Coupling Between Heterotrophic Nanoflagellates and Bacteria in Fresh Waters: Does Latitude Make a Difference?

Bianca T Segovia,Luiz F M Velho,Carolina D Domingues,Bianca R Meira,Hugo Sarmento,Fabio Roland,Lúcia M Lobão,Paulina Fermani,Fernando M Lansac-Toha,Fernando Unrein

doi:10.3389/fmicb.2016.00114

Abstract

Recent studies reported comparatively lower heterotrophic bacteria (HB) abundances in tropical regions, indicating that factors involved in bacterial losses could be more relevant in the tropics. Heterotrophic nanoflagellates (HNF) are considered the main predators of HB in aquatic ecosystems, and one should expect higher abundances in the tropics because of differences in the food web configuration (absence of large daphnids). However, there are no comprehensive studies comparing HB and HNF abundances in a latitudinal gradient. We hypothesized that HB abundance would be lower in the tropics because HNF abundance would be higher, resulting in a tighter HNF–HB coupling. To test this hypothesis, we compiled a large dataset of HB and HNF abundances from tropical and temperate freshwater environments. We found that both HB and HNF abundances were lower in the tropical region, and that HNF-HB coupling does not differ between temperate and tropical regions. The lower HNF abundance and lack of coupling may be explained by a strong top-down control on HNF and/or their herbivory preference. Besides, no relationship was found between bacterial specific growth rate and either chlorophyll-a and HB abundance, indicating that bacterial losses may have an important role in tropical freshwaters. Thus, we found that HNF is likely not the main controllers of HB abundance, and that grazing by ciliates and cladocerans, together with the physiological effects of higher temperatures, may explain the high bacterial loss rates in the tropics.

Highlights

Inland aquatic ecosystems play a relevant role in the global carbon cycle (Cole et al, 2007; Tranvik et al, 2009; Raymond et al, 2013)
The dataset consists of 1047 observations of heterotrophic bacteria (HB) and Heterotrophic nanoflagellates (HNF) abundances from the literature in both tropical (Ntrop = 381) and temperate (Ntemp = 666) freshwater inland aquatic ecosystems
We found that both HNF and HB abundances were lower in the tropics and that there is no difference in the HNF-HB coupling between those regions

Summary

Introduction

Inland aquatic ecosystems play a relevant role in the global carbon cycle (Cole et al, 2007; Tranvik et al, 2009; Raymond et al, 2013). It is essential to identify the patterns and drivers of bacterial abundance, production and respiration across latitudinal gradients In this way, recent studies pointed out that, despite the slightly higher bacterial production in lower latitudes (Amado et al, 2013), the bacterial abundance found in those regions is lower, compared to temperate environments (Roland et al, 2010; Sarmento, 2012). Recent studies pointed out that, despite the slightly higher bacterial production in lower latitudes (Amado et al, 2013), the bacterial abundance found in those regions is lower, compared to temperate environments (Roland et al, 2010; Sarmento, 2012) This indicates that factors involved in bacterial loss would be more important in the tropics, since bacterial biomass does not seem to increase with increasing bacterial production in similar rates in both regions (Billen et al, 1990). These differences in HB abundance at different latitudes have been attributed, at least in part, to a higher top–down control of rotifers, ciliates, and nanoflagellates in warmer regions (Roland et al, 2010; Sarmento et al, 2010; Sarmento, 2012; Vázquez-Domínguez et al, 2012; Amado et al, 2013)

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