Changes in bioturbation of iron biogeochemistry and in molecular response of the clam Ruditapes decussates upon Perkinsus olseni infection.

A series of artificial microcosms was used to test the effect of clam density on benthic iron biogeochemistry and, subsequently, if the response of clam Ruditapes decussatus to infection with Perkinsus olseni, a common opportunistic parasite known to be iron dependent, was correlated with the dynamics of iron sediment pore waters within the chambers. Three series of benthic microcosms were used in the experiment, comparing similar densities of clams (none, one, two, three, or four individuals/chamber) between a control set (no deliberate infection) and two parallel sets of clams that were deliberately infected with the parasite after 10 days of incubation. Fifteen chambers were used simultaneously and the experiment was conducted for 35 days. In order to avoid spurious effects of differential organic loading and clam feeding efficiency on the oxidative state of the sediment, the iron balance was tentatively shifted during incubation toward decreased dissolved iron in pore water. This was done by applying a constant flow of air to all chambers and refraining from supplying extra organic matter during the experimental run, which led to the reduction of benthic oxygen demand as the experiment progressed. Results showed that microcosms bearing both higher clam densities and lower infection levels were able to exert a quantitative influence in iron biogeochemistry through bioturbation activity. This effect was significantly depressed in chambers hosting clams with high infection levels. In addition, analysis of molecular markers responsive to iron and parasite stress revealed an upper regulation of HSP70 and ferritin in infected clams, thus suggesting a role of those molecules on both host protection and response to parasite presence by limiting iron availability. Together, these findings suggest a correlation between the expression of clam molecular iron/stress markers and iron bioavailability, which can be modified by the presence or absence of Perkinsus infection. In turn, we propose that clam lethargy in response to parasite invasion might help to combat infection by reducing iron mobilization in the surrounding sediment through a decrease in bioturbation activity, thus reducing its availability to the parasite.