Abiotic and Biotic Factors Affecting the Ingestion Rates of Mixotrophic Nanoflagellates (Haptophyta)

Abstract

Mixotrophic haptophytes comprise one of several important groups of mixotrophic nanoflagellates in the pelagic environment. This study aimed to investigate if phagotrophy in mixotrophic haptophytes is regulated by light or other factors in the surface (SE) and bottom (BE) of the euphotic zone in the subtropical northwestern Pacific Ocean. We estimated the rates of bacterial ingestion by haptophytes using fluorescently labeled bacteria (FLBs) and fluorescence in situ hybridization. Haptophyte diversity and abundance were also investigated in the same sampling area. The annual mean abundance of haptophytes was 419 ± 85.6cellsmL-1 in both SE and BE. Cells 3-5μm in size were the dominant group in all haptophytes and accounted for majority of bacteria standing stock removed by haptophytes (53%). Most haptophyte ingestion rates (IRs) were not significantly different between the two layers (average SE ingestion rate: 12.5 ± 2.29bacHap-1h-1; BE: 14.7 ± 3.03bacHap-1h-1). Furthermore, the haptophyte IRs were negatively correlated with nitrate concentrations in the SE and positively correlated with bacterial abundances in the BE, which accounts for the significantly high IRs in August 2012 and 2013. These findings imply that mixotrophic haptophytes in this region had different factors affecting phagotrophy to adapt to the ambient light intensity alterations between SE and BE.