Characterization of bacterioplankton communities and quantification of organic carbon pools off the Galapagos Archipelago under contrasting environmental conditions.

Nataly Carolina Guevara Campoverde,Astrid Gärdes,Pier Luigi Buttigieg,Christiane Hassenrück

doi:10.7717/peerj.5984

Abstract

Bacteria play a crucial role in the marine carbon cycle, contributing to the production and degradation of organic carbon. Here, we investigated organic carbon pools, aggregate formation, and bacterioplankton communities in three contrasting oceanographic settings in the Galapagos Archipelago. We studied a submarine CO2 vent at Roca Redonda (RoR), an upwelling site at Bolivar Channel (BoC) subjected to a weak El Niño event at the time of sampling in October 2014, as well as a site without volcanic or upwelling influence at Cowley Islet (CoI). We recorded physico-chemical parameters, and quantified particulate and dissolved organic carbon, transparent exopolymeric particles, and the potential of the water to form larger marine aggregates. Free-living and particle-attached bacterial communities were assessed via 16S rRNA gene sequencing. Both RoR and BoC exhibited temperatures elevated by 1–1.5 °C compared to CoI. RoR further experienced reduced pH between 6.8 and 7.4. We observed pronounced differences in organic carbon pools at each of the three sites, with highest dissolved organic carbon concentrations at BoC and RoR, and highest particulate organic carbon concentrations and aggregate formation at BoC. Bacterioplankton communities at BoC were dominated by opportunistic copiotrophic taxa, such as Alteromonas and Roseobacter, known to thrive in phytoplankton blooms, as opposed to oligotrophic taxa dominating at CoI, such as members of the SAR11 clade. Therefore, we propose that bacterial communities were mainly influenced by the availability of organic carbon at the investigated sites. Our study provides a comprehensive characterization of organic carbon pools and bacterioplankton communities, highlighting the high heterogeneity of various components of the marine carbon cycle around the Galapagos Archipelago.

Highlights

Bacteria play a major role in the marine carbon cycle as primary producers, as well as organic matter degraders
We observed pronounced differences in measured water parameters and the components of the organic carbon pool in the water column between the sampling sites at Cowley Islet (CoI), Roca Redonda (RoR) and Bolivar Channel (BoC). Both RoR and BoC exhibited temperatures elevated by approximately 1 to 1.5 ◦C compared to CoI, where values of about 22 ◦C were recorded (Table 1)
During upwelling, which constitutes regular conditions at BoC, temperatures drop to about 18 ◦C, and inorganic nutrient and chlorophyll a concentrations are expected to be considerably higher than any observed in this study (Liu et al, 2014)

Summary

Introduction

Bacteria play a major role in the marine carbon cycle as primary producers, as well as organic matter degraders. While the majority of the organic carbon produced by photosynthesis is respired by heterotrophic organisms and released back into the atmosphere as CO2 (Hutchins et al, 2017), a small portion escapes remineralization and sinks to deeper waters as particulate organic carbon (POC) in the form of marine aggregates (Alldredge & Silver, 1988; Ducklow et al, 2001; Giering et al, 2014) The formation of these marine aggregates may be enhanced by the presence of transparent exopolymeric particles (TEP; Passow, 2002), which can be formed by cellular exudation from living or lysed cells (Passow et al, 2001; Bhaskar et al, 2005), and which, due to their sticky nature, function as biological glue facilitating aggregation (Jackson, 1995; Engel, 2000). The fate of organic carbon pools in the ocean is tightly coupled to the composition and activity of the FL and PA bacterioplankton community

Objectives

Methods

Discussion

Conclusion