Daphnia meal is yummy

We have already discussed extensively the problem of diseases and antibiotics in fish farming and aquaculture. In the last seminar we talked about an alternative composition of practical diet for Asian seabass (Lates calcarifer). In the study by Debasis et al. (2015) certain gut bacteria were added to the diet to enhance primarily growth performance and digestibility. Following this I would like to present one more paper investigating the benefits of alternative feed compositions for Asian seabass, though regarding disease resistance.  

Shieh-Tsung et al. (2015) investigated the benefits of diets containing Daphnia similis meal. Daphnia is considered to be a source of nutrient-rich food with high protein content. Further, the carapace and the resting eggs of Daphnia contain a high amount of chitin and chitosan. Previous studies have shown that chitin and chitosan are both effective in immune stimulation and disease resistance for aquaculture organisms.

Asian seabass larvae were fed with three different experimental diets. Diets contained 0% (control), 5% (D5) and 10% (D10) Daphnia meal. Fish meal and fish oil decreased in the diets D5 and D10 as the amount of Daphnia meal increased.

All three experimental treatments were challenged with the pathogenic bacteria Aeromonas hydrophila to examine the disease resistance. Further, non-specific immune parameter were measured. Mx gene expression in the spleen and head kidney were analysed by using real-time PCR with SYBR green to examine target mRNA.

Daphnia meal had no negative effects on the general survival rate of the larvae. All three treatments showed no significant difference in survival after being fed for 42 days. Final weight, feed efficiency and percentage weight gain showed no significant difference between the control and D5 treatments as well. Only D10 treatments had a significant lower final weight, percentage of weight gain and feed efficiency. Apparently the lack of nutrients in D10 due to less fish meal caused a worse growth performance.

Daphnia meal showed a positive effect on resistance to A. hydrophila. The mortality rate of fish larvae was highest in the control treatment and decreased with higher Daphnia meal addition. Further, respiratory burst activity increased in D5 and D10 treatments with D10 having the highest rate. Phagocytic activity increased as well in Daphnia meal treatments, but this time with D5 treatments having the highest rate. Mx proteins which are expressed in response to viral infection also increased in Daphnia meal treatments. All in all the author suggest that a diet containing 5% Daphnia meal is suitable for further use since it has no negative impact on growth performance, though showed significant improvement in non-specific immunity and resistance.

I think the findings are promising with a potential for further use. It would be interesting to combine the gut bacteria of Debasis et al. (2015) and the Daphnia meal to see if that would have an even bigger positive effect. Although, one problem I could think of is the high amount of chitin, since some Vibrio species have a preference for organisms with chitin.

Chiu, Shieh-Tsung; Shiu, Ya-Li; Wu, Tsung-Meng u. a. (2015): „Improvement in non-specific immunity and disease resistance of barramundi, Lates calcarifer (Bloch), by diets containing Daphnia similis meal“. In: Fish & Shellfish Immunology. 44 (1), S. 172-179, DOI: 10.1016/j.fsi.2015.02.002.

Viral Heroes

When people mention viruses, the first thing that springs to mind is often disease and mortality, however relatively little is known about viruses existing within the marine environment or their effects on, and interactions with other marine organisms. The majority of the work is currently based on commercially important species which have suffered mass mortalities at the hands of viruses which we don’t yet seem to fully understand, for example White Spot Syndrome Virus of Penaeid shrimp. However, with recent technological advances in sampling techniques and identification methods, more studies on viruses seem to be possible.

