The Ocean's Thin Slice: Drivers of the Community Composition of the Sea Surface Microlayer

The sea surface microlayer (SML) is defined as the top 1 mm of the ocean’s surface and is biologically and chemically distinct from the underlying water (UW) beneath. The SML is primarily composed of dissolved organic carbon and transparent exopolymer particles (TEP), a gelatinous mix of polysaccharides derived from phytoplankton. This concoction of carbon supports high amounts of biomass in the SML, with the inhabitants of this thin slice of the sea (known as the neuston) often found associated with TEP. However, the communities of bacterioneuston in the SML can be altered depending on abiotic factors for example temperature and wind speed, or biotic factors such as nutrient availability. The study presented here aimed to better understand the bacterioneuston communities and the mechanisms that controlled these communities off the Peruvian coast.

SML samples and UW samples were collected from 11 stations during the SO243 cruise to a Peruvian upwelling. Various abiotic and biotic parameters were measured during the cruise, such as wind speed, temperature, salinity, the concentration of TEP and available nutrients (phosphate, nitrate, silicate, and carbohydrates). The bacteria from both depths (SML and UL) at each station were identified by Illumina high-throughput sequencing of 16S rRNA sequences and total bacterial abundances were determined by flow cytometry. Finally, the most abundant families were tested for enrichment or depletion in the SML.

Twenty-four families that made up over 1% of the communities were found across the 11 stations, none of which were found to be significantly depleted in the SML. Of the 24 families, 4 (unknown Flavobacteriales, Flavobacteriaceae, Crymorphaceae, and unknown Bacteroidetes) were significantly enriched in the SML but their abundances were similar at both depths. Overall, the community compositions and bacterial abundances were similar between both depths within a station but more different between stations. Negative correlations of the enriched families were found for temperature and wind speed as this can cause disruption of the SML and prevent enrichment. The enriched families did show positive correlations with nutrient concentrations and TEP, suggesting that the increases in abundance were likely found at upwelling stations. This is not surprising as the upwelling will bring nutrients up from deeper waters that the bacteria can utilise and aid with the transport of TEP and TEP associated bacteria, like the Flavobacteriales, to the surface.

However, the authors suggest that TEP has a limited influence on the community structure of the SML, given that TEP concentrations and community structures were similar at the SML and UL. This was quite surprising to me, as it was my understanding that TEP was the major player in determining SML communities. However, from this study, it is difficult to determine if TEP enhances the growth of bacterioneuston or, is simply a vehicle for some bacteria to float to the surface. It may be that the availability of nutrients (nitrate, phosphate, silicate) coupled with a favourable external environment with low wind speed and suitable temperature could be more important to the enrichment of SML communities.

Paper reviewed:

Zäncker, B., Cunliffe, M., & Engel, A. (2018). Bacterial community composition in the sea surface microlayer off the Peruvian coast. Frontiers in Microbiology, 9, 1-11.

Mutation, Mutation, Mutation: Roseobacter needed a lifestyle change

Rapid adaptation of organisms in stressful environments is vital for survival, especially with today’s warming climate. Marine bacteria have shown that they can quickly adapt to such adverse conditions; given their intrinsic function in global biogeochemical cycling, it is important to understand any ramifications of such adaptations on the physiology and lifestyle of these organisms. 

Kent et al. (2018) used experimental evolution to assess the rapid adaptation in physiology and lifestyle of a member of the abundant marine bacterial clade Roseobacter, under chronic high temperature conditions (33˚C). Roseovarius sp. TM1035 were isolated from a Chesapeake Bay dinoflagellate culture and propagated for 500 generations under optimal (25˚C) and high (33˚C) temperature regimes.

Through genomic and physiological assessment, they found that High Temperature-adapted Lines (HTLs) of Roseobacter had a higher number of gene mutations. A restriction enzyme assay was used to quantify these changes in sequence and allele frequency in the final colonies, compared to the initial ancestor isolated, and calculate the selection rate constant. Mutations found included alterations in genes controlling gas transfer, growth rate, exopolysaccharide secretion and quorum sensing. 

