Unusual is important: how rare Archaea may contribute more to communities than we think.

Recent innovations in molecular metagenomics have improved understanding of the composition and function of the microbial plankton. Archaea are an often overlooked part of the microplankton, and while we appreciate that they significantly contribute to biogeochemical cycles, little is known about their community structure and dynamics. Comprised of three main grroups: Thaumarchaeaota marine group (MGI), and two groups of Euryarchaeota, MGII and MGIII; each group possesses several distinct clusters, potentially indicating ecotypes or evolutionary lineages. 

Whole communities or metabolically active communities can be studied in environmental samples using 16S rDNA and 16s rRNA respectively. This differentiation is important as it gives an indication of growth: often revealing a correlation between abundance and activity. 

 “Rare” OTUs are defined by <1% total reads in the sample; frequently overlooked, the rare community may still have an important role. By utilising metagenomic techniques to characterise the total surface archaeal communities in the Mediterranean sea over 3.5 years, this study from Hugoni et al. (2013) aimed to describe the structure and infer the potential function of the rare archaeal community in different seasons. 

Surface seawater (≈3m) samples were collected every month; from these 16S rDNA/rRNA were co-extracted and rRNA was reverse-transcribed using random primers. Pyrosequencing was carried out on the rDNA and rRNA using universal archaeal primers. Archaeal sequences were used to construct phylogenetic trees. OTUs present were identified and their abundances calculated; they were considered rare if they comprised <1% of sequences and were present in only one sample, otherwise they were abundant. 

Analysis showed that major archaeal groups, MGI, MGIIA and B, were comprised of separate clusters with different levels of activity across the seasons. This could represent adaptation to a variety of niches; different metabolisms are hypothesised to correspond to different ecotypes, as shown in Bacteria (Campbell et al., 2011). Their abundance and activity may differ due to contrasting strategies and competition for resources. 

There were three main factions of OTUs detected in the rare archaeal community. The first OTU group represents the local seed bank of Archaea, which, when rare, had poorly correlated 16S rRNA and rDNA sequences, suggesting that they were inactive. When OTUs became seasonally more abundant, there was a much closer correlation, showing their increase in activity. The variation in activity could represent change rates of metabolic processes over time. 

The second group of OTUs were always rare and inactive. They had a low sequence similarity compared to the existing database of the area, and were considered aliens to the pelagic ecosystem studied. Perhaps they came from deep marine sediment, and other understudied systems, with episodic environmental disturbances initiating dispersal of this non-local seed bank. The different physico-chemical parameters of the surface waters may be a limiting factor of activity, but they may have potential to colonise should amenable conditions arise. 

The third OTU group consisted of Archaea that were always rare but constantly active; their phylogeny and seasonal dynamics resembled those of active and abundant groups. Their higher rate of activity indicates that they are not dormant; they are simply maintained at a low abundance, potentially due to inferior competitive strategy, or a life strategy of rarity to minimise predation. Alternatively, they could be more susceptible to viral attacks than the other groups of Archaea. 

This paper opened the door to discovery of sheer diversity in phylogeny, life histories and activities in the rare Archaeal biosphere. Their potential for impact is immense, and not yet fully understood; and may well be vital to the function of our oceans, especially with regards to biogeochemical cycling through the seasons. Furthermore, it emphasises once more that the use of modern molecular genomics techniques is intrinsic to advancing our understanding of marine microbial community structure and function. Perhaps these powerful techniques should now be used to delve into the more neglected players in marine microbial communities: inherent parts of the puzzle may still remain undiscovered.

Definitions:

Metagemonics - The study of all of the environmental genetic material present, using massively paralleled genetic sequencing.

Active community - The section of a (microbial) community that is contributing to the environmental RNA pool, and therefore is actively transcribing its DNA, producing proteins for growth and metabolism. 

Total community - The total DNA in the environment, some of which may not be actively transcribed and used for growth and metabolism. Used to show which organisms are present, but not necessarily active (ie no differentiation for dormant cells).

