Uptake of manganese into the exoskeleton of the swimming crab Liocarcinus depurator (L.) in relation to biomonitoring and biosorption.
PhD thesis, University of Glasgow.
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The swimming crab Liocarcinus depurator (L.) is a common member of the benthic fauna in Scottish waters, and is often caught as bycatch from the common lobster fishery grounds. This study aims to employ the species in relation to the biomonitoring potential for Mn in the Scottish inshore waters and in UK monitoring programmes by choosing Loch Fyne in the west coast of Scotland as a naturally high Mn area and the Clyde Sea area as the reference area. The ability of the crushed carapace to remove Mn from aqueous solution in a biosorption column system in the remediation of contaminated waters was also investigated as an attempt to turn this un-commercial species into beneficial use.
Measured using standard atomic absorption spectroscopy (AAS), the concentrations of Mn in the tissues of the swimming crab L. depurator from Loch Fyne were consistently higher than in the tissues in crabs from the Clyde Sea area. The metal concentration differed according to sexes, and to the tissues in the order of the exoskeleton (carapace, gills) > hepatopancreas > crusher claw muscle and gonads. The trend observed in L. depurator was comparable to the shore crab, Carcinus maenas which is an established biomonitor for metals collected within the vicinity of both study areas.
A series of different exposures of L. depurator to Mn in sea water (10ppm and 20ppm) for up to 21d, followed by a depuration period of 47d were performed under controlled laboratory conditions. Temporal changes in Mn concentrations in the exoskeleton of individual crabs were monitored by autotomizing a walking leg at weekly intervals. Mn concentrations in other tissues at given sampling points were obtained by sacrificing a batch of crabs at each sampling time. The hard tissues (dorsal carapace and leg exoskeleton) irreversibly accumulated Mn from the water whereas the soft tissues both accumulated Mn when exposed, and eliminated Mn after a period of depuration in clean sea water. As a result, the use of the exoskeleton of autotomized legs to represent Mn accumulation in the whole exoskeleton of a crab was established, and the accumulation of Mn from the water into the crabs tissues particularly the exoskeleton was confirmed.
The ability of dried and crushed carapace particles from the swimming crab L. depurator to remove Mn from aqueous solutions was studied using a packed bed up-flow biosorption column system. From a batch experiment carried out at room temperature on fine carapace particles with a diameter of less than 300µm and 100ml of 80ppm Mn in distilled water, the data fitted the Freundlich adsorption isotherm with an adsorption capacity KF=22.82 mg.g-1. The breakthrough curves generated from a series of up-flow biosorption experiments (constant flow rate of 100ml.h-1, 72h sorption) indicates the great potential of the crab carapace particles to remove Mn from a solution. The removal depended greatly on the initial concentrations of the solution and the amount of carapace particles used which can be expressed as mass or the height of the column beds. Greater column bed heights increase Mn removal capacity which in the columns could be observed visually through progressive colour change of the beds. Experiments repeated using the carapace particles of the Norway lobster, Nephrops norvegicus (L.) generated similar trends and comparable data with the ones observed for L. depurator.
Scanning electron microscopy (SEM) and SEM combined with electron dispersive analysis of x-ray (SEM-EDX) were applied to examine the morphology of the dorsal carapace and gills of L. depurator, and also the form and site of Mn deposition onto both the intact carapace of L. depurator and onto Mn-biosorbed carapace particles. The dorsal carapace of L. depurator resembles the typical crustacean cuticle with three main layers, the epi-, exo- and endocuticles made up primarily by CaCO3. Mn deposited onto the carapace especially on exposed inner layers and broken edges of the carapace particles in the form of Mn-rich nodules, which in 72h could create a layer visually observed as blackening of the particles. Deposition onto the gill surface took the form of fine particles scattered on the lamellae. Exposure of an isolated carapace to 80ppm Mn solution indicated the barrier-effect played by the epicuticle to Mn deposition onto the external surface. This barrier was lost when the surface is abraded. The membranous layer on the internal side did not act as a barrier, and penetration of Mn up to approximately 50µm into the endocuticular layers was detected. These results correlated with AAS measurements which indicated that internal exposure of the carapace resulted in a three times higher concentrations of Mn compared with external exposure.
Based on the wide distribution of L. depurator around the UK coast and its habit of resting on the bottom sediment, the results of this study propose L. depurator as a biomonitor species for Mn in the bottom water, particularly in the Scottish waters. Given the abundance of the species in common lobster fishery grounds in Scotland and often caught as bycatch in the trawls, L. depurator could provide a continued source of materials if the carapace is to be converted into a good Mn-removing agent in Mn contaminated waters.
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