These mutations corresponded to the increased production of biofilm at high temperatures at the water’s surface. This was quantified between experimental lines, with the aid of a crystal violet stain and a spectrophotometer. These results are particularly interesting as an increase in biofilm formation could alter the community structure and composition of the sea-surface microlayer: a vital component particularly of the open ocean ecosystem. It is intrinsic in governing gas exchange, aerosolization and production of Cloud-condensation nuclei, affecting many important ocean processes. If the results of this paper are applicable to a real-world scenario of ocean warming, the precise nature of the sea-surface microlayer may be destabilised. However, some may argue that high temperature events in the ocean are likely to be short term; temperature would fluctuate over 500 generations in a more natural system, potentially yielding different results than seen in this highly controlled experiment.

In addition to increased biofilm formation, some colonies also displayed wrinkly morphotypes, and both these observations were almost entirely confined to the HTLs. Furthermore, the HTL wrinkly morphotypes were experimentally competed against the ancestor and Low Temperature-adapted Lines (LTL) and this was used to independently calculate the selection rate constant. The results yielded from this were consistent to those from the restriction enzyme assay.

The changes in physiology and lifestyle were linked to the thermally-driven decline in oxygen tension. This was assessed in a secondary investigation, in which the HTLs grew relatively better under conditions of low gas transfer compared to the LTLs and ancestor lines. Increased biofilm production may facilitate access to oxygen, sequestering it into a relatively stable matrix, therefore allowing the Roseobacter to grow more efficiently in the thermally-limited low oxygen conditions. HTLs were also shown to have an increased selection rate constant under low oxygen tension, suggesting that this stress induced directional selection: a means for quick adaptation. Kent et al. highlight that the warming climate presents multi-faceted challenges, and indirect effects of adaptation may initiate significant modification to lifestyle and ecosystem function of ubiquitous and crucial marine microorganisms; understanding this is the key to more accurately predicting the effect of warming on our microbial seas. The rapid nature of the adaptations observed here are simultaneously comforting and alarming. While it seems that the survival of a major biogeochemical powerhouse is not at risk, the significant modification of its lifestyle could have severe implications for the delicately balanced sea-surface micro layer: yet another facet of ocean warming to be considered.

Definitions:

Quorum sensing: The ability of organisms to sense and respond to nearby cell density; this can include the alteration of phenotype expression according to the density of the local population. In turn it can inform biofilm formation along with virulence factor expression, motility and many more processes.

Reference:

Kent, A. G., Garcia, C. A., & Martiny, A. C. (2018). Increased biofilm formation due to high-temperature adaptation in marine Roseobacter. Nature microbiology, 3(9), 989.

Microplastics in the SML of estuarines

The pollution by industrial contaminants or plastic debris is an example of anthropogenic changes affecting marine ecosystems. Microplastics (MPs), defined as particles of plastic in the size range of 0.05 to 4.5 mm, are of major interest in present studies. Most recent findings suggest that MPs impact the marine environments in a great extend which is not fully understood. Therefore it is highly necessary to gain more data about the effect of plastic debris of all sizes. Usually MPs have a lower density than the surrounding seawater and depending on their composition and the state of biofouling they tend to float near or at the sea surface. Previous studies on coastal ecosystems or on the open ocean suggest a high presence of MPs in the sea surface microlayer (SML). The SML is characterized by a high microbial activity and the presence of marine microgels. It is already known that the accumulation of MPs in the SML can have an effect on the physical and chemical conditions altering the environment of inhabiting organisms.

This study (Anderson et al., 2018) focused on the accumulation of MPs in the SML of estuarine systems. Estuaries play a major component in the transfer of MPs originated of land-based sources to the open ocean. These highly productive ecosystems are being more and more urbanised and industrialised. A rapid increase of fishing and shipping industries has occurred since a few decades. As a result contamination by sewage, urban run-offs and MPs can be observed. Interestingly this study compared two differing estuarine systems, the Hamble estuary and the Beaulieu estuary. Both of a similar size are very close to each other and located in the southern part of the UK but vary in their stage of development. Hamble is a highly industrialised estuarine, the Beaulieu system is more pristine.