OTU - Operational taxanomic unit: a group of closely related organisms, defined as having ≥97% sequence similarity, usually of 16S rRNA sequences. 

Pyrosequencing - A method of massively paralleled sequencing, which uses light released to signify the order of the sequence. 

Seed bank - A dormant community of organisms that can potentially be resurrected under certain environmental conditions.

Reviewed Paper:

Hugoni, M., Taib, N., Debroas, D., Domaizon, I., Dufournel, I. J., Bronner, G., Salter, G., Mary, I., Galand, P. E. (2013). Structure of the rare archaeal biosphere and seasonal dynamics of active ecotypes in surface coastal waters. Proceedings of the National Academy of Sciences, p201216863

References

Campbell, BJ., Yu, L., Heidelberg, JF., Kirchman, D.L. (2011). Activity of abundant and rare bacteria in a coastal ocean. Proceedings of the National Academy of Sciences 108:12776–12781.

Happily never after: the relationship between the marine microalgae Emiliania huxleyi and the roseobacter Phaeobacter gallaeciensis

Misfortune tests the sincerity of friends – or so thought the Greek fabulist Aesop two millennia ago. The notion of this notorious human trait has grown immense popularity and has been adopted in numerous storylines. This time, however, we’ll take a slightly different approach to it.

The main protagonists in this narrative are the marine microalgae Emiliania huxleyi and the roseobacter Phaeobacter gallaeciensis. E.huxleyi is a ubiquitous coccolithophore, which plays an important role in global carbon cycling by removing CO₂ from the ocean and sequestering it as CaCO₃. It forms massive blooms during which its population expands in great densities over vast areas of the upper ocean (Holligan et al. 1993). The discovered correlation between the microalgal blooms and the roseobacter predominance has prompted Seyedsayamdost et al. (2011) to research deeper into their relationship.

In the first, mutualistic, phase of the interaction P.gallaeciensis synthesizes phenylacetic acid, an auxin which promotes algal growth. It also produces the antibiotic tropodithietic acid (TDA) and thiotropocin which protect the algal host from bacterial pathogens. E.huxleyi, on the other hand, provides the roseobacter with nutrients and an ideal surface for colonization. However, this happy relationship is not meant to last, and the honeymoon phase comes to an end as P.gallaeciensis transforms into a pathogen and kills E.huxleyi. P-coumaric acid (pCA), a lignin breakdown product generated by E.huxleyi, causes P.gallaeciensis to start producing toxins- algaecides, called roseobacticides. As pCA could be interpreted either as algal senescence or as increased algal population density, this rapid shift may occur to give the bacteria access to the food source provided by the aging algae cells, and to give it a chance to find a new healthy host. Seyedsayamdost et al. (2011) also suggests that the duplicitous lifestyle of P.gallaeciensis may be caused by the compliance of phenylalanine. Both tropone and phenylacetic acid are synthesized from phenylalanine, as is pCA, the induction signal and virtually all the components of cell wall lignin.

Beyersmann et al. (2017), on the other hand, considers TDA to be the key component to induce the switch from mutualism to pathogenesis, because of its dual function as an antibiotic and a quorum sensing (QS) mediator in the bacterium Phaeobacter inhibens. Quorum sensing is a type of cell-to-cell communication mediated by signaling molecules and used to coordinate gene expression according to the density of the local population (Marx 2014). The study proposes that the attachment of P.inhibens on its host is reduced by the activation of QS, after which biofilm-associated genes are down-regulated, which results in dispersion of the roseobacter. In a more recent approach to the topic Bramucci et al. (2018) discovered there is a dependence of the algaecidal activity of P.inhibens on the algal cell type of the host. They found that P.inhibens selectively kills two from the three examined host cell types. These cell types, however, differ from the ones targeted by algaecidal viruses. The study also claims that P.inhibens doesn’t use a roseobacticide-dependent mechanism (as described in Seyedsayamdost et al. (2011)) and must be therefore producing additional algaecidal compounds or virulence factors to kill E.huxleyi.

