Marine microbes see a sea of gradients
Far from being the homogenous medium of randomly distributed nutrients that the oceans have traditionally been viewed as, this review by Roman Stocker provides evidence to support a highly heterogenous environment of 'hot spots' within and between each millilitre of seawater. Stocker pools information from various studies and argues that marine bacteria play a major role in influencing the biogeochemistry and productivity of the oceans on a global scale.
The microarchitecture of the water column consists of dynamic nutrient gradients emanating from various sources, creating microscale heterogeneity. For example, the 'phycosphere' is the immediate area surrounding a microplankton cell, consisting of dissolved organic matter (DOM) and oxygen gradients attracting heterotrophic bacteria. As highlighted in Malfatti & Azam (2013), this can lead to intimate associations between bacterial and algal cells in possible symbioses & increased primary production. Other gradients include DOM plumes associated with sinking marine snow aggregations, viral cell lysis, faecal pellets & exopolymers, all creating hot spots of nutrients for chemotactic motile bacterial cells to swarm to and non-motile cells to maximise their uptake of.
The ratio of motile to non-motile cells in the oceans is not clear and whilst some non-motile species such as Pelagibacter ubique (SAR11 clade) are known to be highly abundant, nutrient enrichment can increase the percentage of motile cells in a microenvironment from less than 10% to more than 50% in 12 hours. Non-motile cells are randomly distributed and only a tiny fraction of cells will find themselves within a nutrient hot spot by chance, but many motile cells will traverse the gradient and cluster within tens of seconds of the onset of diffusion. The patch diffuses to background levels typically within minutes, permitting only a limited window of opportunity however evidence suggests that the next nutrient hot spot is only between 100 to 1000 μm away. Chemotaxis into the hot spot accelerates the rate of nutrient uptake by motile bacteria compared with a composition of solely non-motile bacteria, but the entire patch will be consumed in either case. Stocker questions whether this will purely affect the time scale for remineralisation of DOM or also the quantity.
To answer this question, different mechanisms for nutrient gradient uptake, other than time scales, must be considered. Bacterial growth efficiency (BGE – amount of carbon taken into biomass) increases with growth rate and mathematical models predict an increase in growth rate of 50% for cells associated with nutrient patches and a 10-fold increase in growth rate for cells associated with DOM plumes. Copiotrophs (motile cells) have higher maximum growth rates than oligotrophs (non-motile) and this coupled with higher concentrations of copiotrophs within hot spots may cause greater BGE’s in copiotrophs. If correct, then DOM consumed by copiotrophs would direct more carbon into the microbial loop than oligotrophs. In addition, when motile bacteria cluster around phytoplankton, as well as faster remineralisation rates, they may impart some of their inorganic nutrients and so aid phytoplankton primary production (as seen using atomic force microscopy in Malfatti & Azam, 2013).
Optimal foraging theory allows the utilisation of nutrient patches to be considered alongside the energy costs of motile bacteria, in an effort to understand the extent that bacterial responses to microscale gradients affect ocean biogeochemical properties. This framework will compliment the tools currently available to study interactions on a microscopic scale, which include epifluorescence, flow cytometry, genomics and atomic force microscopy.
Given the abundance and diversity of marine microbes, I think it highly probable that microbes play an incredibly important role in the composition of ocean biogeochemistry, productivity and heterogeneic microarchitecture throughout the oceans. It was beyond the remit of this review to encompass all aspects of the paper and I urge anyone interested to follow it up.
Stocker, R. (2012). Marine microbes see a sea of gradients. science, 338(6107), 628-633.