An experimental study of nanoflagellate bacterivory

Abstract

Heterotrophic nanoflagellate Pseudobodo tremulans (4.8 to 7.0 µm) and heterotrophic bacteria, isolated from coastal waters in Ubatuba, SP, Brazil, were used in experiments to analyze quantitatively the relationships between bacteria and nanoflagellates. The meaning of these results for the role of heterotrophic nanoflagellates in the Ubatuba coastal ecosystem is discussed.Keys words: nanoflagellate, heterotrophic bacteria, coastal ecosystem, bacterivory It has been widely accepted that a highly dynamic microbial loop consisting of pelagic bacteria, autotrophic pico-and nanoplankton, heterotrophic nanoflagelattes and microciliates is an integral part of planktonic food webs (3). Protozoa in the size range between 2 -20µm (nanoplankton) are the major consumers of free-living pelagic bacteria in the sea (15). Several investigations on the dynamics of microbial loop have used a combination of laboratory and field methods (2, 12). The present investigation aims to quantify the bacterivory by a heterotrophic nanoplanktonic flagellate in in vitro conditions in order to get some indication of the importance of bacteria and nanoflagelattes in the Ubatuba coastal water ecosystem.Surface seawater samples were collected in Ubatuba coastal region (southeast of Brazilian coast -23 o S 25 o W). The system is considered to be mesooligotrophic and is characterized by temperatures ranging from 14 to 24 o C and salinities between 35 and 36. The primary production is low and nitrogen has been considered to be its main limiting factor (1).Water samples were enriched with sterile rice grains and incubated at 20 o C during 10 days before use. A heterotrophic nanoplanktonic flagellate (4.8 to 7.0 µm) identified as Pseudobodo tremulans was isolated from these enrichment cultures by micropipeting and a heterotrophic bacteria was isolated from the same enrichment by direct plating onto rice agar (9). The isolated microorganisms were maintained in stock cultures using a broth culture media (9) prepared with natural seawater and sterile rice grains, in the dark at 20 o C (ambient water temperature at sampling time).The experiments were carried out in duplicate and represented two different situations. In the first experimental procedure, the two microorganisms were grown without Cycloheximide, whereas in the second, microorganisms were grown in the presence