Driving microbes round the bend: Chemotaxis and sodium-ion and proton motors

Many microbial assemblages utilise chemotactic capability to detect nutrients across a gradient in tandem with a ‘run and tumble’ style of moving, whereby the bacteria will typically use a flagellum to move it around it’s medium in straight lines, a ‘run’. It will periodically ‘tumble’ by reversing the direction of the flagellum which will change its direction. When a microbe senses it is following a nutrient gradient it will perform fewer ‘tumbles’.

Within the marine environment, however, nutrients are generally patchy and being able to chemotactically find them is much more difficult, as a result, microbes have to move in excess of 200µm s^-1. To help with this many marine microbes have modified the usual ‘run and tumble’ mechanism to include a reversal where by the microbe can reverse its direction so that it can move back into a nutrient patch if it otherwise would have drifted away.

The flagella of flagellate bacteria have embedded within their cell wall and cell membrane a rotary molecular motor. There are 2 mechanisms which power these motors, sodium-ion motors and proton motors. They drive the flagella using an electrochemical gradient involving either sodium ions or protons respectively.

The research of the authors of this particular paper focused on how much of a contribution the 2 main motor mechanisms for driving the flagella in marine flagellate microbes made in terms of high speed motility. In order to do this, inhibitors of the mechanisms were introduced to microbes collected from the water column and microbes from recent isolates. This was done for realism and future applicability which allows this study to be applied in a wider context although I would be wary of doing so as comparing a natural sample of microbes to an isolate does not really give a good view of how the motor mechanisms are affected as an isolate sample will not necessarily react in the same way as a natural sample.

The inclusion of an amiloride tolerant, unidentified isolate just to provide a link and comparison to the amiloride literature seemed unnecessary as they had already identified that amiloride was not being used in preference of the sodium-ion uncoupler monensin, mainly on the grounds that the monensin was less toxic to the cells and would work at a thousandth the concentration that amiloride would. This inclusion would only really serve to show that monensin was less toxic than amiloride which had already been identified beforehand. With the isolate being tolerant to amiloride anyway i’m not entirely certain what the authors were trying to accomplish by it. However, later on in the paper it was mentioned that amiloride had indeed been used to test whether or not the sodium-ions present were responsible for the motility of the community.

Although this paper does have very good applications in the research of chemotactic ability in flagellate bacteria, and in showing that many bacteria can maintain the speeds required for chemotaxis within the marine environment by use of 2 particular motors, I feel that this paper should not have drawn inferences or conclusion from inconclusive data concerning the toxicity of the uncouplers for the sodium-ion motors and its effects.

Overall this paper has highlighted some interesting areas for future research in areas such as microbial physiology and the interactions it has with the marine environment and exactly how marine microbe assemblages manage to maintain the high speeds they do within the viscous media that they inhabit.

Mitchell J.G., Barbara G.M., 1999, High speed marine bacteria use sodium-ion and proton driven motors, Aquatic Microbial Ecology, 18, 227-233

SAR86, what’cha metabolising there?

SAR86 is a ubiquitous clade of 16S rRNA which is highly resistant to cultivation in the lab. In previous studies the genomes of some sub-classes have been shown to include genes for rhodopsin, allowing the bacteria to use light to supplement ATP production. However no work has been done on the metabolic capabilities of the clade.

SAR86 genomes were sequenced by combination of flowcytometry and PCR, and then gene fragments from across the clade were compared to genetic libraries to investigate their metabolism. This revealed four main assemblies of genes for the clade, named A, B, C and D. The genomes of A and B were 90% complete, whereas the genomes for C and D were roughly 50% complete. Comparing gene segments to the GOS metagenomic library (sampled from around the world) revealed ecotypes for each group; A segments were found in the open ocean, B segments were found in warm coastal waters, gene fragments from both C and D were found in the colder coastal sites. This is similar to what Colin was teaching, a small streamlined genome reducing the overall maintenance of the genes, but reducing the adaptability of the bacteria leaving a narrow environment in which it can live.

