Factors to Consider in Biogeochemistry

A review of: Strom SL (2008) Microbial Ecology of Ocean Biogeochemistry: A Community Perspective. Science, 320: 1043-1045.

Marine biogeochemical processes which in turn influence atmospheric climate are regulated predominantly by marine microbes that cycle organic carbon, nitrogen, phosphorus and sulphur. Microbial ecology and its influence on resource availability and biogeochemistry is central to many ocean based studies, although some methods are deemed to provide limited perspective due to ‘top-down’ and ‘bottom-up’ approaches. A good example of a’ top-down’ approach would be the ocean iron fertilisation experiments conducted in the last 20 years.

Ecological data and genomic findings indicate a broader view is needed to understand organism interactions and the impact it has on process regulation and microbial distribution and function. Factors such as heterotrophic grazing, lytic viruses, allelopathy and symbioses have a strong influence in shaping communities and must be considered when applying modern research methods in elucidating whole-community structure and function, to gain greater insights into biogeochemical cycling as well as the derivation and roles of the significant genetic diversity contained within a given community.

Biomass accumulation in the marine environment is insubstantial when compared to terrestrial ecosystems because microbial production turns over in days to weeks from mortality processes which consequently apply selective pressure and continually reshape a population, resulting in adaptations in favour of reducing mortality. This is supported by metagenomic surveys that have identified numerous genes related to community interactions, such as genes for antibiotics and polysaccharide synthesis which may impart cell surface defences.

Strom (2008) describes various examples which underscore the boundaries in our understanding of environmental gradients, environmental tolerance, nutrient limitations, nutrient uptake and storage, competitive ability, susceptibility to mortality, etc. of organisms involved in cycling carbon, silicon, nitrogen and dimethyl sulphide (DMS), including Prochlorococcus, Synechococcus, Emiliania huxleyi, diatoms, nitrifying and denitrifying bacteria.

DMS is produced by marine microbes through cleavage of dimethyl sulfoniopropionate (DMSP) which is then released into the atmosphere and act as cloud condensation nuclei, resulting in increased cloud cover. DMS fluxes have been thoroughly investigated but difficult to predict. Community interactions play a role in DMS production, such as in E. huxleyi blooms which produce DMSP to inhibit grazing by protists. DMSP is metabolized by bacteria and the propensity for a bacterial community to produce DMS or demethylate DMSP is dependent on community composition, hence knowledge of community interactions is essential for a mechanistic and predictive understanding of broad-scale processes.

The role of the cell surface in mediating resource acquisition, defence and mortality is poorly understood in marine microbes. Host specificity of viral infections depends on cell surface recognition and accordingly impact mortality therefore further investigations may explain how organisms adapt to selective pressures. In addition, allelopathic interactions in marine microbial communities are not well defined and may also be an effective variable in shaping community structure and function.

The genomic diversity that underlies microbial activity, microbial distribution, community processes and ecosystem function amalgamated into a holistic approach is required to comprehend the complexity of microbial ecology and biogeochemistry.