Introduction
Bivalves are usually cultured in land tanks connected directly to estuarine and coastal waters. This type of aquaculture exposes the cultured species to possible toxic blooms that often affect these natural ecosystems . The effects of toxic algae on humans have been the subject of many studies. However, information on the effects of these toxic species on bivalve species have been less explored.
Gymnodinium catenatum is a dinoflagellate which produces a saxitoxin like compound, being responsible for paralytic shellfish poisoning. It has been shown to cause an increase in antioxidant enzyme activities and oxidative stress in some bivalve species after exposures of 6-12 hours. However, the effect of such contaminants to short-time exposures has not been extensively studied . Shorter exposure times are important to evaluate possible effects into aquacultures, as these types of systems usually allow to close the connection with external water, thereby reducing the exposure of the cultured animals .
Besides toxic algae , o ther species can negatively affect the reared species. Algae like Skeletonema spp. Are known to damage the gills, leading to several health complications in marine animals (Esenkulova et al., 2022) . In addition to physical damage, Skeletonema marinoi produces polyunsaturated aldehydes (PUA) when exposed to grazing (Vidoudez et al., 2011) which are toxic to many marine species (Romano et al., 2011; Tosti et al., 2003) . Nevertheless, S. marinoi is commonly used to feed bivalves in aquacultures (Guéguen et al., 2008).
It has been observed that antioxidant enzymes respond to several contaminants (Cereja et al., 2018; Dias et al., 2019) including algae toxins (Cao et al., 2018) and thus can be used as a physiological stress indicator for to toxic algae exposure.
The aim of this study was to assess the physiological effect of short-term exposure to G. catenatum and S. marinoi in cultured Magallana angulata.
Material and methods
Oysters were collected at an aquaculture farm in the Sado Estuary, Portugal and carried to MARE facilities where they were allowed to acclimate in 70L tanks. During the acclimatization period, all oysters were fed with ≈2*109 cells L-1 of a mixture of Tetraselmis sp. and Phaeodactylum sp. For the exposure treatments, both Gymnodinium catenatum and Skeletonema marinoi , were obtained from the algae culture collection of the Lisbon University (ALISU).
In the experiment, ≈2x107 cells L-1 of Tetraselmis sp. for the control group, ≈4x107 cells L-1 of S. marinoi and ≈1x104 cells L-1 of G. catenatum for the exposure treatments were added to three tanks, each containing 6 oysters , and allowed to filter for two hours. Afterwards, the six organisms of each treatment were opened by cutting the adductor muscle. The gills, the digestive gland and the adductor muscle were removed and stored at -80 oC.
Subsequently, the s amples were homogenized in PBS buffer saline solution, centrifuged (15 min, 10,000×g at 4°C) and the supernatants used to quantify superoxide dismutase (SOD), catalase (CAT), glutathione S-transferase (GST) , acetylcholinesterase (AChE) and total ubiquitin (UBI) and the data was analysed using a PerMANOVA.
Results:
The results showed significant differences in both treatments: tissues and treatments. Gills were the tissue showing higher biomarkers while the adductor muscle had the lowest levels . Regarding the comparison between treatments, the control treatment presented higher levels of the analysed biomarkers. S. marinoi treatment presented higher CAT and GST than G. catenatum treatment.
Discussion:
The higher biomarker levels determined in the control treatment were hypothesized to be a consequence of reduced metabolic rates in the two other treatments, although such relation must be confirmed in future studies . The higher CAT and GST activities determined in the G. catenatum treatment in comparison to the S. marinoi treatment was hypothesized to result from detoxication process. These results suggest that G. catenatum may affect the physiology of M. angulata even in short-term exposures and that live S. marinoi , which is usually used as aquaculture feed, may also impact the M. angulata physiology, probably due to the production of PUA. Further studies must be performed to confirm this possibility and if such effect is also observed when using dead S. marinoi as PUA production may differ.
References
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