Geoscience Reference
In-Depth Information
CHAPTER 5
Markus G. Weinbauer, Xavier Mari, and Jean-Pierre Gattuso
5.1 Introduction
topic. Then the research that has been performed to
assess the effects of ocean acidii cation on the diver-
sity and activity of heterotrophic marine microor-
ganisms is reviewed. Finally, scenarios are developed
and potential implications are discussed.
Microbe-mediated processes are crucial for biogeo-
chemical cycles and the functioning of marine eco-
systems (Azam and Malfatti 2007). If these processes
are affected by ocean acidii cation, major conse-
quences can be expected for the functioning of the
global ocean and the systems that it inl uences, such
as the atmosphere. In contrast to phytoplankton,
which have been relatively well studied (see
Chapter 6), there is comparatively little information
on the effect of ocean acidii cation on heterotrophic
microorganisms. Two reviews on the potential
effects of ocean acidii cation on microbial plankton
have recently been published ( Liu
et al.
2010 ; Joint
et al.
2011) . In a recent perspective paper, Joint
et al.
(2011) concluded that marine microbes possess the
l exibility to accommodate pH change and that
major changes in marine biogeochemical processes
that are driven by microorganisms are unlikely.
Narrative reviews, which look at some of the rele-
vant literature, are potentially biased and could lead
to misleading conclusions (Gates 2002). Meta-
analysis was developed to overcome most biases of
narrative reviews. It statistically combines the results
(effect size) of several studies that address a shared
research hypothesis. Liu
et al.
( 2010 ) used a meta-
analytic approach to comprehensively review the
current understanding of the effect of ocean acidii -
cation on microbes (including phytoplankton) and
microbial processes, and to highlight the gaps that
need to be addressed in future research. In the fol-
lowing, a brief digest on oceanic microbes and their
role is provided for readers unfamiliar with this
5.2 Microbes in the ocean
5.2.1 Structural and functional diversity
of microorganisms
Microorganisms are dei ned as organisms that are
microscopic, i.e. too small to be seen by the naked
human eye, and mostly comprise single-celled
organisms. Viruses are sometimes also included in
this dei nition but it is hotly debated whether viruses
are alive or not (Raoult and Forterre 2008). The cur-
rent phylogeny considers three domains of cellular
life, the Bacteria, the Archaea and the Eukarya. Since
Archaea are more closely related to Eukarya than to
Bacteria, the old term 'Prokaryota' is phylogeneti-
cally invalid for Bacteria and Archaea (Pace 2009).
However, the term 'prokaryotes' can still be used in
a non-phylogenetic context for these two domains
(Whitman 2009) as is done in this chapter.
The development of molecular tools was an
important step for assessing marine microbial
diversity. Previously, the assessment of species
diversity was done by the characterization of iso-
lated bacteria. However, it is well known that only a
small fraction of the cells (often less than 1%) can be
isolated (great plate count anomaly; Staley and
Konopka 1985). The advent of molecular tools
has considerably changed the perception of the
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