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.
When this paper mentions nutrient hotspots, is it talking about inorganic, organic, human pollution? Just whilst reading a paper the other day (my most recent blog post), it suggested that organic nutrients release by viral lysis into the DOM are not all accessible to bacteria (some take 1000's of years). I noticed that it was suggested in this review that 'the entire patch will be consumed in either case'. Just wondered if you had any idea on this?
ReplyDeleteHi Ethan, apologies for the delay in replying! After looking back over the paper, Stocker primarily discusses DOM plumes associated with phytoplankton and marine snow aggregations. The way I understand it, it is the plumes that are generating the 'hotspots' and gradients, which, due to varying factors including turbulence and diffusion, are limited in the amount of time that they are concentrated in a particular spot.
DeleteThis paper is jam-packed full of concepts and ideas relating to gradients and heterogeneity in the oceans, well worth a look.
This is a really interesting paper and it reminded me of something i saw while searching for papers the other day. In 2010 Grossart et al. published a paper; "Bacteria dispersal by hitchhiking on zoo plankton". In it they show that bacteria were literally moving vertically in the water column by attaching to diurnally migrating zooplankton. The study was performed using Daphnia species from a freshwater lake but after a little more digging i found a researcher at Woods Hole (Amalia Aruda) who is looking into this process using copepods but nothing has been published yet that i could find. I wonder if the bacteria, in particular the less motile varieties take advantage of this to help them find the hot spots? and if so what mechanisms and/or cues cause them to attach and detach?
ReplyDeleteHi Lucy, clever bacteria! It would certainly seem advantageous for a non-motile bacterium to hitch a ride through the heterogenous water column, rather than waiting to stumble across one! However, I would imagine that it would depend on the genome of the bacteria; for example the non-motile oligotroph, Pelagibacter ubique, has such a reduced genome that it would not be able to cope with a rich source of nutrition.
DeleteIn relation to attaching to other organisms, Vibrio alginolyticus are commonly associated with chitin and found on crab carapaces, attaching using chitin binding proteins (Pruzzo et al, 1996). Maybe a similar process is going on with the hitchhikers - particularly so for the copepod associations.
Hitching a ride would probably be more advantageous for a non-motile bacteria, but if you are non-motile then you would be much less able to find and make contact with a suitable zooplankton ride. I remember from last year that the flagellum was usually a bacterium's first point of contact with a surface; so maybe motile bacteria are better suited to hitching rides by using their flagella as a hitch hiker's thumb?
DeleteQuite possibly! It may be an energy conserving measure - maximising nutrient uptake without having to pay the energetic cost involved in locomotion. Genome sequencing may answer some of these questions, along with FISH analysis to determine whether specific adhesive genes or even genes for flagella are being expressed. It will make interesting reading when Woods hole do publish their studies.
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