down into the sediment where they use it as electron acceptor for the oxidation of
sulfide, growing autotrophically or mixotrophically (Fossing et al. 1995; Jørgensen
and Gallardo 1999). Similar nitrate vacuoles were found in marine Beggiatoa
filaments found at a Monterey Canyon cold seep and at the Guaymas Basin hydrothermal vents where sulfide concentrations are high and dissolved oxygen is low.
Nitrate concentrations in the cells were estimated at 0.13–0.16 M, being 3000- to
4000-fold higher than ambient levels (McHatton et al. 1996). Another giant sulfuroxidizing bacterium is Thiomargarita namibiensis. With cells measuring up to 0.75
mm, it is the prokaryote with the largest cells ever recorded. It was discovered in
dense populations in Namibian shelf sediments underlying the oxygen minimum
zone of the Benguela Current upwelling system. Like its close relative Thioploca, its
cells possess a central vacuole in which nitrate is accumulated to a concentration of
~0.8 M (Schulz et al. 1999).
10.6 Cable Bacteria: Oxidizing Sulfide Using Remotely
Located Oxygen
One of the most surprising findings in microbial ecology in the past decade is the
discovery that oxidization of sulfide in anaerobic marine sediments can be coupled
with reduction of oxygen in the overlaying aerobic layers, up to several centimeters
higher. Filamentous “cable bacteria” can transport electrons over centimeter distances in sediments. The existence of electrical communication between the reduced
and the oxidized parts of sediments was discovered when it was noticed that altering
the oxygen concentration in the water overlying the sediment resulted in a rapid
change in the sulfide concentration more than 12 mm below the oxic zone. This
change could only be explained by transmission of electrons but not by diffusion of
molecules (Nielsen et al. 2010). The electrical conductors are filamentous bacteria
that transport electrons along their length. These cable bacteria are phylogenetically
affiliated with the sulfate-reducing and sulfur-disproportionating Desulfobulbaceae
(Deltaproteobacteria), and they probably oxidize sulfide by reversing the canonical
sulfate reduction pathway. The cell envelope contains highly conductive fibers, and
periplasmic cytochromes may be responsible for the electron flow (Kjeldsen et al.
2019; Meysman et al. 2019). Presence of the enzymes of the Wood-Ljungdahl
pathway of CO 2 fixation (see Sect. 10.10) suggests that they may lead an autotrophic
way of life (Kjeldsen et al. 2019). The existence of such living electrical cables
shows that the action of some microorganisms is not limited to the micrometer scale
but can have significant effects centimeters away (Nielsen et al. 2010; Pfeffer et al.
2012).
10 The Grand Microbial Variety Show
169
sulfide, growing autotrophically or mixotrophically (Fossing et al. 1995; Jørgensen
and Gallardo 1999). Similar nitrate vacuoles were found in marine Beggiatoa
filaments found at a Monterey Canyon cold seep and at the Guaymas Basin hydrothermal vents where sulfide concentrations are high and dissolved oxygen is low.
Nitrate concentrations in the cells were estimated at 0.13–0.16 M, being 3000- to
4000-fold higher than ambient levels (McHatton et al. 1996). Another giant sulfuroxidizing bacterium is Thiomargarita namibiensis. With cells measuring up to 0.75
mm, it is the prokaryote with the largest cells ever recorded. It was discovered in
dense populations in Namibian shelf sediments underlying the oxygen minimum
zone of the Benguela Current upwelling system. Like its close relative Thioploca, its
cells possess a central vacuole in which nitrate is accumulated to a concentration of
~0.8 M (Schulz et al. 1999).
10.6 Cable Bacteria: Oxidizing Sulfide Using Remotely
Located Oxygen
One of the most surprising findings in microbial ecology in the past decade is the
discovery that oxidization of sulfide in anaerobic marine sediments can be coupled
with reduction of oxygen in the overlaying aerobic layers, up to several centimeters
higher. Filamentous “cable bacteria” can transport electrons over centimeter distances in sediments. The existence of electrical communication between the reduced
and the oxidized parts of sediments was discovered when it was noticed that altering
the oxygen concentration in the water overlying the sediment resulted in a rapid
change in the sulfide concentration more than 12 mm below the oxic zone. This
change could only be explained by transmission of electrons but not by diffusion of
molecules (Nielsen et al. 2010). The electrical conductors are filamentous bacteria
that transport electrons along their length. These cable bacteria are phylogenetically
affiliated with the sulfate-reducing and sulfur-disproportionating Desulfobulbaceae
(Deltaproteobacteria), and they probably oxidize sulfide by reversing the canonical
sulfate reduction pathway. The cell envelope contains highly conductive fibers, and
periplasmic cytochromes may be responsible for the electron flow (Kjeldsen et al.
2019; Meysman et al. 2019). Presence of the enzymes of the Wood-Ljungdahl
pathway of CO 2 fixation (see Sect. 10.10) suggests that they may lead an autotrophic
way of life (Kjeldsen et al. 2019). The existence of such living electrical cables
shows that the action of some microorganisms is not limited to the micrometer scale
but can have significant effects centimeters away (Nielsen et al. 2010; Pfeffer et al.
2012).
10 The Grand Microbial Variety Show
169