Dupont et al. (2012) found SAR86 to be free living aerobic heterotrophs with a buffering capacity of phototrophic ATP (via proteorhodopsin). All SAR86 were found to be largely auxotrophic in regards to vitamins and some animo acids (except from sub-class B having a Vitamin B1 biosynthesis pathway). However the authors point out that this could be due to either; the use of alternative biosynthetic routes yet undiscovered or the genes coding for the proteins were on fragments lost in analysis.

Two clades which recruited the largest portions of GOS were SAR86 and SAR11 which are often found in the same environment, so the authors made comparisons between the two most abundant bacterial clades. SAR11 uses protein compounds as primary energy source, whereas SAR86 utilises fats and carbohydrates as primary energy source. Both SAR11 and SAR86 contained sulphur reducing metabolic pathways, but SAR86 has a putative transporter for glutathione and ϒ-glutamyl transferases (allows the break down of glutathione into cysteine) which means SAR86 has a relatively larger pool of organic sulphur to metabolise than SAR11.

To conclude Dupont et al. (2012) draws attention to the relatively small overlap between the niches of both SAR11 and SAR86, and ties the abundance of both Clades to the ratios of protein, carbohydrate and lipids found within the sea.

Genomic insights to SAR86, an abundant and uncultivated marine bacterial lineage

Coral-mucus Vibrio Integrons are Evolutionary Hotspots

Coral-mucus-associated Vibrio integrons in the Great Barrier Reef: genomic hotspots for environmental adaptation

Aside from the well-known symbiotic associations between corals and dinoflagellate zooxanthellae (corals being largely dependent on zooxanthellae for certain nutrients), the role of prokaryotes in the coral microbiodome has been the target of recent study. The authors of this study (Boucher et al, 2011) focus on Vibrionaceae, a family of proteobacteria, which they obtained from the mucus of the scleractinian coral Pocillopora damicornis from the Great Barrier Reef and investigate their associations within the coral microbiodome.

Some corals exhibit an apparent immunity to pathogens, although this is not the same adaptive immunity observed in vertebrates, and the hypothesis aiming to explain this immunity is known as the coral probiotic hypothesis. The authors make use of the ideas behind this hypothesis to explore the relationship between Pocillopora damicornis and its associated Vibrios.

The coral probiotic hypothesis suggests that the prokaryotic microbiome provides pathogen resistance (notably to pathogens that may cause coral bleaching) to the host coral and that this immunity insinuates a dynamic pattern of antimicrobial production by non-pathogenic (commensal) bacteria, enabled by horizontal gene transfer.

The genetic element behind this immunity and its transfer (and also facilitating antimicrobial resistance among pathogens), known as the integron, is a system consisting of an integrase gene (intI) and an associated integration site (attI), where an integrase protein (IntI) catalyzes the insertion and removal of gene cassettes. Gene cassettes typically consist of a single gene and recombination site (attC) and there may be up to 100 cassettes per integron array.

Integrons are vital in the spread of both antimicrobials and antimicrobial resistance among commensal bacteria and pathogenic bacteria. Their importance is evidenced by a rise in proportion of Vibrio 16s rRNA genes sequenced by both commensal and pathogenic bacteria during coral bleaching events.

The authors cultivated coral mucus samples and amplified, cloned and sequenced the genes present in the samples using PCR. They then acquired datasets from which to draw comparisons via a variety of methods including screening the colonies for IntI, constructing fosmid libraries, sequencing Vibrio housekeeping genes, performing a taxonomic assignment of Vibrio cultivars by recA phylogeny and more as detailed in the article.

            Their main findings were that a diverse variety of Vibrio species were contained within the mucus of P. damicornis and that the mucus Vibrio-cassette arrays were found to be highly dynamic (in that around 90% of their integron associated gene cassettes were being actively shared, leaving around 10% at most in common between cultivars). This meant that mucus Vibrio-cassette arrays could evolve more rapidly in comparison to chromosomal genes, and the high mobility of genes allows the integrons of mucus Vibrio to be viewed as strong evolutionary hotspots in genomes. Comparisons to free living Vibrios exemplified this observed high gene mobility. A direct link between the Vibrio cultivars, Vibrio coral pathogens and human pathogens was demonstrated by the exchange of a subset of integron associated gene cassettes (associated with antimicrobial resistance), exemplifying the extensive scale of cassette sharing between microbial niches. This link may be useful in further understanding how resistance antimicrobial and antibacterial drugs spreads in a medical context.

