Effect of subsurface nutrient supplies on the vertical migration of Euglena proxima (Euglenophyta)

Euglena proxima Dangeard inhabits intertidal sand flats and displays a tidal rhythm in vertical migration. During daytime low tides when the sand flat is emersed, millions of cells are visible on the sediment surface, but the population remains below the surface at all other times. An earlier study demonstrated that the extent of downward migration of E. proxima is reinforced by the presence of a subsurface layer of black sediment. The present study was designed to test the hypothesis that the higher availability of inorganic nutrients or organic substrates in or above the black layer is responsible for the enhancement of downward migration in E. proxima. This hypothesis was tested experimentally by manipulating the bottom water in 24 mesocosm containers in a tidal tank. Six replicates of each of the following nutrient treatments were tested: seawater control; deep porewater collected from 70 cm below the sediment surface; seawater enriched with ammonium, nitrate, and phosphate; and seawater enriched with acetate, glucose, and the preceding inorganic nutrients. Multivariate analysis of variance revealed that the chl a biomass and chl a-weighted mean depth of the population at high tide were significantly greater for replicates receiving inorganic nutrients. There was no difference between those receiving only inorganic nutrients and those enriched with inorganic nutrients, acetate, and glucose. These findings represent the first experimental evidence that subsurface nutrients are an important resource that reinforces the maintenance of vertical migration behavior in benthic microalgae.