hypothesized to explain the result, whereby A. stutchburyi stressed by low oxygen produce and excrete more ammonia waste, increasing their risk of mortality and subsequent lethal ammonia concentrations. Water samples taken across the 45-day experiment support the hypothesis, with ammonia concentrations differing between treatments with bivalves in hypoxic treatments producing more ammonia than normoxic treatments. Siphon activity and respiration rate responses to hypoxic stress also demonstrated the importance of hypoxia as a primary driver of stress responses by A. stutchburyi. These findings highlight the threats to estuarine ecosystems with bivalve populations, whereby, extensive degradation of algal mats results in increased oxygen consumption from microbial respiration, which leads to hypoxia and shallowing of the oxygenated layer. Consequently, chemical reduction in sediments can be associated with an efflux of ammonium into the water column. The experimental results presented here on interactions between stressors highlights the substantial detrimental implications for bivalve populations and subsequent ecosystem functions given increased prevalence of warming and eutrophication in estuaries and coastal environments.

Eutrophication is an increasing problem worldwide and can disrupt ecosystem processes in which macrobenthic bioturbators play an essential role. This study explores how intraspecific variation in body size affects the survival, mobility and impact on sediment organic matter breakdown in enriched sediments of an infaunal bivalve. A mesocosm experiment was conducted in which monocultures and all size combinations of three body sizes (small, medium and large) of the Sydney cockle, Anadara trapezia, were exposed to natural or organically enriched sediments. Results demonstrate that larger body sizes have higher tolerance to enriched conditions and can reduce survival of smaller cockles when grown together. Also, large A. trapezia influenced sediment organic matter breakdown although a direct link to bioturbation activity was not clear. Overall, this study found that intraspecific variation in body size influences survival and performance of bioturbators in eutrophic scenarios.

As a secondary consequence of the high productivity of the upwelling system, organisms inhabiting Peruvian coastal bays are frequently exposed to hypoxic conditions. The aim of the present paper was to investigate the effects of daily-cyclic-severe hypoxia on energetics of a species presenting little escape ability when facing hypoxia. For this purpose, juvenile Peruvian scallops (Argopecten purpuratus) were exposed to four experimental conditions: fed and starved, combined or not to nightly severe hypoxia (5% oxygen saturation) for ≈ 12 h over a 21-day experiment. In both fed conditions, clearance rate was measured by the mean of an open-flow system. Our results indicate that the Peruvian scallop is able to maintain an active filtration even at low oxygen saturation, at least during expositions up to 12h. During the first phase of exposure to hypoxia, clearance rate decreased abruptly when oxygen saturation dropped below 10%, but rapidly recovered to values close to those found under normoxia. As a consequence of this ability to feed during hypoxia, no difference in soft tissue dry weight (digestive gland not included) was observed at the end of the experimental period between oxic conditions among fed scallops. However, shell growth was negatively affected by hypoxic condition. Starved individuals exhibited similar weight loss between hypoxic and normoxic conditions indicating no or little effect of oxic condition on maintenance costs. Considering the observed responses for feeding, growth and maintenance, we can hypothesize that this species presents metabolic/bioenergetic efficient adaptations to deal with hypoxic conditions that are recurrent in Peruvian coastal bays. We hypothesize that the small observed effects might be modelled in the context of the Dynamic Energy Budget theory as a restriction of reserve mobilization under hypoxic conditions.