            The diversity of cassette genes discovered in the coral mucus may show a cooperative sharing of resources and a mutually beneficial association within the coral microbiodome. This diversity and association allows quick adaptation in response to threats from pathogens that may be viewed as immunity as mentioned earlier. While beneficial to the coral microbiodome, this rapid adaptation to threats does act as a selective pressure to pathogens, facilitating an evolutionary arms race on both sides.

Boucher, Y. et al., 2011. Coral-mucus-associated Vibrio integrons in the Great Barrier Reef: genomic hotspots for environmental adaptation. ISME Journal (2011) 5, pp.962–972. Available at: http://www.nature.com/ismej/journal/v5/n6/full/ismej2010193a.html

Viruses have proteorhodopsin genes too!

Genomic techniques can reveal information on the evolution of proteorhodopsin

As Colin explained in our lecture this week, viral genomes can encode proteins which are not directly part of virus reproduction.  These extra proteins can modify functional systems of the infected cell to somehow aid, or boost, virus reproduction; a well-documented example of this being cyanophage which contain genes for photosynthesis. Viruses are thought to get these extra genes via horizontal gene transfer.

Yutin & Koonin (2012) have very recently found evidence that giant marine viruses have genes for proteorhodopsin, another light-dependant system with two different functions. The first being a light-driven proton pump which generates ATP, the second being a light-sensitive signal receptor potentially involved in phototaxis. It is useful to note at this point that it was only in the last decade that proteorhodopsins were discovered to be widespread in a diverse range of marine microbes and recognised as an important metabolic process in the oceans.

The authors analysed alignment of conserved sequence blocks in the rhodopsin super family (across viral, bacterial, archaeal and eukaryotic forms of the protein) and found that the viral sequence is highly conserved; however, it does not include the conservation of a proton donor. Therefore they conclude that without a proton donor, the function of the protein in the viral infected host is likely to be for sensory purposes, such as phototaxis, as opposed to being used as a light-driven proton pump. The alignment data then was used to construct a phylogenetic tree based on sequence similarity, authors were able to visualise distinct clades which infer that the giant virus acquired proteorhopsin from bacteria, or more likely eukaryotes, via horizontal gene transfer.

The authors quite rightly conclude that viral proteorhodopsins, regardless of their speculated function (signalling or proton pump), could be major players in virus-host ecology in the ocean. Looking at the bigger picture, maybe what these findings could potentially tell us about the evolution and conservation of proteorhodopsins is more interesting… It is clear that rhodopsins are an important protein as they are so highly conserved across different lineages, from the human eye, to bacterial proton pumps and now also found in viruses. Whilst it is likely that this protein evolved independently in both the eukaryotes and bacteria (and then diverged via horizontal gene transfer), more comparative and experimental work is needed in order to follow the evolution of this system; questions over its independent origins (or lack of) remain to be answered.

Yutin, N., & Koonin, E. V. (2012). Proteorhodopsin genes in giant viruses. Biology direct, 7(1), 34.

N.B. this paper has only just been accepted and is still "in press", only a provisional copy is available.

http://www.biology-direct.com/content/7/1/34/abstract

 

 

The Aftermath of Iron Fertilization from a Microbial Perspective

The idea of being able to decrease atmospheric CO₂ by boosting phytoplankton seemed promising when the idea of iron fertilization first emerged in the 1990s. Several in situ experiments have shown since then that iron is the limiting factor for phytoplankton growth in high nutrient, low chlorophyll regions (HNLC) and that the addition of iron results in vast plankton blooms. However, a concomitant controversy on the efficiency as carbon sink and on the ecological consequences was unavoidable. 

Thiele et al. (2012) focused on the very central part of the biological pump, namely the microbes, and monitored the response of microbial communities to iron fertilization, by looking especially for the characteristic succession patterns observed during natural plankton blooms. It is unquestionable that this knowledge is essential to understand the fate of fixed carbon following iron fertilization.

