Abundance and bacterivory of heterotrophic nanoflagellates in the meromictic Lake Suigetsu, Japan

Abstract

The abundance and bacterivory of heterotrophic nanoflagellates (HNF) were seasonally followed in the oxic and anoxic layers of the meromictic Lake Suigetsu between May 2008 and November 2010. The HNF abundance in the anoxic layer was always lower than in the oxic layer during the study period. Ingestion of fluorescently labeled 0.5 µm diameter beads by the anaerobic HNF in the anoxic layer indicated bacterivory by HNF. The specific ingestion rates in the anoxic layers were generally similar to those taken from the oxic layer, with some exceptions. Our data thus suggested that anaerobic HNF were bacterial consumers with high potential bacterivory comparable to that of aerobic HNF in Lake Suigetsu. Bacterial turnover rates by HNF grazing in the oxic layer were estimated to be as high as ~10% d −1 of the bacterial standing stock. In contrast, the rates in the anoxic layer were <1% d −1 due to the low density of HNF in the anoxic layer. Our data thus provide a valuable contribution to understanding the structure and function of microbial food webs in anoxic aquatic environments.

Growth and Grazing Kinetics of the Facultative Anaerobic Nanoflagellate, <i>Suigetsumonas clinomigrationis</i>

“…Heterotrophic nanoflagellates (HNF) are important bacterial consumers in the microbial loop (17, 23). HNF have also been detected in the anoxic bottom waters of stratified lakes (11, 18, 21) and in anaerobic cultures from anoxic habitats (3) and deep-sea hydrothermal vents (1). However, HNF ecology in anoxic environments currently remains unclear because of limited data on the abundance and bacterivory of HNF under these conditions (18, 22).…”

“…However, HNF ecology in anoxic environments currently remains unclear because of limited data on the abundance and bacterivory of HNF under these conditions (18, 22). We recently detected the high potential bacterivorous activities of HNF in the anoxic layers of meromictic Lake Suigetsu, indicating that anaerobic HNF act as bacteria consumers in the microbial loop of anoxic environments (18). …”

“…Water temperatures in the epilimnion change seasonally, ranging from less than 5°C in the winter season to ca. 30°C in the summer, while the temperature below the metalimnion is constant at 15°C throughout the year (10, 13, 18). Thus, S. clinomigrationis grow in the whole water column of Lake Suigetsu throughout the year, except for the epilimnion in winter.…”

“…However, the μmax anoxi (0.5±0.0 d −1 ) of S. clinomigrationis was approximately 1/7 that under oxic conditions, showing that these conditions are disadvantageous for its growth. The numbers of predators of HNF such as ciliates and metazoan zooplankton are negligible or below the detection limit in the water of the anoxic deeper layer of Lake Suigetsu (9, 18). Therefore, S. clinomigrationis may be able to grow and maintain its population in anoxic environments because of the absence of predation pressure.…”

“…Thus, we cannot compare the values for bacteria ingestion rates obtained in this study with those of other anaerobic HNF. We examined grazing kinetics in our culture under optimal growth conditions, with various levels of bacterial abundance in the water column of Lake Suigetsu (18). Bacterial abundance in the anoxic layer of the lake was 2.0–8.9×10 6 cells mL −1 , and the highest abundance of bacteria (5.0–18×10 6 cells mL −1 ) was detected in the oxic–anoxic interface layer.…”