There are thousands of species, and even whole phyla which do not yet have any related viruses described for them, one of these phyla being Echinodermata. This study by Gudenkauf et al. (2014) works to find and identify and viruses associated with 3 different species of sea urchin off the coast of Hawaii. The team used a viral metagenomics approach to sample viruses in the urchin tissues, nearby sediments and the surrounding seawater. Over 80% of the viral DNA was attributed to phages, while the remaining DNA came from megaviruses, phycodnaviruses and densoviruses, with the densoviruses receiving the most attention. Of the relatively little work done on viruses in the marine environment, densoviruses have received notable attention, having mixed effects on various species of shrimp. In some species, the presence of densoviruses has been associated with delaying the mortality of its host when infected with WSSV, alternatively, in a different species of the same genus, densoviruses are known to cause stunted growth and lesions. In the case of the 3 echinoderms studied, all seemed very healthy and none were displaying any symptoms of diseases, therefore until further work is carried out in this field, I think it could be assumed that the viruses present here are of no threat to their hosts (though it must be said the sample size was very small).

I believe this work highlights the importance of further work on viral associations with marine organisms. I am sure that most, if not all marine organisms will have some sort of interaction with viruses throughout their lifetime. I think it would be interesting to consider the possibility that many organisms may play host to resident viral communities, as they do bacterial communities, which may also aid in a small part of their normal functioning. There are few previous studies discussing the effects of densoviruses on various Penaeid shrimp, but from what I have read, at least in these few cases, viral activity/behavior seems to have many different effects and can be influenced by the presence of other viruses. So while viruses tend to be overlooked if they are not causing problems or being actively searched for, I think they could actually be more significant than previously thought.

B. Gudenkauf, J. E. (2014). Discovery of urchin-associated densoviruses (family Parvoviridae) in coastal waters of the Big Island, Hawaii. Journal of General Virology, 652-658.

Available at: http://jgv.microbiologyresearch.org/content/journal/jgv/10.1099/vir.0.0607800#tab2 

(Marine) Snow is falling.

Marine snow is a phenomenon where particulate organic matter (POM) sinks through the water column. It plays important roles in nutrient and carbon cycling to the deep sea. Motile heterotrophic micro-organisms may detect these nutrient rich sediments and colonise them to acquire nutrients. Despite knowing that bacteria move towards and "feed" from them, relatively little has been done to investigate microbial populations associated with these particles. Fontanez et al. carried out a study to do just that.

The study was performed by deploying free drifting sediment traps at 150, 200m, 300m and 500m in the North Pacific subtropical gyre. One set of these cylinders contained only sterile seawater, deemed "live", and the other was "poisoned" with a solution that inhibited further growth and preserved the DNA of micro-organisms that fell into it. Once collected, the sediments were analysed with epifluorescence and confocal microscopy. DNA from the sediments was also collected and analysed.

The most abundant bacterial genera differed between "live" and "poisoned" traps: Alteromonadales (e.g. Alteromonas and Pseudoalteromonas) in the live traps and Vibrios in the poisoned traps. The paper does not discuss this difference but considering the poisoned traps inhibited any further growth, it is possible the particles trapped were preserved at an earlier stage of succession whereas "live" populations were able to grow and become more complex. This is reinforced by the data that showed poison traps populations collected at 500m were more balanced and not so dominated by Vibrios; it is possible that Vibrios are part of the colonising population on marine snow and as it sinks, the population becomes more diverse.

The study showed that live trap assemblages had a high bacterial genetic diversity such as genes for motility, vitamin synthesis and metabolism of carbohydrates and amino acids such as those from algae and diatoms. The bacterial genes from the poison traps coded largely for siderophores and iron acquisition, which according to the paper are linked with lifestyles associated to eukaryotes. Virulence factors and quorum sensing genes were also enriched in the “poison” traps; all of which were associated with Vibrios. “Poison” trap bacteria were also associated with eukaryotic surfaces and anaerobic intestinal tracts as pathogens, saprophytes or symbionts.

Genes for chitin attachment and degradation were expressed in both traps, but of different bacterial origin.

I think this is a nice study, marine snow is a major import of nutrients into the ocean depths and carbon cycling. This study- understandably- only sampled relatively shallow particles; I would be interested in see how populations and metabolisms further progress at even greater depths along with the degradation state of the POM itself so we get a better understanding of the nutrients that sink into the deep sea. 