Water samples from different depths, at time intervals from 4-5 days were taken using Niskin bottles from inside and outside the fertilization area during the LOHAFEX experiment in the Southern Atlantic Ocean. Thymidine and leucine incorporation rates, into DNA or proteins respectively, were used to assess microbial productivity, whereas CARD-FISH (remember that is the enzyme catalysed version of fluorescence in situ hybridization) allowed to quantify and identify community members, using both general and clade-specific probes. 

The iron addition caused a phytoplankton bloom stretching over 300 km² mainly composed of Prymnesiophytes (containing the maybe better known group coccolithophorids) and not of diatoms, because of silicate depleted waters.  The data analysis showed a significant increase in total microbial cell number inside the fertilization patch, along with significant increases in thymidine and leucine uptakes. Moreover the CARD-FISH revealed that the Bacteria, rather than the Crenarchaea, were responsible for the significant increase in cell numbers within the fertilized patch. In fact, the SAR11 clade accounted for 50 % of total cell counts and increased significantly at day 18, then remained stable. Also Roseobacter and Bacteroidetes were significantly more abundant within the fertilized area than outside. However, no changes were observed for Gammaproteobacteria. 

Three different community richness indices were applied: whereas Chao-1 values decreased till day 9 before increasing till the end of the experiment, this trend was not reflected by Shannon and Simpson indices. 

So far, so good.  But then I read their discussion, where after having listed innumerous results of significant increases, the authors conclude “total cell numbers of bacterioplankton and of the major clades are rather constant”. What does “rather constant” even mean, relative to what!? Moreover, they state that their results are concordant with similar studies showing that iron fertilization is not followed by a change in microbial communities. If they had made reference to the usual scale of community shifts during other plankton blooms, the reader would maybe be able to come to the same conclusion. Instead, the three diversity indices are not even mentioned again in the conclusion, nor a possible explanation for the observed trend in the Chao-1 values.  The increase in cell numbers might have been small, but it was statistically significant, still no biological explanations are proposed. Instead the authors relate the apparently constant numbers of cells to flagellate grazing and top-down control, an idea that had emerged from previous iron fertilization experiments. 

By no means am I saying that the results and the conclusion of this study are wrong, but I think the paper is lacking some essential biological information to be able to come to the same conclusion, and too many assumptions were made (suddenly an hypothesised pre-experiment plankton bloom makes its appearance in the discussion to explain inconvenient results) . In my opinion, the authors should have made an advance and tested the hypothesis of a negative correlation between bacterial abundance and abundance of heterotrophic nanoflagellates “hinted” by previous studies, instead of basing their results on the same assumptions as their colleagues.

Thiele, S., Fuchs, B., Ramaiah, N., Amann, R., 2012. Microbial community response during the iron fertilization experiment LOHAFEX. Applied and environmental microbiology, (October). Available at: http://www.ncbi.nlm.nih.gov/pubmed/23064339

Co-culturing reveals shift in gene expression levels compared to solo cultures

Co-Cultures of Pseudomonas aeruginosa and Roseobacter dentrificans Reveal shifts in Gene Expression levels compared to solo Cultures

Interactions among diverse microbial species are dynamic and most likely the basis for many adaptations that allow the occupation of diverse niches. These interactions may be beneficial, forming a mutualistic relationship such as symbiosis (Rosenberg & Zilber-Rosenberg 2011) or antagonistic through competition for resources or space etc (Long & Azam 2001; Rypien et al. 2010). The molecular basis of some ecological interactions have been linked to the production of secondary metabolites noted by (Allen et al. 2010), who also discusses there different uses, such as intraspecies signalling or defence.  Many biologically active secondary metabolites have potential to be used for future medicines (Nunnery et al. 2010), and so the need for reliable biosynthesis of these metabolites is high. However, pure cultures are often unreliable in the yield, or consistent biosynthesis, of secondary metabolites (muscholl-Silberhorn et al. 2008).