Now the pure moral of this story is that abandoning your friends, because they are old or sick, is indeed spineless. Yet scientifically speaking there are two key points in this review to take home: algae-bacteria interactions play an essential role in shaping species composition in pelagic environments and they are largely unexplored. As we can see bacteria have the ability to influence the microalgal cell-type composition by selective pathogenesis, thereby altering its populations and its bloom-bust lifestyle. These novel findings have a great potential in aiding us tackle microalgae-induced issues, such as harmful algal blooms (HABs). However, we still have a long way to go, until we can fully grasp the processes driven in and from bacteria in their interactions with algae.

Reviewed paper:

Seyedsayamdost, M. R., Case, R. J., Kolter, R., and Clardy, J. (2011). The Jekyll-and-Hyde chemistry of Phaeobacter gallaeciensis. Nat. Chem. 3, 331–335. doi: 10.1038/nchem.1002

References:

Beyersmann, P. G., Tomasch, J., Son, K., Stocker, R., Göker, M., Wagner-Döbler, I., et al. (2017). Dual function of tropodithietic acid as antibiotic and signaling molecule in global gene regulation of the probiotic bacterium Phaeobacter inhibens. Sci. Rep. 7, 1–9. doi: 10.1038/s41598-017-00784-7

Bramucci, A., Labeeuw, L., Orata, F. D., Ryan, E. M., Malmstom, R. R., Case, R. J. (2018). The bacterial symbiont Phaeobacter inhibens shapes the life history of its algal host Emiliania huxleyi. Front. Mar. Sci., 29 May 2018. doi: 10.3389/fmars.2018.00188

Holligan, P.M., Groom, S.B., Harbour, D.S., 1993. What controls the distribution of the coccolithophorid Emiliania huxleyi in the North Sea? Fish. Oceanogr. 2, 175-183.

Marx, V., 2014. Stop the microbial chatter. Nature 511, 493–497

Pond aquaculture: potential reservoirs for antibiotic resistance

Increasing attention has been given to antibiotic usage in aquaculture, with their addition to the environment being a potential source for the proliferation of antibiotic resistance genes (ARGs). These genes could then be spread to human pathogens via horizontal gene transfer. Antibiotic resistance is exacerbated by anthropogenic activity in fields such as medicine, animal farming, and waste treatment. Thus, adding to growing fears over how we may be able to treat future infectious diseases.

Although the aquaculture industry is widespread, a vast amount of aquaculture production is carried out in southeastern Asia, with China accounting for 71% of aquaculture production globally. Agriculture-aquaculture is common practice in China with terrestrial farmland being in close association with aquaculture ponds. Animal manure is directed into the pond system promoting phytoplankton proliferation to feed the fish stock. The manure often contains antimicrobial growth promoters which have not been fully digested by farmland animals. This leads to the spread of antibiotic resistance and supports antibiotic-resistant bacterial communities.

The study presented here aimed to investigate the concentrations of antibiotics, the abundance of ARGs and the bacterial communities within the water and sediments of aquaculture ponds in Guangdong, China. This is the first study to use culture-independent methods to address these aims.

Paired water and sediment samples were taken from four different ponds. Antibiotic (tetracyclines, sulphonamides and (fluoro)quinolones) concentrations were analysed by ultra-performance liquid chromatography electrospray tandem mass spectrometry. DNA was extracted from the water and sediment samples, DNA was amplified by PCR to determine the presence of ARGs followed by a qPCR reaction to quantify the abundance of ARGs. Finally, the bacterial community composition was analysed by amplifying 16s rRNA by PCR, the amplicons were then sequenced using Ion Torrent to allow for bacterial identification.