Fontanez, K.M. Eppley, J.M. Samo, T.J. Karl, D.M. and DeLong, E.F. (2015) Microbial community structure and function on sinking particles in the North Pacific Subtropical Gyre.  Frontiers in Microbiology, 6, article 469

So, should we be worried about Shewanella?

The marine environment is largely considered an uncommon reservoir for the acquisition of human-related pathogens. In fact, only a small number of bacterial species are usually associated with the infection of humans, the most notable obviously being in the Vibrio genus. However, recently, strains of the genus Shewanella are increasingly being implicated as a growing cause of maritime-associated disease. A study by J. M. Janda therefore attempts to corroborate information on the genus, in order to examine both its pathogenicity, and its mechanisms for infection.

The Shewanella genus itself has grown in size from a single type species in 1980 to more than 60 accepted or proposed species.  And, with this increased understanding of the genus’ taxonomy, has come the realisation that there is a great deal of diversity within the Shewanella. For instance, some species have been implicated as sources of bioremediation while others have been posited as future sources of microbial fuel cells. While this indicates a potential benefit of Shewanella, there are, on the other hand, some major pathogenic species in the genus that have the capability to cause human disease.

The understanding of Shewanella species as disease-causing agents has been revolutionised by the use of 16S sRNA sequencing. In fact, until the early 1990s it was thought that only one species of the genus, S. putrefaciens, was causing disease in humans. However, molecular techniques have now revealed that a large proportion of the strains causing human infection actually belong to S. algae. In addition to this, there have been reports of S.baliotis causing necrotisingfasciitis similar to cases of V. vulnificus. There have also been incidences where S. xiamenensis has been implicated in health care-associated peripancreatic infection. More recently, Shewanella has even been implicated as having a potential role in gastrointenstinal disease.

Human infection of Shewanella is directly, or indirectly, associated with contact with the marine ecosystem and its inhabitants due to the ubiquitous nature of the genus in the marine environment. In fact, the most common cause of infection is bacterial introduction through abraded urfaces. This can include a number of things such as penetration by a sea urchin spine, swimming in the ocean or occupational exposure from fishing and crabbing, among others.

Of course, the real question is; should this be a cause for concern? And the answer is, no, not for now. However, it is important to use modern molecular techniques to further the research the genus. Techniques like 16s rRNA sequencing may be key in monitoring the pathogenicity of the genus as well as aiding its identification as a causative agent of human disease. By expanding on current knowledge of the genus it should be possible to not only minimise its threat as a disease factor, but also maximise its potential as a human rescource.

Janda, J. M. (2014). Shewanella: a Marine Pathogen as an Emerging Cause of Human Disease. Clinical Microbiology. 36 (4), 25-29.

The Ascidian Army

Ascidians, sessile filter-feeding chordates, are found globally and are used in bio-applications such as drug discovery due to their production of secondary metabolites. Described metabolites range from defensive compounds, e.g. polyketides, to protective compounds, e.g. mycosprine amino acids which protect against UV irradiation. Bacteria are the only known sources of ascidian secondary metabolites; cases have shown bacteria to produce defensive compounds where host animals do not. Metabolites appear to increase survival and be species and location specific, suggesting chemical and environmental drivers are involved in assemblage selection. Both horizontal and vertical transmission of symbionts takes place. In tropical species, Prochloron producers of cynobactins (toxic cyclic peptides) and mycosprine (amino acids which act as a UV radiation protectant) seem to be obligate for host animals despite dominance of horizontal transfer.  Contrastingly, the polyketide producing relationship between ascidians and alpha-proteobacteria Endolissoclinum fulkneri has led to genome reduction, vertical transmission and co-speciation.

This study aimed to identify factors underlying production of secondary metabolites in ascidians globally by investigating the microbiomes and metabolomes of tropical, sub-tropical and temperate ascidians. Specifically, the study focussed on the link between secondary metabolites and symbiotic bacteria with the hope of discovering and understanding bio-active natural products and understanding symbiotic interactions with ascidians to reveal the true producer of ascidian associated secondary metabolites.