The microbiological responses of two bivalves species from Tagus estuary, Venerupis pullastra (native clam) and Ruditapes philippinarum (exotic clam) were investigated during 48h of depuration and subsequent simulated transport in semi-dry conditions at two temperatures (4 and 22°C) until reaching 50% lethal time (LT50). Regardless of temperature and species, the maintenance of clams in water for 48h (depuration period) did not affect LT50 during transport. R.philippinarum showed higher survival rates than V.pullastra, always reaching LT50 later, especially at 4°C. Significant differences between clams' species were found in almost all microbiological parameters. This can be related with clams' biological activity and habitat environmental conditions since both clams do not coexist in Tagus estuary. Depuration was efficient to reduce the bacterial load, particularly Escherichia coli, but not efficient to remove Vibrio spp. In both species, the growth of Vibrio spp. was inhibited at 4°C, whereas exponential growth occurred at 22°C. Total viable counts significantly increased in most treatments, while E.coli counts significantly decreased to undetected levels, except for non-depurated R.philippinarum simulated transported at 4°C. Thus, this study highlights the importance of clams depuration for at least 24h in polluted estuarine areas, followed by transport at low temperatures (4°C).

Chemometrics and Intelligent Laboratory Systems, Volume 138, 2014, pp. 133-141

The Adair equation is used to model biological macro-molecule reactions. This equation relates the saturation rate to the free ligand concentration. But, the latter is not a variable completely under the control of the experimenter. The ligand is a random variable depending on an initial ligand added by the experimenter, which can be designed, but the dependence of the saturation rate on the initial ligand has not been considered in the literature. In this paper a transformed model based on the Adair model of first order (monomer) is derived in order to obtain a proper model that depends on the initial ligand. This model will allow proper fitting and optimal designs using the initial ligand. It will be called the transformed Adair model (TAM). Optimal designs as well as seven-point quasi-optimal designs forced to follow a harmonic, geometric or uniform progression, are computed. The parameters are estimated and compared for simulated data from these designs. A sensitivity analysis against the choice of nominal values of the parameters is also performed for the TAM. The analytic version of the transformed model is only possible for the first class model. But the good efficiencies of the optimal designs obtained directly from the monomer model for fitting the TAM justify doing something similar for the second order model (dimer). Designs were computed numerically in this case.

Encyclopedia of Spectroscopy and Spectrometry, 2017, pp. 95-102

As a new research field, high-temperature mass spectrometry was established in the early 1950s. Of special value was the combined use of the classic Knudsen effusion method and mass spectral study of the evaporation products of inorganic compounds (the KEMS technique). The first review appeared as early as 1959 and contained data for approximately 120 systems and compounds that had been studied by that time.

Aquaculture, Volume 528, 2020, Article 735512

The objectives of this study were to 1) determine the influence of environmental factors (water temperature, food availability [chlorophyll-a], salinity) on nacre deposition rate and tablet thickness at different positions in shells of two strains of pearl oyster, Pinctada fucata (Gould, 1850)—one from Japan, and one a Hybrid between this Japanese strain and another from China; 2) compare nacre deposition rates and tablet thickness in different shell positions within and between these two pearl oyster strains; and 3) determine relationships between shell nacre deposition rate and tablet thickness in host oysters of these two strains with those of their pearls. Nacre and tablet thickness were measured at three positions along the mantle pallial line (anterior, middle, and posterior), and one position close to the hinge, on the left valve of each oyster. Water temperature and chlorophyll-a were positively correlated (p<.001, Spearman's rho >0.75) and salinity was negatively correlated (p<.001, Spearman's rho < −0.7) with nacre deposition rate at all shell positions in both pearl oyster strains. Only water temperature below 13°C influenced nacre tablet thinning. Significant differences in total nacre deposition occurred between the four sampling positions on the shell in both strains (p<.001), with deposition at the hinge area the slowest for both Hybrid (373.212μm) and Japanese (569.248μm) oysters. Nacre tablet thickness in the middle position on the shell increased most steadily over time, and was thinnest in the coolest months (0.151μm and 0.167μm for Hybrid and Japanese oysters, respectively). No significant difference (p>.05) in nacre tablet thickness was apparent between strains. Correlation analysis of nacre deposition rate between the shell and pearls showed that all shell positions were highly correlated with their respective pearls (p<.001, Spearman's Rho >0.75). Environmental factors influence shell nacre deposition and thickness, and because shell nacre deposition is related to pearl nacre, pearl growth also. This influence varies in different shell positions, with nacre deposition in the posterior position on the shell the fastest, and the middle position showing the most stable and thin nacre tablet. Japanese pearl oysters exhibit better nacre deposition than Hybrid oysters and would be more appropriate as host oysters in Ago Bay, Japan, should culture occur in environmental conditions comparable to those in this study.

