Given the climate change crisis, it is urgent to reduce anthropogenic CO2 emissions
and explore climate geoengineering opportunities. The ocean plays critical roles in
global carbon cycling and may provide a solution for climate change mitigation (Buesseler
et al., 2008; Yoon et al., 2018). An accurate understanding of the processes and mechanisms
of the marine carbon cycle and its interactions with human-driven climate change is
fundamentally important. The Aquatic Microbiology Section made important contributions
of new knowledge and insights to this research field, including focused investigations
on marine primary production, organic matter biodegradation and biotransformation,
and microbial responses to natural and anthropogenic environmental gradients and stressors
(e.g., Bullerjahn and Post, 2014; Labbate et al., 2016; Daniel et al., 2018; Gutierrez
et al., 2018; Mayali, 2018; Villar-Argaiz et al., 2018; Wilson and Church, 2018; Dang
et al., 2019; Murillo et al., 2019; Sala et al., 2019; Wietz et al., 2019).
Coastal vs. Oceanic Blue Carbon Sinks
Vegetated coastal ecosystems (VCEs), including mangrove forests, salt marshes, seagrass
meadows, and seaweed beds, constitute intense blue carbon sinks at the land–ocean
transition zones (Nellemann et al., 2009; Macreadie et al., 2019). Conserving and
restoring VCEs for maintaining and enhancing blue carbon sequestration have been proposed
as an integral part of strategies for climate remediation (Geraldi et al., 2019).
However, the VCEs are also hotspots of non-CO2 greenhouse gas emissions due to anthropogenic
eutrophication-enhanced microbial activities including CH4 and N2O production (Angell
et al., 2018; Dang and Li, 2018). Although VCEs may contribute to climate change mitigation
at the local and national scales, they occupy a very limited spatial extent on Earth
and thus their climate remediation potential is small at the global scale (Taillardat
et al., 2018).
The ocean is a huge carbon sink and of enormous climate remediation potential. It
absorbs atmospheric CO2 via both physical and biological processes. Unfortunately,
excess CO2 absorbed via physical processes causes ocean acidification (Doney et al.,
2009). Decreasing seawater pH and carbonate saturation states change the food web
structure and biogeochemical cycles of the ocean, causing loss of marine ecosystem
stability and services (Cooley et al., 2016; Hurd et al., 2018). Ocean acidification
may even trigger a sixth mass extinction event (Veron, 2008). Therefore, it is inappropriate
to inject more CO2 into the ocean via physical methods for climate remediation (IPCC,
2005; Reith et al., 2019). On the contrary, biological CO2 fixation converts inorganic
carbon into organic matter, providing a more favorable mechanism of the ocean to absorb
atmospheric CO2 (Falkowski and Raven, 2007; Siegel et al., 2016).
Biological Carbon Pump vs. Microbial Carbon Pump
Marine ecosystems contribute half of all biological carbon fixation on Earth. However,
in order for long-term carbon sequestration, photosynthetically fixed carbon needs
to be transported to and stored in deep ocean waters and sediments. The biological
carbon pump (BCP) helps fulfill this function, transporting particulate organic carbon
(POC) from the ocean surface to its interior and thus contributing to climate modulation
on geological time scales (Falkowski and Raven, 2007; Le Moigne, 2019). The microbial
carbon pump (MCP) is another biological carbon sequestration mechanism (Jiao et al.,
2010). The essence of MCP is the microbial transformation of labile dissolved organic
carbon (LDOC) into recalcitrant dissolved organic carbon (RDOC) that is resistant
to further biological degradation and thus maintained in the ocean for decades to
millennia (Ogawa et al., 2001; Jiao et al., 2014). Both structural recalcitrance and
low concentration of DOC molecules contribute to their persistence (Jiao et al., 2014,
2015; Arrieta et al., 2015). However, there are debates on the relative contributions
of these two distinct mechanisms to the formation of the huge RDOC pool in the ocean
(Arrieta et al., 2015; Zark et al., 2017; Wang et al., 2018; Shen and Benner, 2019).
These debates advance studies examining the marine DOC molecular composition (Petras
et al., 2017; Zark et al., 2017). The MCP generates both structural recalcitrance
and a huge molecular diversity of DOC compounds each present at picomolar or subpicomolar
concentrations (i.e., below microbial uptake thresholds) to evade being further consumed
(Mentges et al., 2017; Jiao et al., 2018; Zark and Dittmar, 2018; Noriega-Ortega et
al., 2019).