Ascidian samples from Papua New Guina, Fiji, Vanatu, Catalaina Island, California and Florida Keys were collected during 2006-2011. 18s and 16s RNA were used for phylogenetic analysis of ascidian and bacterial species respectively. Compounds were identified using ultra-high pressure liquid chromatography and mass spectrometry. Ascidians were found to be predominantly didemnid ascidians from the order Aplousobranchia. Similarly to sponges, high microbiome diversity was seen across all sample types and habitats.

Microbiome and chemical diversity were not indicative of each other; instead, chemical diversity seemed to be down to species specific bacterial assemblage and ascidian interactions. Ascidian samples were stable over time and space; most samples had approximately 10 species, described as the ‘Top-10’, of more abundant bacteria which tended to be more novel with less abundant microbiome bacteria tending to be more widespread and correlated geographically to samples. Microbiome and secondary metabolites were strongly species specific. In addition, tropical ascidians were highly bioactive and toxic, subtropical less so, and temperate almost not non-toxic due to the production of patellazoles and pyridoacridines.

This study is the first to compare ascidian metabolmes and microbiomes across species and location making it valuable to fields involving ascidian and bacterial interactions and bio-applications. However, it does not detail any specific relationships. Where compound producers were known, they belonged to the most abundant species of bacteria in that species specific assemblage. The authors suggested the most abundant species of bacteria are the priority for the production of natural products and that there is a strong selection for defensive microbes. This is a useful starting point however less abundant species and other microbes must not be overlooked; they may produce or contribute to production of compounds. Further work must be done to understand the specific interactions between host and microbes.

Tianero, M. D. B., Kwan, J. C., Wyche, T. P., Presson, A. P., Koch. M., Barrows, L. R., Bugni, T. S., Schmidt, E. W. (2015) Species specificity of symbiosis and secondary metabolism in ascidians. ISME. 9:165-628.

Combating diseases in fatty fish with probiotics

Studies have shown that using probiotics to alter gut microbes in order to control diseases in fish have provided positive outcomes. Using probiotics such as Lactobacillus and Bifidobacterium to control diseases that disturb lipid metabolism has become a recent research area, as the gut microbiota is tightly linked with influencing the host metabolism, energy and lipid metabolism, fat distribution, insulin sensitivity and growth performance. The addition of probiotics could therefore prevent such diseases linked with lipid metabolism by decreasing serum lipid content and improving other health factors of fish. Fish use triglycerides (TAG) and cholesterol from their diet as a main source of energy, however in order to be absorbed, they must be resynthesized via two main processes – the glycerol phosphate pathway (which occurs mainly in the liver and adipose tissues) and the monoacylglycerol pathway (which takes place in the intestine).

This study by Falcinelli et al. looks at the effects of supplementation of the probiotic Lactobacillus rhamnosus on the gut microbial community and lipid metabolism of Zebrafish (Danio rerio). By coupling high-throughput sequencing with biochemical, molecular and morphological analysis, the effect of L. rhamnosus on intestinal epithelial structures, total body cholesterol, TAG and non-polar fatty acids and zebrafish larval growth was investigated. Genes involved in lipid metabolism such as those that regulated lipid synthesis, traffic storage, and homeostasis, were used.  

Results showed that there was a significant change in the microbiota community in the Zebrafish digestive tract which was due to the probiotic supplementation. These changes varied the expression of a network of genes involves in the physiological control of lipogenesis, lowered total body cholesterol and triglycerides, increased non-polar fatty acids, improved intestinal epithelium structures (i.e. microvilli and enterocytes) and reduced lipid droplets as well as increasing growth of probiotic treated larvae. The addition of L. rhamnosus reduced the presence of bacteria that contain potential pathogens, along with increasing other lactic acid bacteria. Increased microvilli and enterocyte height indicate the addition of L. rhamnosus expanded the intestinal structures, allowing better gut function and overall health. The decrease in genes agpat4 and dgat2 (both involved in the synthesis pathways of TAG) gave evidence of reduced TAG levels in larvae. TAG together with sterol esters form lipid droplets, which the gene fit2 has a correspondence with. The knock-out of this gene led to a decrease of lipid droplets in the liver and intestine. There was also an up-regulation in the cck gene expression (coding for gallbladder contraction and pancreatic enzyme secretion) with supplementation which led to increased bile production and therefore better breakdowns of lipids.