Aquatic Toxicology, Volume 163, 2015, pp. 121-129

We investigated the effects of three different carbon-based nanomaterials on brine shrimp (Artemia salina) larvae. The larvae were exposed to different concentrations of carbon black, graphene oxide, and multiwall carbon nanotubes for 48h, and observed using phase contrast and scanning electron microscopy. Acute (mortality) and behavioural (swimming speed alteration) responses and cholinesterase, glutathione-S-transferase and catalase enzyme activities were evaluated. These nanomaterials were ingested and concentrated in the gut, and attached onto the body surface of the A. salina larvae. This attachment was responsible for concentration–dependent inhibition of larval swimming, and partly for alterations in the enzyme activities, that differed according to the type of tested nanomaterials. No lethal effects were observed up to 0.5mg/mL carbon black and 0.1mg/mL multiwall carbon nanotubes, while graphene oxide showed a threshold whereby it had no effects at 0.6mg/mL, and more than 90% mortality at 0.7mg/mL. Risk quotients calculated on the basis of predicted environmental concentrations indicate that carbon black and multiwall carbon nanotubes currently do not pose a serious risk to the marine environment, however if uncontrolled release of nanomaterials continues, this scenario can rapidly change.

Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology, Volumes 243–244, 2020, Article 110438

In the ocean the main climate drivers affecting marine organisms are warming, hypercapnia, and hypoxia. We investigated the acute effects of warming (W), warming plus hypercapnia (WHc, ~1800 μatm CO₂), warming plus hypoxia (WHo, ~12.1 kPa O₂), and a combined exposure of all three drivers (Deadly Trio, DT) on king scallops (Pecten maximus). All exposures started at 14 °C and temperature was increased by 2 °C once every 48 h until the lethal temperature was reached (28 °C). Gill samples were taken at 14 °C, 18 °C, 22 °C, and 26 °C and analyzed for their metabolic response by ¹H-nuclear magnetic resonance (NMR) spectroscopy. Scallops were most tolerant to WHc and most susceptible to oxygen reduction (WHo and DT). In particular under DT, scallops' mitochondrial energy metabolism was affected. Changes became apparent at 22 °C and 26 °C involving significant accumulation of glycogenic amino acids (e.g. glycine and valine) and anaerobic end-products (e.g. acetic acid and succinate). In line with these observations the LT₅₀ was lower under the exposure to DT (22.5 °C) than to W alone (~ 25 °C) indicating a narrowing of the thermal niche due to an imbalance between oxygen demand and supply.

Journal of Experimental Marine Biology and Ecology, Volume 522, 2020, Article 151256

The link between the behavior of organisms living within the sediment and benthic-pelagic coupling is critical to our interpretation of impacts of physiological stress. The rapidity with which organisms respond to a stress and recover once a stress is removed impacts the cascade of secondary effects that result from such behavioral changes. Such behavioral responses, although critical to our understanding, are difficult to quantify, particularly for organisms such as bivalves that inhabit sediments, the most common spatial habitat on Earth. Sublethal responses to fluctuating stressors such as salinity are rarely quantified in terms of activity. Here we do so using a combination of direct observations and pressure sensors, which allow us to record burrowing and other hydraulic activities. Based on our field observations and the literature, we predicted that there would be a breakpoint in behavior between salinities of 15 and 20 and that below this breakpoint the animals would significantly reduce activities including burrowing and feeding. The data presented support this prediction; burrowing was reduced in all three species at salinities at or below 15 as were fecal deposition and appearance of siphons above the sediment surface. These data are consistent with the prediction that under conditions of large fluctuations in salinity the magnitude of the link to primary productivity and nutrient availability from the benthos will be significantly reduced.