Antibiotic concentrations in sediments reached as high as 446 μg kg^-1 and 98.6 ng L^-1 in water samples. Although some of the antibiotic concentrations were below bacterial inhibition they are high enough to allow the development and maintenance of antibiotic resistance. A greater proportion of antibiotics were found in sediment samples compared to water samples, indicating sediments are reservoirs for antibiotics and are potential hotspots for the development of antibiotic resistance. ARG abundances were also found to be much higher than those found in pristine environments. The bacterial communities were dominated by the Bacteroidetes, Proteobacteria, and Firmicutes. Alarmingly, opportunistic human pathogens such as Clostridium and Escherichia were identified in all sediment and water samples, with Escherichia being a potential vector for horizontal gene transfer of ARGs to other human pathogens. However, many sequences were unidentifiable showing there is still more to know about these communities while also highlighting a possible downside to a metagenomic approach to identifying community compositions.

There is without doubt, that anthropogenic activities within aquiculture are mediating the development of antibiotic resistance and supporting potentially pathogenic bacterial communities. Better management practices need to be put into place, such as the use of fertilizers free from animal waste contaminated with antibiotics.

Paper reviewed:

Xiong, W., Sun, Y., Zhang, T., Ding, X., Li, Y., Wang, M., & Zeng, Z. (2015). Antibiotics, antibiotic resistance genes, and bacterial community composition in fresh water aquaculture environment in China. Environmental Microbiology, 70, 425-432.

Stimulated growth promotion via signal exchange in microalgal-bacterial symbiotic relationships

Microalgae interact with bacteria on many different levels and form symbiotic relationships with specific bacterial species. Synergistic mutualism between the organisms can be used to benefit the aquaculture industry by promoting growth and improving overall biomass production. There are two main kinds of mutualism occurring between microalgae and bacteria, the exchange of materials and resources and the communication of signals between the species.

Early studies in the literature have focussed on the exchange of material and resources in order to benefit their survival. Some species rely on symbiotic relationships to aid metabolic mechanisms such as bacteria providing vitamin-B12, to many species of microalgae that don’t possess the gene that codes for vitamin-B12. The bacteria will provide this vitamin in exchange for dissolved organic matter that the bacteria can use for as energy for growth.

Less information is available on the importance of signal exchange in the synergistic mutualistic relationship between microalgae and bacteria. The exchange of substances are used for communication, not as nutrients, and are used to activate or inhibit gene expression or biological activity, resulting in the change in growth and metabolism of cells. Recently, these interactions have been receiving more attention and new important discoveries are being made. It has recently been reported that Chlorella vulgaris secretes a certain signal substance that inactivates the signal substance of the bacterial acyl-homoserine lactones (AHLs), thereby inhibiting the production of bacterial toxins, while Azospirrillum could promote the growth of the microalgae by secreting some hormones such as IAA – Indole-3- Acetic Acid.

The aim of this study was to investigate the relationship between microalgae and symbiotic bacteria and how the hormone IAA can promote the microalgae culture process.This Journal appears to be one of the first experiments to use high-throughput screening for research in microalgae-promoting bacteria. This provides a new method into research on microalgae-bacteria symbiosis. Using this form of screening allowed for the characteristics of the isolated bacteria and the small molecules that they excreted to be studied.

High throughput multiple-well plate method was developed for efficient isolation of bacteria from microalgae (Scenedesmus sp. LX1) cultivation. Scenedesmus sp. LX1 was isolated from tap water and cultivated to be used as the microalgae inoculation and bacterial community associated were obtained using the gradient dilution plate method. For detecting IAA secretion ability, bacteria were cultured under 6 different conditions (including axenic condition) and each was inoculated. The IAA concentration was then measured using an HPLC tandem (High Performance Liquid Chromatography) with a photodiode detector.

The results of this experiment presented that in comparison to the axenic control, most bacteria not only increased microalgal biomass yield but also the growth rate. 16S ribosomal DNA phylogenetic analysis presented that that some of the main isolates present were Pseudomonas and Bacillus. These bacteria strains proved to significantly enhance the growth of microalgae by secretion of IAA, in addition to carbon supply, in Scenedesmus sp. LX1. However, it also showed that simultaneously, microalgae were also secreting other unknown small molecules that seemed to be stimulating the bacteria to secrete IAA, confirming a mutualistic approach.

In summary, IAA is considered to be an important signal substance in plants and bacteria, and the knowledge of this could be potentially used in the aquaculture industries as an economic strategy in large-scale microalgae cultivation to promote blooms that can be used as feed in farm fisheries.