Overall this paper provides in-depth results of how the addition of a particular probiotic bacteria can cause many positive changes to the lipid metabolism of fish through gut microbiota community alterations, as well as enhancing growth and morphological features. The method involves using state of the art technology such as high-performance liquid chromatography for biochemical analysis and real-time PCR for genetic analysis to provide results only specific to the aim. These methods could be used not only on model species for biomedical research, but also commercially important marine species which require knowledge on preventing the spread of diseases in aquaculture systems.

Reference:

Falcinelli, S. et al., 2015. Lactobacillus rhamnosus lowers zebrafish lipid content by changing gut microbiota and host transcription of genes involved in lipid metabolism. Scientific Reports, 5(9336), pp. 1-11.

http://www.nature.com/articles/srep09336

Fungal-like Pathogens of fish: what lies beneath?

Diseases in fish can be cause by a wide array of infectious agents, however, currently emerging are increasing issues with diseases cause by true fungal and fungal-like pathogens. Disease caused by such pathogens is increasing in incidence, geographic range and virulence in recent years and so poses a global biodiversity threat. At this time though the underlying causes of this observed increase are not known and so it is vital to further understanding of the ecological factors driving such change.

A review paper by Gozlan et al therefore attempts to collate information on the current understanding of such fungal pathogens, in order to highlight not only what may be causing infections, but also what direction research should take in order to minimise the ecosystem impacts. This is with the hopes that it may aid in the control of the increasingly infectious outbreaks of disease that have, in places, caused local extinctions.

The review states that the majority of infections in fish caused by fungi are caused by species in the phylum Ascmycota and that, most importantly, the majority of these fungi are opportunistic pathogens. In addition to this, within a clade of fungal-like organisms, the Mesomycetozoea, there are some notable pathogens within the clade Dermocystida, such as Sphaerothecum destruens, which can infect a wide range of hosts. Finally there are a number of oomycete parasites of fish which morphologically resemble fungi but are taxonomically distinct.

One of the most important things to take from this study is the generalist nature of such pathogens. It states that all three grouping of  fungi and fungal-like organisms contain pathogens that are true generalist. This ability to infect and cause disease in fishes across often drives high virulence in vulnerable hosts, and is what makes these diseases such a threat to global fish populations. The study of these diseases therefore becomes imperative, especially given that the current method for getting rid of the diseases in aquaculture is through the use of culling. As such, the review proposes the use of up to date molecular methods like Fluorescence in situ hybridisation (FISH) and PCR to examine the microbial species causing disease.

The power of this study lies in its collaborative nature. By bringing together a wide range of current knowledge on the subject of fungal and fungal-like pathogens it is able to highlight just how little epidemiological data there is for fish pathogens. And, since the fisheries sector employs 33.1 million people, these diseases can be considered a significant threat to a vital ecosystem service. When coupling this with their evidently generalist nature, it is not unfair to suggest that such diseases are emerging as a major concern for global fish populations, thus meaning their study is of great importance.

Gozlan, R. E, Marshall, W. L, Lilje, O, Jessop, C. N, Gleason, F. H. and Andreou, D. (2014). Current ecological understanding of fungal-like pathogens of fish: what lies beneath?. Frontiers in Microbiology. 5, Article 62.

Probiotics not as 'Pro' as we think

A problem which every farmer faces is the onset of disease. Medication such as antibiotics can be used to fight infection, particularly in young stock, however this can help further drive antibiotic resistance. Probiotics are a means to help promote healthy growth in the selected organism, and so may also help combat disease.

