Bringing to attention the importance of extracellular enzymes; and their role in the carbon pump

Cunha, A., Almeida, A., Coelho, F. J. R. C., Gomes, N. C. M., Oliveira, V., & Santos, A. L. (2010). Bacterial Extracellular Enzymatic Activity in Globally Changing Aquatic Ecosystems. Applied Microbiology, 124-135.

This paper is not strictly on marine microbes, but relates to extra cellular enzyme and their potential change in activity due to globally changing aquatic ecosystems (as the title suggests). After my last blog about the discovery of unique properties of Em2L8 it spurred my thinking in relation to bioremediation and the potential of extra cellular enzymes in this context. This intrigue led to the discovery of the above paper, so yes, I am doing a review, of a review.

Heterotrophic microorganisms are key to the microbial loop process; nutrient cycling and carbon flow through aquatic food webs. The main sources of organic matter to the microbial loop are exudates from phytoplankton, algae and bacterial cell material due to grazing. In addition to this river flow and terrestrial deposition also provide organic matter input. These factors combined are a large reservoir of energy for heterotrophic microbes.

Bacterial cell membranes are semi permeable by passive transport, and are mainly restricted to the passage of simple chemical compounds (low molecular weight). The architectural properties of gram negative and gram positive cell wall constructs are different, but are also similar in regards to their membrane selectivity of specific chemical compounds. Having stated this it is known that gram positive cell wall is not as discriminating as gram negative. Particular organic matter and dissolved organic matter consist mainly of high molecular weight compounds. To allow transport across outer membranes of cells large molecular substrate complexes must be hydrolysed outside of the cell. This step is mediated by extracellular enzymes allowing heterotrophic bacteria to gain nutrition from the diverse range of organic substrates that would otherwise be in an impermeable form. Taking this into consideration any disruption to the functioning of extracellular enzymes would potentially detrimentally impact the rate of metabolism of bacteria, consequentially limiting nutrient and remineralisation cycling. If this occurred to the carbon cycle, it could increase the Co2 in the atmosphere and potentially accelerate global climate change.

The extracellular enzymes production and activity is specific to the hydrolytic action e.g. oxygenases and peroxidases. Some extracellular enzymes are not truly separate from the host and are bound to the exterior wall or contained in the peri-plasmic space. True extracellular enzymes catalyse reactions in a detached form from their producers.

Cunha et al. highlight that the identities of particular members of mixed assemblages of microbes are capable of producing mixtures of extracellular enzymes and their structure and action is largely unknown. This is due to the fact most molecular analysis focuses on rRNA sequences, that provide little information about the degradable capabilities of uncultured microorganisms. This indicates to me that further methods need to be developed for investigating the degradable faculties of microbes or methods of analysing bacteria and substrate complexes such as marine snow. It is known that marine snow aggregates are colonized by heterotrophic microbes which express high levels of hydrolytic processes. As it is thought that they are epicentres of carbon remineralisation. Ocean acidification may have an effect on bacterial extracellular activity and negatively affect primary producers, which would affect the heterotrophs and consequentially inhibit the vertical transport of particular organic carbon to deeper ocean layers (biological carbon pump). Due to the severe implications of such an event, it seems obvious it is pivotal to further investigate the complexities of biogeochemical cycling and the consequences climate change may have on extra-cellular enzyme activity. Keeping people up to date on the developments in understanding of biogeochemical cycling responses to global climate change, in theory would lead to more effective environmental management; therefore I would advocate further research in this filed.