References 
Dao, G.H., Wu, G.X., Wang, X.X., Zhang, T.Y., Zhan, X.M., & Hu, H.Y. (2018). Enhanced microalgae growth through stimulated secretion of indole acetic acid by symbiotic bacteria. Algal Research , 33, 345-351.
https://www.sciencedirect.com/science/article/pii/S2211926418301334

Cho, D.H., Ramanan, R., Heo, J., Lee, J., Kim, B.H., Oh, H.M., & Kim, H.S (2015). Enhancing microalgal biomass productivity by engineering a microalgal-bacterial community. Bioresour. Technol. 175, 578-585.

Trichodesmium: a nitrogen fixing cyanobacteria

Trichodesmium is a genus of globally distributed nitrogen fixing marine cyanobacteria. It provides up to 50% of new nitrogen in some regions. They can form large macroscopic colonies that host an association of diverse microbes distinct from that in the surrounding sea water.

Interactions between the cyanobacteria and its community are recognised as being important by the way they influence the physiology and nitrogen fixation of the Trichodesmium host. When located in equatorial regions there is a lack of heterocysts in the host. It is suggested that with the host provisioning carbon and nitrogen aiding the growth of the heterotrophs, they intern respire resulting in microenvironments of decreased oxygen concentration. These sub oxic environments become a haven for the host and in turn benefit the community through more nitrogen fixation.

Due to their provision of a substantial proportion of new nitrogen there have been studies on its response to changes in the ocean. Under increased CO₂ conditions nitrogen fixation also increases. The community increase in respiration, seen through dedicating more transcription of relevant proteins, due to the rise in nitrogen fixation.

Another study¹ showed that increasing CO₂ without any other changes increased growth of Trichodesmium. Low growth rates are attributed to high energy demands to establish the carbon concentrating mechanism which in turn limits energy for nitrogen fixation. At high CO₂ the mechanism is fully saturated allowing for upregulation of nitrogen fixation, indirectly increasing growth.

Conversely CO₂, and associated ocean acidification has been shown by other studies² to be detrimental to the cyanobacteria. Low pH causes fixation rates to drop as the pH in the cytosol where nitrogen is located decreases with that of the surrounding sea water, thus lowering the efficiency nitrogenase. Effects of low pH differ depending on what is the limiting factor. If carbon is not limiting it can dominate over the effects of acidification. It is possible that when in a microbial community even if carbon is not too high, the heterotrophs could provision enough or through lowering oxygen still enable reasonable levels of nitrogen fixation at low pH.

The community is made up of organisms which are key in the cycling of DOM including Bacteriodetes, gammaproteobacteria and alphaproteobacteria. Bacteriodetes aid the maintenance of low oxygen environments through contributing to the extracellular matrix as it restricts oxygen diffusion. Alphaproteobacteria dominated by rhodobacterales and rhizobiales demonstrated significantly higher gene expression of amino acid transport, corresponding with their nature of consuming the amino acid component of marine DOM.  It is likely not a coincident that these major taxanomic groups that can together degrade DOM are dominantly found in association like Trichodesmium. This association means that how the climate will affect this cyanobacteria may also affect these DOM processing bacteria, and so it is important to look into these community structures and how they respond to elemental fluxes to better understand individual taxa responses.

Reviewed Paper

Lee, M., Webb, E., Walworth, N., Fu, F., Held, N., Saito, M. and Hutchins, D. (2017). Transcriptional Activities of the Microbial Consortium Living with the Marine Nitrogen-Fixing Cyanobacterium Trichodesmium Reveal Potential Roles in Community-Level Nitrogen Cycling. Applied and Environmental Microbiology, 84(1).

References

1.Boatman, T., Lawson, T. and Geider, R. (2017). A Key Marine Diazotroph in a Changing Ocean: The Interacting Effects of Temperature, CO2 and Light on the Growth of Trichodesmium erythraeum IMS101. PLOS ONE, 12(1), p.e0168796.