This study attempted to induce, measure and track the expression of microbial genes while they grew in mixed cultures, in order to mimic antagonism and interaction in the natural environment. Two model bacteria were chosen Pseudomonas aeruginosa (P.a) PAO1 and marine Roseobacter denitrificans (R.d) Och114, due to the availability of their complete genomic sequence. All cultures and co-cultures (growth of >1 bacterial species within one flask), were sampled for standard RNA extraction at different time points, and levels of specific gene expression were tracked and quantified by using real-time quantitative PCR, using the SYBR green detection (Ginzinger 2002; Livak & Schmittgen 2001). Two genes from the two model bacterial genomes were chosen, and used to create the gene specific primer design (using PRIMER BLAST), these were; PhzA, RhdA, Betalact and DMSP. Gene expression in solo and co-cultures were compared using qPCR at intervals, with the solo cultures acting as controls.

Conway et al found that P.a, when co-cultured with R.d, had a much lower gene expression of both RdhA and PhzH when compared to the solo, control culture of P.a. However, when the gene expression of R.d co-cultured with P.a was measured a different pattern emerged.  Both Betalact and DMSP were lower than solo culture levels during the initial stages but after 30 minutes rose by a factor of 2 and then levelled off. After 2 hours both Betalact and DMSP decreased below solo culture levels. These results show that gene expression of certain target genes could be reproducibly induced, or affected, by systematic co-culturing in multistrain growth conditions.

Although not measured directly, Conway et al went on to suggest that quorum sensing (QS) may have played roles in the co-culture gene expression in this study. QS is the regulation of gene expression in response to changes in cell-population density, QS bacteria produce and release chemical signal molecules, called autoinducers, which increase in concentration with cell density (Miller & Bassler 2001). The detection of a minimal threshold stimulatory concentration of the autoinducer leads to an alteration in gene expression. It is possible that after the initial mixing of P.a and R.d, any autoinducers released by either species was diluted by at least a half and may have fallen below the minimum stimulatory concentration. On the other hand, it may be possible that interactions in mixed cultures involve the degredation, or modification, of autoinducers produced by other members of the community. Though they do state there are other other possible explanations for the patterns seen in these results, but it is clear that these patterns resulted from the mixed species co-culturing.

I originally came about this study during my project research on interactions between bacterial communities. I thought it was particularly interesting as it combined bacterial interactions with gene expression, instead of just stating whether one bacterium inhibited the other.

Here is a link to the study if anyone is interested; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3330761/

Allen, H.K. et al., 2010. Call of the wild: antibiotic resistance genes in natural environments. Nature reviews. Microbiology, 8(4), pp.251–9. Available at: http://www.ncbi.nlm.nih.gov/pubmed/20190823 [Accessed March 3, 2012].

Ginzinger, D.G., 2002. Gene quantification using real-time quantitative PCR: an emerging technology hits the mainstream. Experimental hematology, 30(6), pp.503–12. Available at: http://www.ncbi.nlm.nih.gov/pubmed/12063017.

Jaiswal, P., Singh, P.K. & Prasanna, R., 2008. ` SE REVIEW / SYNTHE Cyanobacterial bioactive molecules — an overview of their toxic properties. , 717, pp.701–717.

Livak, K.J. & Schmittgen, T.D., 2001. Analysis of relative gene expression data using real-time quantitative PCR and the 2(-Delta Delta C(T)) Method. Methods (San Diego, Calif.), 25(4), pp.402–8. Available at: http://www.ncbi.nlm.nih.gov/pubmed/11846609 [Accessed October 3, 2012].

Long, R. & Azam, F., 2001. Antagonistic interactions among marine pelagic bacteria. Applied and Environmental Microbiology, 67(11), pp.4975–4983. Available at: http://aem.asm.org/content/67/11/4975.short [Accessed October 19, 2012].

Miller, M. & Bassler, B., 2001. Quorum sensing in bacteria. Annual Reviews in Microbiology. Available at: http://www.annualreviews.org/doi/pdf/10.1146/annurev.micro.55.1.165 [Accessed October 23, 2012].

