Abstract:
Mixotrophic algae are algae that combine photosynthesis with phagotrophy to
satisfy nutritional requirements. Mixotrophic algae have been found to dominate the
nanoplankton assemblage in some aquatic environments, and algal phagotrophy can
account for a significant fraction of total bacterivory on occasion. The distribution and
abundance of mixotrophic algae have not been widely characterized, however, and the
factors that control their abundances and phagotrophic activity in aquatic environments
are poorly understood. The vertical distribution and abundance of mixotrophic
nanoplankton was therefore documented in a variety of aquatic environments including
the oligotrophic open ocean (Sargasso Sea), nine freshwater seepage ponds (Adirondack
Mountains, NY) and three coastal salt ponds (Falmouth, MA) using epifluorescence
microscopy and fluorescently labeled prey to trace ingestion by algae. Field experiments
designed to examine the influence of prey density, nutrient availability and light intensity
on mixotrophic nanoplankton abundances and their phagotrophic activity were also
carried out to examine the adaptive significance of phagotrophy for mixotrophic algae in
surface waters of the Sargasso Sea and in the epilimnion of one freshwater seepage pond.
Mixotrophic nanoplankton abundances in the Sargasso Sea ranged from less than
a few cells mi-l to more than 140 cells mi-l, and they were more abundant in surface
waters. On two occasions rnixotrophic algae comprised more than 50% of the total algal
nanoplankton assemblage in surface waters. The results of field experiments indicate
that phagotrophy may provide mixotrophic algae in surface waters of the Sargasso Sea
with major nutrients and may enable them to compete with purely phototrophic algae
during periods of low nutrient concentrations.
Mixotrophic nanoplankton abundances comprised a small fraction of the total
algal nanoplankton ( <5%) in most of the freshwater seepage ponds and in all three
coastal salt ponds. Mixotrophic algae in the epilimnion of one freshwater pond,
however, accounted for -25% of the total algal nanoplankton, and Dinobryon dominated
the rnixotrophic nanoplankton in this pond (-2x104 Dinobryon ml-1). Results from field
experiments support the hypothesis that nutrient acquisition is an important secondary
function for phagotrophy in Dinobryon, but that the primary function may be to provide
essential growth factors.