2.H. Hong et al., Science 10.1126/science.aal2981 (2017). 

Microplastics as a Novel Mode of Transportation for Potentially Dangerous ‘Hitchhiking’ Vibrio Spp.

 As the rate of global plastic production continues to increase, so too does public awareness of plastic pollution within the marine environment. Whilst the hazards surrounding larger plastic debris have received considerable attention from researchers, relatively little is known about the threats posed by much tinier ‘microplastics’ and how they might interact with marine microbial communities.

 Microplastics are generally classified as plastic fragments less than 5mm in diameter and occur when larger plastics break down due to weathering. As with larger plastic debris, microplastics in the marine environment become swiftly colonised by a mosaic of bacteria and other microscopic organisms, conglomerating to form complex biofilms. Previous studies have highlighted how marine plastic debris-associated microbial populations usually differ in composition from the surrounding bacterioplankton. As such, the term ‘Plastisphere’ has arisen, recognising plastic surfaces as novel habitats for their microbial constituents.

 Being lightweight and small in size, microplastics are suspended and transported readily by waves and currents, allowing their microbial colonisers to hitch rides over considerable distances. Accordingly, it is plausible that microplastics may serve as novel vectors for the transportation and dispersal of microbes.

 One recent study performed by Kirstein et al. investigated the occurrence of potentially human pathogenic Vibrio bacteria colonising microplastics within the North and Baltic Seas. The primary focus of the study was to detect the presence of V. cholerae, V. parahaemolyticus and V. vulnificus; known to cause disease in humans.

 Microplastic particles were collected from surface waters using a fine mesh net and the surrounding surface seawater was also sampled so that the occurrence of Vibrio spp. could be compared between microplastic-attached and bacterioplankton populations. Microplastic and filtered seawater samples were incubated individually at 37°C to allow for the selective enrichment of mesophilic Vibrio spp. Subsequently, samples displaying growth were plated onto a vibrio-selective agar media and incubated further. Resulting colonies resembling V. cholerae, V. parahaemolyticus and V. vulnificus were analysed via MALDI-TOF mass spectrometry and PCR amplification of species-specific genes in order to confirm their identification.

 Of the collected microplastics analysed within this study, 13% were found to exhibit colonisation by cultivatable Vibrio spp. Whilst V. cholerae and V. vulnificus were detected only within seawater samples, potentially pathogenic V. parahaemolyticus was isolated from a number of microplastic particles. Where V. parahaemolyticus was detected, it was generally present both attached to microplastics and within the surrounding seawater, suggesting that colonisation of microplastics by Vibrio spp. may occur from the surrounding bacterioplankton. Furthermore, sampling efforts within the Baltic Sea were unable to detect V. parahaemolyticus within surface waters yet identified it associated to a single microplastic particle. Such an observation may serve as an illustration of Vibrio spp. utilising microplastic to 'hitch' a ride, persisting into an environment where it may have otherwise been absent.

 Ultimately, Kirstein et al. have made an important contribution to the understanding of the hazards surrounding microplastic pollution by providing the first definitive proof of potentially human pathogenic Vibrio spp. within marine microplastic biofilms. As average sea surface temperatures continue to rise, the occurrence of Vibrio infection is increasing in frequency and geographical range. Moreover, as larger plastic items repeatedly fragment within the natural environment, marine microplastic concentrations are set to increase. Accordingly, global warming coupled with the emerging problem of marine microplastic could prove to be a perilous combination. In order to fully comprehend the hazards posed by microplastic-attached Vibrio spp., subsequent research could perhaps employ a metagenomic approach in order to gain a clearer understanding of Vibrio abundance, avoiding possible cultivation bias.              

Reviewed Paper:

Kirstein, I. V., Kirmizi, S., Wichels, A., Garin-Fernandez, A., Erler, R., Löder, M. & Gerdts, G. (2016). Dangerous hitchhikers? Evidence for potentially pathogenic Vibrio spp. on microplastic particles. Marine Environmental Research, 120: 1-8.