Nunnery, J.K., Mevers, E. & Gerwick, W.H., 2010. Biologically active secondary metabolites from marine cyanobacteria. Current opinion in biotechnology, 21(6), pp.787–93. Available at: http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=3034308&tool=pmcentrez&rendertype=abstract [Accessed March 13, 2012].

Rosenberg, E. & Zilber-Rosenberg, I., 2011. Symbiosis and development: the hologenome concept. Birth defects research. Part C, Embryo today : reviews, 93(1), pp.56–66. Available at: http://www.ncbi.nlm.nih.gov/pubmed/21425442 [Accessed October 18, 2012].

Rypien, K.L., Ward, J.R. & Azam, F., 2010. Antagonistic interactions among coral-associated bacteria. Environmental microbiology, 12(1), pp.28–39. Available at: http://www.ncbi.nlm.nih.gov/pubmed/19691500 [Accessed July 17, 2012].

Chemotactic response to extracellular products of Cyanobacteria

Chemotactic response to extracellular products of Cyanobacteria

Dissolved organic carbon (DOC) is released into the water column through extracellular exudation and cell lysis by cyanobacteria, such as the two important marine groups Synechocococcus and Prochlorococcus, which often constitute the bulk of photosynthetic biomass and are therefore responsible a significant proportion of primary production in oligotrophic waters. DOC is essential to the growth of heterotrophic bacteria and while they are expected to cluster around living phytoplankton cells, using chemotaxis to take advantage of the exuded DOC, yet they had not previously been examined quantitatively in association with prokaryotic phytoplankton. Seymour et al. (2010) tested whether the exuded chemical products of Synechococcus and Prochlorococcus are chemoattractants for three heterotrophic marine bacterium strains; Pseudoalteromonas haloplanktis, Silicibacter TM1040 and Vibrio alginolyticus. Global Ocean Sampling data from 15 open ocean sites showed that they occurred in 100%, 100% and 93% of samples, respectively, and are therefore likely to co-occur with Synechococcus and Prochlorococcus.

Synechococcus exudants were found to induce a strong chemotactic response from all three strains, with P. haloplanktis accumulating in concentrations up to 9-fold higher than the background levels. While all three strain exhibited a marked response, this was far stronger in P. haloplanktis than either of the other two strains, but was significantly different from only V. alginolyticus. Response time is driven by differences in the chemotactic velocity; typically a fraction of the maximum swimming speed but it also depends on the sensitivity of their chemoreceptor’s. Similar results were found for Prochlorococcus, with P. haloplanktis exhibiting the strongest response, but again this was not significantly different from Silicibacter TM1040. Although a slight chemotactic response was observed in V. alginolyticus, it was not significantly different from the control. Interestingly, V. alginolyticus was found to have a higher mean swimming speed than Silicibacter TM1040 (54 and 52µm s^-1 respectively), so perhaps it may much less sensitive chemoreceptor’s than Silicibacter TM1040. P. haloplanktis was found to have a much higher mean swimming speed than either of the other strains; 85µm s^-1.

The DOC values used here were 3-5 fold higher than commonly found in the oligotrophic ocean, however they were still within an environmentally relevant range as they are within concentrations that would be observed during a bloom event or large, localised aggregation of cells. Rapid chemotactic responses to the DOC released by these cyanobacteria are likely to provide a strong competitive advantage for heterotrophic bacteria, which will gain exposure to the nutrients before competitors. The rapid response of P. haloplanktis is likely to give it a strong competitive advantage in comparison to Silicibacter TM1040 and V. alginolyticus.

The chemotactic behaviour exhibited here provides a potential mechanism for the development of associations, recently found to commonly occur in the open ocean (in a separate study), between Synechococcus and heterotrophic bacteria. These associations may have major implications for nutrient cycling rates and microbial competition.