A recent paper by Skjermo et al. (2015) has looked into the previously understudied colonisation of probiotic bacteria in the microbiota of cod larvae (Gadus morhua L), in order to determine the most effective time to introduce the strains, and how they grow over time. Four strains were cultivated over a period of weeks and were used to inoculate cod larvae: Microbacterium (ID3-10), Ruegeria (RA4-1), Pseudoalteromonas (RA7-14) and Vibrio (RD5-30). These strains were previously identified in another paper from cod intestines, and were introduced to the live feed in the water over a period of 24 hours for each treatment time: 0, 2, 4, 8, 16, 30, 45 days post-hatching. No repeat doses were given.

The strains were measured using 16S rRNA sequencing and DNA extraction of randomly selected fish, as well as real-time PCR, so that they could be quantified and identified. The results were interesting, as only ID3-10 was still present in significantly high amounts after 45 days, while the other three strains were significantly less after 11, and had faded to background levels. It was also interesting to see greater concentrations of ID3-10 after only one day post-hatching in the larvae than in the surrounding water or feed. This had suggested that the strain had taken hold in the gut; however the levels soon dropped after around four days.

None of the strains could establish themselves in the microbiota during any stage, as the levels decreased steadily. This may have been due to the selective pressures the larvae face whilst developing, as well as competition within the microbial community itself. These results have shed light on how difficult it is to introduce new strains of probiotics to a system, especially without continuous doses. Unless repeated inoculation occurs, it is hard to create new strains within the cod larvae. It also appeared that introducing the strains later on in larvae development yielded the best results, however it would require intensive and continuous administration of the probiotics, which would be labour intensive and potentially costly.

While it seems that using probiotics could be the solution to antibiotic overuse, the problems with introducing an effective strain to a species are yet to be solved. A few studies have provided promising results, however this paper highlights the need for further optimisation of these methods in order to increase the use of microbes in aquaculture. I believe that this study has pointed out the difficulties in a way that is beneficial and so others can build upon this research to forward this growing industry.

Skjermo, J., Bakke, I., Dahle, S.W. and Vadstein, O., 2015. Probiotic strains introduced through live feed and rearing water have low colonizing success in developing Atlantic cod larvae. Aquaculture, 438, pp.17-23.

http://www.sciencedirect.com/science/article/pii/S0044848614006504

"Jamming the radar"

Over the last two decades aquaculture has become the fastest growth sector of agribusiness worldwide. However, aquafarming is facing one fundamental problem: Diseases. The aquaculture industry loses billions each year because of diseases caused mainly by microbial infection. In order to counter this dilemma the use of antibiotics has been widespread. Their excessive use has generated unwanted side effects. Therefore, an alternative solution is necessary. One potential solution could be quorum quenching (QQ). Many species of bacteria use quorum sensing (QS) to regulate their gene expression in response to cell-population density. Behaviors such as virulence controlled by QS only occur at high bacterial cell densities. Bacteria use autoinducers like N-acyl-homoserine lactone (AHL) for intraspecific communication. QQ enzymes which interfere with the QS system by degrading signal molecules can shut down the expression of pathogenic genes.

Recent studies have shown that there are quite a few marine bacteria with the enzymatic potential of QQ. Zheng et al. (2016) conducted a study with focus on the Pacific white shrimp (Litopenaeus vannamei). The main focus of this study was to compare the bacterial composition of different health statuses. Further, they looked at the bacterial composition of distinct larval stages. 240 bacterial strains where identified in total with a significant shift between samples from healthy and diseased populations and also within different growth stages.

What I found most interesting was that they screened the 143 strains which they isolated from healthy shrimp and water samples for their AHL degrading activity. 18 out of 143 strains showed reduction of the normalized β-galactosidase activity. The AHL level is proportional to β-galactosidase activity. Ten out of 18 strains showed strong AHL degrading potential. The dominant species with strong degrading potential were Tenacibaculum mesophilum (nine strains) and Microbacterium aquimaris (two strains). They suggest that the identified bacteria with strong degrading potential could potentially be test to control diseases in aquaculture.

