Oct 20, 2012

Sinking global warming - will it be a unicellular solution?

Possible influence of bacterial quorum sensing on the hydrolysis of sinking particulate organic carbon in marine environments  

FROM: Environmental Microbiology Reports (2011) 3(6), 682–688

Possible influence of bacterial quorum sensing on the hydrolysis of sinking particulate organic carbon in marine environments

Laura R. Hmelo,† Tracy J. Mincer and Benjamin A. S. Van Mooy*
Department of Marine Chemistry and Geochemistry,
Woods Hole Oceanographic Institution, MS #4, Woods Hole, MA 02543, USA

A central component of the ocean’s biological carbon pump is the export of sinking, photosynthetically derived, particulate organic carbon (POC). Bacteria colonize these particles and produce enzymes that hydrolyse sinking POC thereby acting as one of the major controls on the biological pump.

Here we provide evidence that a bacterial cell–cell communication mechanism, quorum sensing (QS), may influence the activity of hydrolytic enzymes on sinking particles.
(Continued below image.)
 We collected sinking POC from a site off Vancouver Island, Canada and found that it contained acylated homoserine lactones (AHLs), a suite of well known bacterial communication molecules. Furthermore, we observed that the addition of exogenous
AHLs to incubations containing sinking POC affected the activity of key hydrolytic enzymes involved in POC degradation in some cases.

Our results suggest that AHL-based QS could play an important role in regulating the degradation of sinking POC and that variability in AHL-triggered POC hydrolysis is a heretofore unrecognized process that impacts the marine biological carbon pump.
Carbon fixed by photosynthesis is transferred to the deep ocean via sinking particulate organic carbon (POC) where it is removed from the atmosphere for hundreds to thousands of years. Sinking POC flux declines significantly in the upper few hundred meters of the ocean (Martin et al., 1987; Buesseler et al., 2008), but there is significant
spatial and temporal variability in this attenuation (Buesseler et al., 2007). 

It is generally accepted that POC - attached bacteria contribute significantly to organic
carbon degradation during sinking (Steinberg et al., 2008), but parameterizing their contribution remains a major obstacle to the development of accurate carbon cycle models. Currently, flux attenuation is represented by empirical (Martin et al., 1987) or chemical models (Armstrong et al., 2001), which do not explicitly incorporate the
activities of particle-attached bacteria.

Many bacteria use a cell-density dependent signalling system, ‘quorum sensing’ (QS) to coordinate the expression of genes encoding behaviours that benefit cells at high population density (e.g. extracellular hydrolytic enzyme production, luminescence, biofilm formation).

Extracellular hydrolytic enzymes are used by particle attached bacteria in the conversion of sinking POC to suspended POC or dissolved organic carbon (DOC) in the environment (e.g. Smith et al., 1992). Bacteria that produce the particular class of QS signal molecules, acylated homoserine lactones (AHLs), are readily isolated
from marine environments where bacterial population densities are high (Gram et al., 2002; Wagner-Dobler
et al., 2005).

At this time there are only a limited number of cultivation-independent reports of AHLs from natural marine environments (e.g. Decho et al., 2009).

Although micromolar concentrations of AHLs have been reported in bacterial cultures (Boettcher and Ruby, 1995; e.g. Puskas et al., 1997), many bacteria can respond to concentrations in the nanomolar range (Kaplan and Greenberg, 1985; Boettcher and Ruby, 1995; Burton et al., 2005).

In the marine environment, AHLs are most likely to accumulate in microenvironments where bacterial densities are high (Hmelo and Van Mooy, 2009), such as within sinking POC.

Direct measurements of AHLs in environmental samples require extremely sensitive and selective analytical tools, which are only now becoming available to the oceanographic community.

Although bacterial cultivars from marine particles have ben shown to possess the capacity for Quorum Sensing (Gram et al., 2002), in situ QS activity has yet to be demonstrated in sinking POC

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