Seymour, J., Ahmed, T., Durham, W. & Stocker, R. (2010) Chemotactic response of marine bacteria to the extracellular products of Synechococcus and Prochlorococcus. Aquatic Microbial Ecology. 59, 161-168
http://web.mit.edu/romanstocker/publications/SeymourEtAl_AME_2010.pdf

Separate study mentioned is Malfatti, F. & Azam, F. (2009) Atomic force microscopy reveals Microscale networks and possible symbioses among pelagic marine bacteria. Aquatic Microbial Ecology. 58, 1-14http://www.int-res.com/articles/feature/a058p001.pdf

Marine Snow: Whole versus Fragmented

Marine Snow can be described as sinking particles of organic matter or macro-aggregates, which are aggregates greater than 5mm in size. The marine snow collects at the surface water, where there is an abundance of bacteria and organic matter creating aggregates. The aggregates increase in density and eventually sink to the sea bed where due to the lack of light in the deep sea the aggregates are the primary source of carbon as many organisms are unable to photosynthesise. As the particles sink to the seafloor it is exposed to the currents of the ocean, the feeding and swimming of passing organisms; they are preyed upon by phytoplankton, zooplankton, bacteria, protists. These processes produce not only a plume but smaller particles (daughter particles) which still have the same composition, the division or fragmentation of the particles creates a greater surface area for bacteria to colonise. Daughter particles have a reduced density, so they sink at a slower rate. This means that smaller particles are more likely to remain in the surface area and according to Goldthwait et al (2005) remineralization is likely to occur.

Goldthwait et al (2005) tested two consequences of first time fragmentation on macroaggregates. 

•  The immediate release of dissolved organic carbon (DOC) and interstitial nutrients into surrounding seawater,

• The elevated solubilisation and remineralization of daughter-particle carbon due to the increased available surface area for bacterial colonization.

Approximately 200-300 aggregates of marine snow were collected by scuba divers at surface waters of Santa Barbara Channel in California during the summer months of 2002 -2003. Two types of experiments were carried out; 1 fragmentation experiment and 2 aggregate remineralization experiments. Each experiment was tested on the two treatments of aggregates (whole and fragmented) as well as a seawater control.

The seawater control was used to simulate the 10 aggregates which were added to 11 glass stopper BOD bottles filled with 300ml of unfiltered seawater. 4 replicates held the whole aggregates, 4 replicates held the fragmented and 3 held the control.

The fragmentation experiment analysed filtered particles for POC (particulate organic carbon) and PON (particulate organic nitrogen). Then Nutrient samples were collected from the filtrate, and analysed for their concentration of phosphate, nitrate/nitrite and ammonia.

3 replicates of 40ml DOC samples from the filtrate were stored at -20 then analysed using high temperature combustion, to determine the amount of the carbon dioxide present.

The Aggregate remineralization experiments consisted of 2 experiments, which were performed on 11 bottles kept at surface temperature and rotated end to end to keep aggregates in suspension

        Experiment 1: the bottles were re-suspended in unfiltered seawater, incubated for 3 days

        Experiment 2: the bottles were re-suspended in 0.2 µm filtered seawater and incubated for 5 days.

The amounts of solubilisation and remineralisation in whole and fragmented aggregates were compared, by assessing the pools of carbon from each. POC and DOC were measured and as a result TOC (total organic carbon) was determined (POC+DOC=TOC).

The authors concluded that fragmentation results in the immediate release of interstitial DOC and macronutrients to surrounding seawater. Solubilision rates were the same for whole and fragmented aggregates suggesting the surface of aggregates do not regulate bacterial colonization. Fragmentation causes a decrease in the changes of aggregate-associated carbon that come from the DOC release and slower sinking rate of daughter particles. Fragmentation also does not seem to accelerate POC degradation.

This paper originally interested me because it explained more about the presence and importance of marine snow in deep oceans. The fact that an aggregate of marine snow can divide into two smaller daughter particles and keep the same composition is incredible. Aggregate fragmentation is one of the primary removal mechanisms for sinking particles (Goldthwait et al 2005), this paper looks at its biochemical outcome and impact on the carbon cycle and this information and results could be useful in situations of large scale pollution and attempts at removing pollution.

Goldthwait S. A, Carlson C. A, Henderson G. K, Alldredge A. L,(2005) Effects of physical fragmentation on remineralization of marine snow, Marine Ecology Progress Series, Vol. 305: 59–65, Published December 23

http://www.int-res.com/articles/meps2005/305/m305p059.pdf