Depuration kinetics and persistence of the cyanobacterial toxin microcystin-LR in the freshwater bivalve Unio douglasiae

Distribution of microcystins in a lake foodweb: no evidence for biomagnification

Microcystins, toxins produced by cyanobacteria, may play a role in fish kills, although their specific contribution remains unclear. A better understanding of the eco-toxicological effects of microcystins is hampered by a lack of analyses at different trophic levels in lake foodwebs. We present 3 years of monitoring data, and directly compare the transfer of microcystin in the foodweb starting with the uptake of (toxic) cyanobacteria by two different filter feeders: the cladoceran Daphnia galeata and the zebra mussel Dreissena polymorpha. Furthermore foodwebs are compared in years in which the colonial cyanobacterium Microcystis aeruginosa or the filamentous cyanobacterium Planktothrix agardhii dominated; there are implications in terms of the types and amount of microcystins produced and in the ingestion of cyanobacteria. Microcystin concentrations in the seston commonly reached levels where harmful effects on zooplankton are to be expected. Likewise, concentrations in zooplankton reached levels where intoxication of fish is likely. The food chain starting with Dreissena (consumed by roach and diving ducks) remained relatively free from microcystins. Liver damage, typical for exposure to microcystins, was observed in a large fraction of the populations of different fish species, although no relation with the amount of microcystin could be established. Microcystin levels were especially high in the livers of planktivorous fish, mainly smelt. This puts piscivorous birds at risk. We found no evidence for biomagnification of microcystins. Concentrations in filter feeders were always much below those in the seston, and yet vectorial transport to higher trophic levels took place. Concentrations of microcystin in smelt liver exceeded those in the diet of these fish, but it is incorrect to compare levels in a selected organ to those in a whole organism (zooplankton). The discussion focuses on the implications of detoxication and covalent binding of microcystin for the transfer of the toxin in the foodweb. It seems likely that microcystins are one, but not the sole, factor involved in fish kills during blooms of cyanobacteria.

Assimilation and depuration of microcystin-LR by the zebra mussel, Dreissena polymorpha

Zebra mussels (Dreissena polymorpha) are an important component of the foodweb of shallow lakes in the Netherlands, amongst others in Lake IJsselmeer, an international important wetland. Large numbers of ducks feed on these mussels in autumn and winter. The mussels are filter feeders and are exposed to high densities of cyanobacteria in summer and autumn. Mussels and cyanobacteria both thrive in Lake IJsselmeer. Apparently the mussels are somehow protected against accumulation of harmful quantities of cyanobacterial toxins. In this study, we investigated the assimilation of the cyanobacterial toxin microcystin-LR (MC-LR) in zebra mussels when fed the toxic cyanobacterium Microcystis aeruginosa as sole food or in a mixture with the eustigmatophyte Nannochloropsis limnetica. After 3 weeks of assimilation we studied the depuration of MC-LR during 3 weeks when the food of the mussels was free of cyanobacteria. These assimilation/depuration experiments were combined with grazing experiments, using the same food treatments. Microcystins were analyzed using liquid chromatography-mass spectrometry (LC-MS); in addition, covalently bound MC were analyzed using the MMPB method. The mussels showed higher clearance rates on Microcystis than on Nannochloropsis. No selective rejection of either phytoplankton species was observed in the excretion products of the mussels. Zebra mussels fed Microcystis as single food, assimilated microcystin-LR relatively fast, and after 1 week the maximum value of free unbound microcystin assimilation (ca. 11 microg g DW(-1)) was attained. For mussels, fed with the mixed food, a maximum of only 3.9 microg g DW(-1) was recorded after 3 weeks. Covalently bound MC never reached high values, with a maximum of approximately 62% of free MC in the 2nd week of the experiment. In the depuration period microcystin decreased rapidly to low values and after 3 weeks only very low amounts of microcystin were detectable. The amount of toxin that accumulated in the mussels would appear to be high enough to cause (liver) damage in diving ducks. However, death by exposure to microcystin seems unlikely. Mussels seem efficient in minimizing the assimilation of microcystin. If it were not for this, mass mortalities of ducks in shallow lakes in the Netherlands would presumably occur on a much more widespread scale than is currently observed.

Accumulation and depuration of the cyanobacterial toxin cylindrospermopsin in the freshwater mussel Anodonta cygnea

Cylindrospermopsin (CYN) is a toxic alkaloid produced by several genera of freshwater cyanobacteria. This compound has been implicated in outbreaks of human sickness and the death of domestic and wild animals. Given that several of the cyanobacterial genera known to produce CYN are common components of the phytoplankton of freshwaters including aquaculture facilities, we studied the accumulation of CYN in the freshwater mussel (swan mussel) Anodonta cygnea. Anodonta were exposed to CYN-producing cultures of the cyanobacterium Cylindrospermopsis raciborskii for 16 days and were found to accumulate the toxin to concentrations up to 2.52 microg g tissue dry weight(-1). There was considerable variation in the concentrations of CYN detected in different parts of the body. At the end of a 2-week accumulation period the distribution of CYN in the body of Anodonta was as follows: haemolymph (68.1%), viscera (23.3%), foot and gonad (7.7%) and mantle (0.9%). No CYN was detected in the gills or adductor muscle of any animals. Following a 2-week depuration period, approximately 50% of the toxin remained in the tissues. Based on the recently derived guideline value for CYN in human drinking water (1 microg l(-1)) and the concentrations of this compound in animal tissues reported here, there is a clear need for the increased monitoring of this compound in organisms grown for human and animal consumption.