Samples where taken on two days using a multiplicity of methods allowing the extraction of sea surface water or sub surface water samples. The main aims were to evaluate the relatively novel dipped glass plate method for the characterization of the SML and to compare these to sub surface samples regarding the content in MPs. The glass-based method is very applicable for the characterization of the SML; this technique allows taking samples from a depth of 100 to 200 µm representing the SML precisely.

With the use of these methods MPs could be extracted from different water layers. Afterwards a characterization of the MPs took place regarding their colour, length and surface texture using a standard light microscope and a scanning electron microscope (SEM). Mainly fibres of MPs were identified during the two sampling days, as it is the most common type of microplastic in those estuaries. Reading the study I wondered for what reason the colour and size of the fibres were of importance if it doesn’t allow predicating the original source of contamination? In another study on microplastics researchers used Raman spectroscopy in order to determine the composition of plastic (Imhof et al., 2012), possibly leading to the source identification.

Using the dipped glass plate method the highest concentration of MPs was sampled at the Hamble site, which supports the hypothesis that in the SML MPs accumulate. But generally both sites had significant microplastic concentrations highlighting the ubiquitous existence of plastic particles in relatively pristine aquatic environments.

Generally the study provides a first insight in the accumulation of MPs in the SML in estuarine systems but an adequate comparison between estuaries and sampling days was not possible due to very small number of samples. Much more data would be needed to allow any comparisons or even any biological interpretations.

However this study approved the dipped glass method as a highly suitable SML sampling method.

Article Reviewed

Anderson, Z. T., Cundy, A. B., Croudace, I. W., Warwick, P. E., Celis-Hernandez, O., & Stead, J. L. (2018). A rapid method for assessing the accumulation of microplastics in the sea surface microlayer (SML) of estuarine systems. Scientific reports, 8(1), 9428.

References

Imhof, H. K., Schmid, J., Niessner, R., Ivleva, N. P., & Laforsch, C. (2012). A novel, highly efficient method for the separation and quantification of plastic particles in sediments of aquatic environments. Limnology and oceanography: methods, 10(7), 524-537.

Seasonal time bombs: dominant temperate viruses affect Southern Ocean microbial dynamics.

Polar marine ecosystems are very sensitive to effects of warming and temperature change because of the significant influence of sea ice on ecosystem dynamics. Rapid warming in the highly productive area of the Western Antarctic Peninsula (WAP) region of the Southern Ocean has affected multiple trophic levels ranging from ecosystem foundational microbes to high-level consumers such as krill and penguins. Microorganisms, especially in these locations are known to play significant roles in polar ecosystem changes and the abundance of key microorganisms are suggested to predict carbon cycling and climate feed backs on global-scale models. However, viruses are also known to play a substantial role in marine ecosystems through their alteration of microbial communities by causing bacterial mortality through viral infection and altering the biogeochemical cycling through the release of cellular contents via lysis. Recently published studies from a global-scale analysis of viromes suggest that viral diversity in this region is lower than that observed at lower-latitude locations. (Brum et al., 2015).

Despite the role these organism play, viral influences on microbial processes and ecosystem function remain highly unstudied in the Southern Ocean compared to other marine environments. Most studies focus on the spatial and temporal variability of community viral abundance via microscopy.

Quantitative examinations of these viral roles in nature is challenging, however, recent methodological advances through optimized sample-to-sequence pipeline are being used to generate quantitative double-stranded DNA (dsDNA) viral metagenomes (viromes). These new methods increase the knowledge of viral genomic diversity, niche differentiation and ecological drivers of variability.

Viral infections can either be lytic, where viral takeover of cellular machinery results in new viral progeny and lysis of the host or may involve a lysogenic stage in the case of temperate viruses where in viral DNA is maintained within the host as a prophage until induced to replicate lytically. Viruses in polar systems are thought to have diverse replication strategies that aid in the survival of the species in low temperature environments. The current paradigm based on cultivated temperate virus-host systems, is that they primarily utilise lysogeny when bacterial production is low (such as winter conditions) and switch to lytic replication when bacterial production increases (such as spring phytoplankton blooms), therefore temperate viral dominance offers a mechanism for survival in harsh winter conditions.

