Dissolved Organic Carbon Source Influences Tropical Coastal Heterotrophic Bacterioplankton Response to Experimental Warming.

Christian Lønborg,Cátia Carreira,Federico Baltar,Xosé Anxelu G Morán

doi:10.3389/fmicb.2019.02807

Abstract

Global change impacts on marine biogeochemistry will be partly mediated by heterotrophic bacteria. Besides ocean warming, future environmental changes have been suggested to affect the quantity and quality of organic matter available for bacterial growth. However, it is yet to be determined in what way warming and changing substrate conditions will impact marine heterotrophic bacteria activity. Using short-term (4 days) experiments conducted at three temperatures (−3°C, in situ, +3°C) we assessed the temperature dependence of bacterial cycling of marine surface water used as a control and three different dissolved organic carbon (DOC) substrates (glucose, seagrass, and mangrove) in tropical coastal waters of the Great Barrier Reef, Australia. Our study shows that DOC source had the largest effect on the measured bacterial response, but this response was amplified by increasing temperature. We specifically demonstrate that (1) extracellular enzymatic activity and DOC consumption increased with warming, (2) this enhanced DOC consumption did not result in increased biomass production, since the increases in respiration were larger than for bacterial growth with warming, and (3) different DOC bioavailability affected the magnitude of the microbial community response to warming. We suggest that in coastal tropical waters, the magnitude of heterotrophic bacterial productivity and enzyme activity response to warming will depend partly on the DOC source bioavailability.

Highlights

Heterotrophic bacteria are key players in many marine biogeochemical processes mainly due to their ability to degrade organic matter
The calculated temperature sensitivity showed that in all experiments, except in the control (p = 0.17, n = 3), warming led to higher bioavailable dissolved organic carbon (DOC) (BDOC) values, with the resulting activation energies varying between 0.44 ± 0.01 eV (Q10 = 1.8 ± 1.0) in the mangrove and 1.45 ± 0.15 eV (Q10 = 6.4 ± 0.8) in the seagrass experiment (Table 2)
Determining the impact of temperature on heterotrophic bacterial degradation of DOC is essential to predict the response of tropical coastal waters to climate change (Ducklow et al, 2010; Lønborg et al, 2018b)

Summary

Introduction

Heterotrophic bacteria are key players in many marine biogeochemical processes mainly due to their ability to degrade organic matter. Climate models predict a mean increase in sea surface temperatures between 1 and 3◦C by the end of this century (Rhein et al, 2013), with the warming rates of coastal zones exceeding by up to an order of magnitude those of the open ocean (Amos et al, 2003). Tropical coastal waters are global hotspots of biogeochemical activity, receiving and processing approximately one order of magnitude more carbon, nitrogen, and phosphorus than temperate and polar regions (Brunskill, 2010), with the high activity being sustained by the relatively high river discharges, elevated temperatures and sunlight levels (Nittrouer et al, 1995). Despite the importance of tropical coastal waters, to date no study has investigated in detail how warming will impact biogeochemical processes in these low latitude systems. Low latitude coastal waters are the ideal place to test these contrasting views, since presently they are characterized by the highest ocean surface temperatures on Earth

Methods

Results

Conclusion