In my opinion these findings are very promising. Previous studies have revealed that the use of organisms with the potential of QQ can be an effective way to block QS systems of bacteria. E.g. QQ can be used to control biofilm formation and therefore prevent biofouling (Joint et al. 2007). Additionally, the author mentioned recent studies where QQ was successfully applied to marine organisms such as the giant freshwater prawn. Anyway, further research is necessary. More bacteria with QQ potential for other farmed species have to be identified. Further, the efficiency in commercial aquaculture has yet to be proven.

If you are interested in that topic: I found two useful reviews about QQ enzymes and QQ marine agents:

Tang et al. (2014), Quorum Quenching Agents: Resources for Antivirulence Therapy, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4071575/

Chen et al. (2013), Quorum Quenching Enzymes and Their Application in Degrading Signal Molecules to Block Quorum Sensing-Dependent Infection, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3794736/

And here’s the reference for this article:

Zheng Y., Yu M., Liu Y., Su Y., Xu T., Yu M., Zhang X. (2016) Comparison of cultivable bacterial communities associated with Pacific white shrimp (Litopenaeus vannamei) larvae at different health statuses and growth stages, Aquaculture,doi:10.1016/j.aquaculture.2015.09.020

http://www.sciencedirect.com/science/article/pii/S0044848615301794

Phytoplankton and neurotoxin. More complicated than it sounds.

Recent studies show that dinoflagellates have acquired a number of important things from cyanobacterial horizontal gene transfer, for example RuBisCo; a crucial component in CO₂ fixation, histone-like proteins and other plasmid-related genes. As certain dinoflagellates are also known to produce harmful compounds known as saxitoxins, Hackett et al. carried out genomic analysis of the dinoflagellate Alexandrium tamarense to look for saxitoxin genes (sxt), testing the hypothesis that they too had been passed on by horizontal gene transfer.

The researchers first grew A. tamarense in media and harvested RNA which was prepared and sequenced. The genetic material was then screened for a particular domain found in cyanobacterial stx genes.

The study showed that A. tamarense had 13 homologous genes for cyanobacterial saxitoxin production, however the genes were not structurally similar. In cyanobacteria, sxtA is composed of two domains fused together, in the dinoflagellate however, the two domains are separate proteins, suggesting the two are not related phylogenetically. Other proteins that showed similarities to cyanobacterial proteins were not calculated to have arisen by horizontal gene transfer to the dinoflagellate. 

These results tie in nicely with another article Laura blogged about recently. Her article said that the dinoflagellate Gymnodium catenatum did not produce saxitoxin without its bacterial community. G. catenatum is mentioned briefly in the discussion of this article as having sxt homologues. So how do we interpret these results? Without being too speculative, I think it may be possible that the dinoflagellate doesn't have all the proteins required to produce saxitoxin and therefore requires its bacterial community to create it instead. 

Another idea put forward in this paper is that the dinoflagellate enzymes have evolved independently of the cyanobacteria and are functionally different despite structural similarities, that is, the dinoflagellate sxts are involved in other processes separate from production of saxitoxin. A next step would be to test the functionality of the genes from these dinoflagellates to see if they do or don’t produce the toxins. 

Here’s a link to Laura’s blog, if you want to read it yourself:

http://2015-mbio322.blogspot.co.uk/2015/11/algae-and-bacteria-toxic-relationship.html

And here’s the reference for this article: 

Hackett, J.D. Wisecaver, J.H. Brosnahan, M.L. Kulis, D.M. Anderson, D.M. Bhattacharya, D. Plumley, F.G. and Erdner, D. (2013) Evolution of Saxitoxin Synthesis in Cyanobacteria and Dinoflagellates. Molecular Biology and Evolution 30(1) pp.70-78.

If you have any questions or comments, do write them below and I’ll respond as best I can.