The data from this study compliments the long-term ecological research in the WAP (Ducklow et al, 2012) suggesting that temperate viruses play a very important role in modulating microbial driven processes in the biogeochemical cycle for this region.

Brum JR, Ignacio-Espinoza JC, Roux S, Doulcier G, Acinas SG, Alberti A et al. (2015). Patterns and ecological drivers of ocean viral communities. Science 348: 1261498.

Brum, J., Hurwitz, B. l., Schofield, O., Ducklow, H.W. & Sullivan, M.B (2016). Seasonal time bombs: dominant temperate viruses affect Southern Ocean microbial dynamics. The International Society for Microbiology Journal. 10, 437-449.

Marine fungi – not so fun for pathogens! Marine fungi show antibacterial activity for pathogens in fish aquaculture.

Aquaculture is the fastest growing industry for food production in the world. Much research has focused on how to manage issues in aquaculture both economically and sustainably – one of these problems is disease control. Heavy use of antibiotics has caused the worldwide issues of: antibiotic resistance, environmental toxicity, aquatic pollution, and high-cost implications. Accordingly, there has been recent investigation into safer and more natural options of biological control – here enter the fungi! In previous studies, both marine fungi and sponges have been highlighted for their antimicrobial activity against fish pathogenic bacteria. Sponges hold a rich diversity of bioactive fungi in their tissues. Sponge-associated fungi produce unique bioactive secondary metabolites and many host-specific toxins. The study by Özkaya et al. (2017) investigated antibacterial activities of fungal isolates, that are associated with sponges, against some commonly occurring pathogens in aquaculture. 

The study focused on four of the most well-known fish pathogenic bacteria that cause disease in aquaculture: Vibrio anguillarum, Yersinia ruckeri, Lactococcus garvieae, and Vagococcus salmonarum. V. anguillarum is the most common cause of disease outbreaks for fish and shellfish, causing vibriosis. Y. ruckeri is the agent responsible for the disease yersinoisis – it has caused major economic loss to the salmonid farming industry. L. garvieae is a major problem for a wide range of wild and cultured fish species – both marine and freshwater. Finally, V. salmoninarum is the cause of coldwater streptococcosis; this is an emerging disease for rainbow trout in the European Union which can cause up to 50% broodstock mortality. 

The study investigated 70 strains of marine fungi, isolated from sponges, and used ethyl acetate extracts to screen against the four previously named pathogenic bacteria. Antibacterial assays were performed using the agar diffusion method. Authors also used the co-culture technique to test the production rates of novel bioactive (antibacterial) metabolites – they tested each fungal strain combined with a mixture of all target bacteria.  

Results showed that a total of sixteen fungal isolates had antibacterial activity against at least one pathogen – all these isolates showed antimicrobial activity against V. salmoninarium. Additionally, three isolates had a strong inhibition of growth against all the tested pathogens, including Penicillum canescens and Aspergillus iizukae. P. canescens was the most active against all bacteria collectively, with an inhibition zone of >13mm. Whereas fungi A. iizukae had the strongest activity in the screening of mono-culture against L. garviae and V. Salmoninarum, with an inhibition zone of 30 – 35mm, respectively. The co-culture experiments found an induction of bioactivity in two isolates. A. iizukae gained broad-spectrum activity with high efficiency against test pathogens when fermented in co-culture conditions. 

The study shows that fungi as a use of antibiotics is extremely promising. The antimicrobial activity of fungi varies greatly between differing properties such as: fungal strains, species of targeted pathogen, sponge host species, habitat salinity, seasonal variations, and an array of chemical and bio-physical parameters (there’s quite a lot of varying factors! But this is expected). The study also found that competition between fungi species was also an important factor – the production of secondary metabolites may initiate a defence or attack reaction by stimulating toxin synthesis. Further research is still required to find out the most effective conditions for the use of fungi for antibacterial control, but the benefits appear worth-while. 

Overall, marine fungal metabolites can be a great alternative to commercially banned antibiotics. Marine fungi appear to be a potential source of eco-friendly and non-costly disease control against fish pathogenic bacteria in aquaculture. 

