Small Changes in pH Have Direct Effects on Marine Bacterial Community Composition: A Microcosm Approach

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From: PLoS ONE(Vol. 7, Issue 10)
Publisher: Public Library of Science
Document Type: Report
Length: 9,023 words
Lexile Measure: 1430L

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Author(s): Evamaria Krause 1 , * , Antje Wichels 1 , Luis Giménez 2 , Mirko Lunau 3 , Markus B. Schilhabel 4 , Gunnar Gerdts 1

Introduction

Since the beginning of the industrial period, the oceans have taken up one-quarter to one-third of anthropogenic CO2 emissions [1], [2]. This has already led to a reduction in surface ocean pH of 0.1 units, which may reach up to 0.7 units assuming the depletion of all fossil fuel reserves during the next three centuries [3]. In contrast, pH has constantly remained above 8.1 for the last 23 million years [4]. By the year 2100, atmospheric pCO2 values of 700 or 1000 ppm may lower mean surface pH in the southern North Sea to 7.82 or 7.67, respectively [5].

The effects of the anticipated rapid reduction in pH on marine organisms, and their ability to adapt, will determine future marine biodiversity and ecosystem functions. Yet the impact of ocean acidification on different groups of marine organisms remains under debate [6], [7], especially regarding heterotrophic bacteria as important players in marine biogeochemical cycles. Joint et al. [8] recently argued that microbe-dependent processes will not substantially change in a more acidic ocean, as marine microbes already experience large regional, temporal and depth-dependent pH variability, and even greater pH ranges are observed in freshwater lakes. This view was challenged by a meta-analysis on microbe-related ocean acidification research, which identified nitrogen fixation, cyanobacterial photosynthesis and elemental ratios as affected by changes in seawater carbonate chemistry [9]. Concerning other microbial processes and especially heterotrophic bacteria however, results have often been inconsistent and Liu et al. [9] concluded that "more research is needed at multi-species and community scales".

What we know about ocean acidification effects on bacterial communities predominantly stems from complex systems such as symbiotic microbial communities of corals or large-scale mesocosm experiments. At reduced pH, coral microbial communities were found to shift to bacteria associated with stressed or diseased hosts [10], [11], which could however not be confirmed at natural CO2 vent sites [12]. Furthermore, a decrease in the relative abundance of Alphaproteobacteria and an increase in the relative abundance of Flavobacteriales were observed in natural biofilms from the Great Barrier Reef [13].

Knowledge on the seawater bacterial community has remained scarce though. In mesocosm experiments, only minor indications of bacterial community shifts with pH were found [14], [15]. Notably, these findings relied on only one or two replicates per pCO2 treatment, which is a common problem in mesocosm studies. Although these experiments are biologically highly complex, involving indirect pH effects through food web interactions, they are usually carried out in low replication, due to logistical challenges and high costs. As a consequence, these experimental designs preclude a robust statistical interpretation. Furthermore, the natural variability of bacterial communities, which is characterized by seasonally recurring patterns [16]-[18], is not taken into account.

Therefore, a straightforward small-scale approach is needed to allow for high replication and the consideration of differently assembled communities. The problem...

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Gale Document Number: GALE|A498257561