Microorganisms shape the marine ecosystems and drive the biogeochemical cycles (Azam
et al., 1983; Azam and Malfatti, 2007; Falkowski et al., 2008). The carbon sequestration
efficiency of both BCP and MCP is mainly regulated by microbial communities (Dang
and Lovell, 2016; Zhang et al., 2018). POC and dissolved organic carbon (DOC) are
the two distinct forms of organic carbon in the ocean, supporting distinct carbon
sequestration processes, respectively. The formulation of the BCP concept started
more than 35 years ago (Volk and Hoffert, 1985), and research on this front has been
being highly active ever since (Siegel et al., 2016; Le Moigne, 2019). Although the
concept of MCP is quite new, its research is gaining recognition and momentum (Zhang
et al., 2018). In spite of the great research efforts, neither MCP nor BCP has achieved
full understanding, regarding their respective quantitative contribution to climate
modulation and the environmental and biological factors that may control their contributions
and dynamics (Boyd, 2015; Robinson et al., 2018).
Primary Production vs. Respiration
Organic matter provides the basis for the BCP and MCP to function. Most organic matter
in the surface ocean is produced by primary producers. Thus, the marine primary production,
which is subject to both top-down and bottom-up controls, is a key factor influencing
BCP and MCP (Lechtenfeld et al., 2015; Siegel et al., 2016). Zooplankton grazing and
viral lysis affect the composition, biomass, productivity, and partitioning of produced
organic matter (particulate vs. dissolved) of the primary producer communities (Jiao
et al., 2010; Sime-Ngando, 2014; Worden et al., 2015; Steinberg and Landry, 2017;
Zimmerman et al., 2020). The availability and chemical speciation of nutrients play
a critical role in determining the composition, abundance, and productivity of the
marine photosynthetic microbial communities as well, and different phytoplankton may
have distinct carbon export potentials (Herndl and Reinthaler, 2013; Richardson, 2019).
Warming and nutrient scarcity may shift the phytoplankton communities, favoring taxa
with small cell sizes, such as picocyanobacteria (Hutchins and Fu, 2017). Prochlorococcus
are the most abundant and productive picocyanobacteria in oligotrophic oceans (Partensky
et al., 1999; García-Fernández et al., 2004). They can hardly sink quickly enough
on their own to directly contribute to BCP. However, this typical view is recently
challenged (Richardson, 2019). Prochlorococcus are a potential source of transparent
exopolymer particles (TEPs) that enhance marine aggregate formation and thus facilitate
the BCP (Iuculano et al., 2017). Heterotrophic bacteria may also play a role in prompting
TEP production and aggregate formation of Prochlorococcus (Cruz and Neuer, 2019).
DOC released from Prochlorococcus via viral lysis and other processes may fuel the
MCP for RDOC production (Zhao et al., 2017). The whole ecosystem structure has recently
been proposed to majorly set the carbon sequestration efficiency (Guidi et al., 2016;
Moriceau et al., 2018; Bach et al., 2019; Henson et al., 2019). These examples highlight
the need of systematic and mechanistic investigations on the marine ecosystem key
players and interactions, particularly in terms of their carbon sequestration roles
and quantitative contributions.
Chemolithoautotrophic bacteria and archaea contribute organic carbon to the ocean
as well (Herndl and Reinthaler, 2013; Dang and Chen, 2017). They not only play a key
role in sustaining the chemosynthetic ecosystems related to deep-sea hydrothermal
vents and cold seeps (McNichol et al., 2018; Dick, 2019) but also may contribute significantly
to food web and energy transfer in non-extreme environments (Herndl and Reinthaler,
2013). Dark carbon fixation may provide substantial primary production in certain
marine waters (Taylor et al., 2001; Yakimov et al., 2011; Celussi et al., 2017; Guerrero-Feijóo
et al., 2018; La Cono et al., 2018). Microbial degradation and remineralization of
marine particulate organic matter (POM) significantly lower the BCP efficiency (Turner,
2015; Dang and Lovell, 2016). However, the same processes regenerate nutrients and
energy sources, likely playing a role in fueling chemolithoautotrophy and partially
offsetting fixed carbon loss during sinking POM remineralization (Wright et al., 2012;
Herndl and Reinthaler, 2013; Dang and Chen, 2017). Chemolithoautotrophy may help fuel
the MCP as well. Ammonia-oxidizing archaea, usually dominant in mesopelagic and bathypelagic
marine waters, release DOC to partially support in situ prokaryotic heterotrophy (Bayer
et al., 2019). The contribution of chemolithoautotrophy to the global ocean's primary
production, BCP, and MCP warrants further investigation.
