Warming and CO2 Enhance Arctic Heterotrophic Microbial Activity.

Dolors Vaqué,Elisabet L Sà,Paul F Wassmann,Alexandra Coello-Camba,Jesús M Arrieta,Iris E Hendriks,Elena Lara,Marta Alvarez,Johnna Holding,Carlos M Duarte,Susana Agustí

doi:10.3389/fmicb.2019.00494

Abstract

Ocean acidification and warming are two main consequences of climate change that can directly affect biological and ecosystem processes in marine habitats. The Arctic Ocean is the region of the world experiencing climate change at the steepest rate compared with other latitudes. Since marine planktonic microorganisms play a key role in the biogeochemical cycles in the ocean it is crucial to simultaneously evaluate the effect of warming and increasing CO2 on marine microbial communities. In 20 L experimental microcosms filled with water from a high-Arctic fjord (Svalbard), we examined changes in phototrophic and heterotrophic microbial abundances and processes [bacterial production (BP) and mortality], and viral activity (lytic and lysogenic) in relation to warming and elevated CO2. The summer microbial plankton community living at 1.4°C in situ temperature, was exposed to increased CO2 concentrations (135–2,318 μatm) in three controlled temperature treatments (1, 6, and 10°C) at the UNIS installations in Longyearbyen (Svalbard), in summer 2010. Results showed that chlorophyll a concentration decreased at increasing temperatures, while BP significantly increased with pCO2 at 6 and 10°C. Lytic viral production was not affected by changes in pCO2 and temperature, while lysogeny increased significantly at increasing levels of pCO2, especially at 10°C (R2 = 0.858, p = 0.02). Moreover, protistan grazing rates showed a positive interaction between pCO2 and temperature. The averaged percentage of bacteria grazed per day was higher (19.56 ± 2.77% d-1) than the averaged percentage of lysed bacteria by virus (7.18 ± 1.50% d-1) for all treatments. Furthermore, the relationship among microbial abundances and processes showed that BP was significantly related to phototrophic pico/nanoflagellate abundance in the 1°C and the 6°C treatments, and BP triggered viral activity, mainly lysogeny at 6 and 10°C, while bacterial mortality rates was significantly related to bacterial abundances at 6°C. Consequently, our experimental results suggested that future increases in water temperature and pCO2 in Arctic waters will produce a decrease of phytoplankton biomass, enhancement of BP and changes in the carbon fluxes within the microbial food web. All these heterotrophic processes will contribute to weakening the CO2 sink capacity of the Arctic plankton community.

Highlights

The Arctic Ocean is warming at two to three times the global rate and is experiencing accelerated ice loss with a historical minimum reached in the summer of 2012 (Parkinson and Comiso, 2013)
Bacterial abundance did not vary too much among the three temperature treatments (Figure 1B p > 0.05 and Supplementary Table S1), we detected occasionally higher values in the 6◦ than in the 1 and 10◦C treatments after day 5 up to day 11, while opposite trends in Chl a concentration were observed in the 6 and 10◦C treatments
A common concern is that short-time manipulations cannot simulate properly the effect of long-term adaptation likely occurring in natural systems and are biased

Summary

Introduction

The Arctic Ocean is warming at two to three times the global rate and is experiencing accelerated ice loss with a historical minimum reached in the summer of 2012 (Parkinson and Comiso, 2013). Previous experimental studies have shown, that warming (beyond a threshold of 5◦C) triggers a decrease of phytoplankton biomass and net primary production in the Arctic (Holding et al, 2013; Coello-Camba et al, 2015) This will favor an increase of bacterial growth which is translated in excess community respiration over gross primary production (Holding et al, 2013). A major effect of warming produce changes in the plankton carbon flow pattern, enhancing bacterial processing of DOC (and resulting increased CO2 production). This has been confirmed by many other experimental studies (e.g., Wohlers et al, 2009). A positive effect of increasing temperature and pCO2 on marine bacterial communities is observed, favoring their growth as a consequence of selecting different more active phylotypes (e.g., Lindh et al, 2013; Piontek et al, 2013)

Methods

Results

Conclusion