Study of reference: 

Özkaya, F.C., Peker, Z., Camas, M., Sazak Camas, A. and Altunok, M., 2017. Marine Fungi Against Aquaculture Pathogens and Induction of the Activity via Co‐Culture. CLEAN–Soil, Air, Water, 45(8), p.1700238.

Deep-Water Horizon oil spill impairs immune function and increases susceptibility to pathogenic bacteria.

The explosion of the Deepwater Horizon oil platform on April 20, 2010 resulted in the largest release of oil in history, entering the northern Gulf of Mexico causing massive amounts of ecological and economical damage to the ecosystem. Up to 1773 km of coastline from Louisiana to Florida experienced some form of oil exposure. The application of dispersant via aircraft, was intended to remove much of the oil from the ocean surface which resulted in a large portion of sinking to sediments. Exposure to crude oil and its individual constituents can have detrimental impacts on fish species, including impairment of the immune response.

In this study, they examined the effects of exposure to high concentrations of hydrocarbons in crude oil on immune function and increased susceptibility to pathogen infection, in the southern flounder (Paralichthys lethostigm), based on the increased incidents of external lesions/ sores on fish species that are indicative of bacterial infections such as vibriosis.

The experiment consisted of a fully factorial exposure design with contaminated sediment and pathogen exposure as main factors.

Juvenile flounder were exposed to oil-contaminate sediment for 7 days, then challenged with a known fish pathogen for 1 hour, Vibrio anguillarum, that is prevalent in the Gulf of Mexico. V. anguillarum is the causative agent of vibriosis, a haemorrhagic disease responsible for severe economic losses, especially in aquaculture fisheries. The juvenile fish were sampled at 24 hours post bacterial challenge with two fish then being sacrificed from each tank, in which the liver, spleen, kidney, intestine and gill samples were removed and preserved for further processing.

DNA was extracted from samples using PowerSoil DNA Isolation Kit and the relationship between microbial communities in intestine and gill tissues of oil, pathogen and co-exposed fish was determined by amplification and sequencing of the gene encoding 16S rRNA. After denoising and chimera checking, sequence data were separated into operational taxonomic units (OTUs) and predictive metagenomic analysis was performed.

Fish in several oil exposed treatment tanks were observed swimming to the surface and gulping for air, despite oxygen levels being monitored and adequate at those times in comparison to fish from the control tanks, which did not exhibit these behaviours. A lowered haemoglobin concentration has been linked to oil exposure in which down-regulation of β- haemoglobin gene expression, a gene important for erythrocyte production and oxygen transport, occurred suggesting a highly negative impact of hydrocarbons on fish health.

Oil exposure also resulted in reduced expression of IgM mRNA, the primary systemic fish antibody and typically one of the first to respond to bacterial infection. In several fish species, a reduced ability to defend against pathogen infection occurred after exposure to oil or its components. The results suggest that the lesions observed in nGOM fish after Deep-Water Horizon oil spill are the result of an immunotoxic response to oil exposure, resulting in an increased prevalence of pathogenic infections. 

Oil exposure resulted in consistent and clear shifts in overall taxonomically distinct bacterial community composition in gills and intestines of fish, with Alcanivorax being found in increased quantities in marine sediments and impacted by Deepwater Horizon oil where it has been demonstrated that, when supplied with adequate nutrients (nitrogen and phosphorus), it will become the predominant bacterial species in seawater containing petroleum.  Imbalances in these communities can have detrimental physiological effects, important for organismal homeostasis.

This study provides evidence that an imbalance or shift in gut microbial communities can result in the successful establishment of lethal infections by pathogenic organisms, leading to possible disastrous impacts on the aquaculture industry and  the overall diversity of the Gulf of Mexico.

Bayha, K. M., Ortell, N., Ryan, C.N., Griffitt, K.J., Krasnec, M., Sena, J., Ramaraj, T., Takeshita, R., Mayer, G.D., Schilkey, F. & Griffitt, R.J (2017) Crude oil impairs immune function and increases susceptibility to pathogenic bacteria in southern flounder. PLoS ONE 12(5): e0176559. https://doi.org/10.1371/journal.pone.0176559

• Thiruvarangan Ramaraj,
• Ryan Takeshita,
• Gregory D. Mayer,
• Faye Schilkey,
• Robert J. Griffitt