Although the ocean's total primary production is very high (up to 50 Gt C/year), only
a small fraction (<10%) is transported to the deep ocean via the BCP and even a smaller
fraction (<1%) is sequestered for millennia (Henson et al., 2011; Bach et al., 2019;
Fender et al., 2019; Giering et al., 2020). The majority of the marine primary production
is converted back to CO2 in the ocean's twilight zone via community respiration, to
which the microbes usually contribute the most (~50% to >90%) (Rivkin and Legendre,
2001; Sanders et al., 2016). Heterotrophic microbes uptake and respire DOC, and many
particle-associated microbes secrete extracellular enzymes to hydrolyze POC into DOC
(Arnosti, 2011; Orcutt et al., 2011; Dang and Lovell, 2016; Baltar, 2018). Respiration
not only significantly lowers the BCP efficiency but also may cause deoxygenation
and acidification in the affected marine waters (Cai et al., 2011; Oschlies et al.,
2018; Robinson, 2019). Respiration may constrain the MCP as well (Robinson and Ramaiah,
2011; Dang and Jiao, 2014).
Although respiration is a fundamental metabolic process and the balance between respiration
and primary production controls the ecosystem carbon storage capacity, respiration
is much less investigated than primary production in the ocean (del Giorgio and Duarte,
2002; Arístegui et al., 2009; Robinson, 2019). This situation hinders our understanding
of the ocean's carbon cycle. For example, the subtropical gyres cover ~40% of the
Earth surface (Karl and Church, 2014). However, whether these oligotrophic open oceans
are overall autotrophic (i.e., net CO2 sinks) or heterotrophic (i.e., net CO2 sources)
is still being debated (Duarte et al., 2013; Ducklow and Doney, 2013; Williams et
al., 2013; Koeve and Kähler, 2016), let alone confident prediction of their climate
modulation potentials.
Perspectives
The BCP (~0.2–0.5 Gt C/year) and MCP (~0.2 Gt C/year) may make similar contributions
to long-term organic carbon sequestration (Guidi et al., 2015; Legendre et al., 2015;
Giering et al., 2020), and both show climate geoengineering potentials (Le Moigne,
2019; Richardson, 2019). However, the BCP export efficiency has reduced ~1.5% over
the past 33 years of climate warming (Cael et al., 2017), and warmer conditions will
induce larger reductions (Boyd, 2015; Barange et al., 2017). The negative response
of BCP to warming constitutes a positive feedback on climate change. The response
of the MCP to climate change is currently not clearly known. Under the impacts of
climate change and other anthropogenic perturbations, the global nitrogen cycle and
ecosystem biodiversity may have already crossed the safe planetary boundaries (Rockström
et al., 2009), exerting further negative impacts on the carbon cycle and climate through
disrupting the coupled biogeochemical cycles (Schlesinger et al., 2011; Boyd et al.,
2015). Research on fundamental processes and mechanisms of the BCP and MCP under varying
oceanographic and climatic conditions is urgently needed, with a particular focus
on integrating the major biogeochemical cycles and the ocean's biological, chemical,
and physical processes for a better understanding of the marine carbon cycle and its
response to climate change (Lucas et al., 2016; Hwang et al., 2017; Bif et al., 2018;
Igarza et al., 2019; Quigley et al., 2019; Romera-Castillo et al., 2019). Mechanistic
insights and implementation strategies have recently been proposed (Robinson et al.,
2018; Yoon et al., 2018; Zhang et al., 2018; Boyd et al., 2019). Advances in upcoming
marine carbon cycling research may also help overcome the uncertainty and difficulty
in developing environmental-friendly ocean geoengineering techniques for climate change
mitigation, the success of which may as well require interdisciplinary collaborations,
strategic planning, technique innovations, and systematic investigations including
both BCP and MCP for integrated ocean carbon sequestration enhancement (Polimene et
al., 2018; Emerson, 2019; Sloyan et al., 2019; Sogin et al., 2019; Zhang et al., 2019).
Author Contributions
The author confirms being the sole contributor of this work and has approved it for
publication.
Conflict of Interest
The author declares that the research was conducted in the absence of any commercial
or financial relationships that could be construed as a potential conflict of interest.