Can a marine diatom be an efficient source of omega-3 alternatively to conventional fish oil?

Omega-3 fatty acids are essential in human nutrition to ensure a well functioning of the metabolism and the cardiovascular system. Two major forms of these fatty acids are EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid). So far the main source of fish oils is the fish industry itself. But with increasing environmental challenges on one hand and economical and ethical issues on the other hand the production of fish oil is facing some difficulties. Therefore scientists are looking into marine microalgae as a potential and sustainable source for omega-3. A few research studies were already conducted showing that microalgae provide a wide range of advantages compared to fish stock but they mainly took place in southern latitudes^,2.

The aim of this study was to evaluate the suitability of microalgae as a source of EPA and DHA under northern climate conditions on a large scale. The microalga of interest was the diatom Phaeodactylum tricornutum, a robust species with some valuable characteristics, such as a high growth rate and high content in EPA.

The study took place in Norway for a period of six months categorized in three different seasons covering spring, summer and autumn. As test subjects three strains of the diatom P. tricornutum were used, two local strains (M28 and B58) and one commercial Spanish strain called Fito. The marine microalgae were cultivated in big outdoor flat panel photobioreactors to enable the observation of strain specific responses to the environmental characteristics. Each panel was observed intensively regarding the pH, the temperature as well as the nitrate and phosphate concentrations. The difficulty of the study consisted in maintaining the conditions in the panel at a steady state. Due to some technical problems related to the choice of equipment and methods the pH level fluctuated heavily at some points of the experiment.

However the study lead to some interesting findings. The highest biomass production with a correlating high content of EPA was observed in the spring season because of the highest irradiance. The three strains differed in their growth rate but weren’t consistent during all three seasons. Compared to M28 and B58, Fito significantly showed the highest EPA content.

The article showed that a cultivation of marine microalgae on a large scale is possible in northern latitudes. However, this is efficient only during half of the year, when the irradiance is high. This raises the question if algal omega-3 is beneficial enough from an economical point of view?

Even if the productivity of diatoms and the content of EPA is stronger in southern located studies a big disadvantage is the cost intensive cooling necessary because of the strong irradiance. In general the cultivation of marine diatoms is still quite expensive compared to the conventional fish industry. I think that a lot of improvements regarding the methods and equipment will be needed. Another study showed that the acyl lipid composition could be influenced by the age of the cells or by the use of biolistic transformation². Maybe this could be used in further attempts to increase the EPA content in the diatoms. In my opinion improvements regarding the experimental set-up could also have a great impact on the efficiency of the study. The use of helical instead of flat panel photobioreactor¹, or even chemostats could be a way to enhance the stability of the cultures.

In sum, more research on this subject will be needed even though the potential of marine microalga as a source of omega-3 is undeniable.

Article Reviewed

Steinrücken, P., Prestegard, S. K., de Vree, J. H., Storesund, J. E., Pree, B., Mjøs, S. A., & Erga, S. R. (2018). Comparing EPA production and fatty acid profiles of three Phaeodactylum tricornutum strains under western Norwegian climate conditions. Algal research, 30, 11-22.

https://www.sciencedirect.com/science/article/pii/S2211926417308846

References

¹ Fernández, F. A., Hall, D. O., Guerrero, E. C., Rao, K. K., & Grima, E. M. (2003). Outdoor production of Phaeodactylum tricornutum biomass in a helical reactor. Journal of Biotechnology, 103(2), 137-152.

https://www.sciencedirect.com/science/article/pii/S0168165603001019

² Alonso, D. L., Belarbi, E. H., Fernández-Sevilla, J. M., Rodríguez-Ruiz, J., & Grima, E. M. (2000). Acyl lipid composition variation related to culture age and nitrogen concentration in continuous culture of the microalga Phaeodactylum tricornutum. Phytochemistry, 54(5), 461-471.

https://www.sciencedirect.com/science/article/pii/S0031942200000844

³ Hamilton, M. L., Haslam, R. P., Napier, J. A., & Sayanova, O. (2014). Metabolic engineering of Phaeodactylum tricornutum for the enhanced accumulation of omega-3 long chain polyunsaturated fatty acids. Metabolic engineering, 22, 3-9.

https://www.sciencedirect.com/science/article/pii/S1096717613001225

More information about the molecular method biolistic transformation:

https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/gene-gun

Quorum sensing: Gram-negative bacteria are positive for Gracilaria dura carpospore liberation. 

