The distribution, abundance and ecology of mixotrophic algae in marine and freshwater plankton communities

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.