Macroalgae release extracellular substances, which provides nutrition for a variety of microorganisms. Bacteria are the most abundant of microbial groups on macroalgae and are found on the external surfaces and internal tissues of macroalgae. Bacterial communities have shown to impact the growth, morphogenesis, and reproduction on the macroalgae: Ulva and Gracilaria spp. – which are the most common macroalgae found across global coastlines. 

Microbial communities do not live in isolation, but rather rely on intercellular interactions to thrive (Johnson et al., 2016). For bacteria, quorum sensing is a method of intercellular communication. Communication is coordinated by the detection and production of signal molecules called autoinducers; autoinducers regulate gene expression in response to bacterial population density. These signal molecules are chemically diverse, but the most widespread are N-acyl homoserine lactones (AHLs), which are found in Gram-negative bacteria. Gram-positive bacteria do not use AHL systems but rely on peptide signals instead, which are highly species-specific.  

Bacterial quorum sensing can impact the development of macroalgae species, which may include: facilitation in the settlement of zoospores, root stimulation, root and shoot growth, root nodule formation, pathogenic resistance, and even plant defence. Additionally, Authors note that prior studies found associations between AHLs on red algae Acrochaetium spp. for carpospore liberation, but knowledge was limited. The study by Singh et al. (2015) aimed to investigate the impact of bacterial quorum sensing on carpospore liberation for green and red macroalgae.  

The study analysed bacterial species abundances and presence of autoinducers from epiphytic and endophytic bacteria on Ulva and Gracilaria spp. Samples were collected from two different locations (Indian coastal areas) and three seasons. Additionally, AHL-containing culture filtrates were measured from cystocarp plantlets of Gracilaria dura, to analyse carpospore liberation. Bacterial isolates were identified by 16S rRNA gene amplification and sequencing. AHL production and separation was measured by a process of inoculation and centrifugation. AHL identification was measured by liquid chromatography electrospray ionization mass/mass spectrometry and electrospray ionization mass spectrometry.  

Gammaproteobacteria occurred across all samples, seasons, and locations. This group is thought to be highly responsible for the formation of biofilms, which can determine the production of quorum sensing-mediated genes. Of the bacteria analysed, seven isolates were found to produce AHLs in G. dura; this includes Shewanella algae, which produced five different types of AHLs alone. The study found that carpospore liberation increases with the relative increase in concentration of C4- and C6-HSL, up to 10 µm. However, C8-, C10-, 3-oxo-C12-HSL, and culture filtrates of Gram-positive bacterium Bacillus flexus showed no influence on carpospore liberation.  

It appears that in this study, the short acyl chain molecules enhanced carpospore liberation when compared to longer acyl chain molecules. Short acyl chain molecules appear to have higher diffusion and solubility rates in water, which allows them to be actively taken up into plant roots and transported through shoots. 

Overall, quorum sensing efficiency does not only benefit bacterial phenotypes, but has an impact on the reproductive output of macroalgal species. The findings of this study infer that the diffusion ability, stability, and availability of AHLs are important factors that help to function carpospore liberation from G. dura. 

Referenced material:  

Johnson, W.M., Soule, M.C.K., and Kujawinski, E.B. (2016) “Evidence for quorum sensing and differential metabolite production by a marine bacterium in response to DMSP.” The ISME journal, 10(9), pp.2304. 

Article reviewed: 

Singh, R.P., Baghel, R.S., Reddy, C.R.K., and Jha, B. (2015) “Effect of quorum sensing signals produced by seaweed-associated bacteria on carpospore liberation from Gracilaria dura.” Frontiers in plant science, 6, pp.117.