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Algae occur naturally in the majority of fresh and salt water. However, climate change – in which industry is a key factor – is causing an increase in the formation of harmful algae blooms (HABs) across the world. Warmer waters, high levels of nutrients from increased rain washing agricultural fertilisers into the water, and sufficient sunlight all contribute to the rise in algal blooms.
Microalgae are highly diverse and abundant in the aquatic environment. As photosynthetic organisms are, as “plants”, they are the most efficient harvesters of solar energy on the planet and form the basis of the marine food web. Diatoms and cyanobacteria are amongst the most abundant groups. Microalgae grow naturally and can form blooms, given the correct conditions. Sometimes blooms can be damaging, being designated as “harmful algal blooms” (HABs), a term coined by the International Oceanographic Commission (IOC) to designate any microalgae proliferation – regardless their concentration – that is perceived as a harmful for its negative effects in human health, fisheries, aquaculture, the tourist industry and other resources. Harmful algal blooms are increasing in frequency, intensity and geographic distribution, depending on the species.
Socioeconomic impact of HAB
The economic impact from HABs is difficult to estimate but it affects multiple sectors, the major slice being commercial aquaculture, with interdiction of production areas due to the presence of toxins; then tourism and recreation due to coastal areas interdiction; and finally the losses resulting from the public and private investment in monitoring and managing of HABs. The impact on the society is also important, being that consumer and public opinion are particularly affected upon episodes of seafood or fresh-water poisoning, decreasing the safety perception of the consumption and its consequent decline, and the increased investment in activities for safety reassurance of water reservoirs or coastal areas.
Current short term procedures to overcome HAB
Massive blooms can be associated with the presence of toxins, discolouration of water, closure of recreational areas and commercial farms, and mortalities of nearby species. In closed environments such as lakes, with an history of high nutrient concentration, the obvious solution has been to reduce nutrients concentration and the concomitant growth of algae. Reducing nutrient inputs has been one of the main control measures in UK, e.g. smearing phosphate removal processes in sewage treatment and adjusting agricultural practices to moderate the run-off of nutrients. Current short-term procedures to overcome HAB embrace dosing algaecides or other chemicals able to adsorb or precipitate dissolved phosphates. The disadvantage here is that when a rapid decay of HAB occurs, toxins are released, and contamination of the water supply occurs. Nevertheless, most of the times HABs anti-measurements are place in action after the occurrences and not as a strategy of active prevention. Despite the advances of molecular detection techniques, imaging detection (satellite, microscopy and cytometry), new techniques for real-time detection are essential for the early detection and warning of toxic species. Satellite detection is still suffering from a lack of resolution, while flow cytometry is very promising for real time and early warning, but it suffers from the disadvantages of a diminutive degree of miniaturisation and for stable operation outside of the laboratory.
“current short-term procedures to overcome HAB embrace dosing algaecides; the disadvantage here is that when a rapid decay of HAB occurs, toxins are released”
Usage of sensors to monitor HABs
Other approaches such as real-time sensors are being developed either by using molecular signatures or physiological characteristics. Despite the efforts, these approaches are not ready for the commercial market and they do not comply with all the needs. With this focus in mind a collaboration has taken place to better understand algae signalling and to innovate by creating and enabling in the future, novel sensing technologies to detect the onset of bloom formation, and consequence of alterations of aquatic ecosystems. These applications are in a high demand due to most frequent changes in global or regional climate patterns and by all the social and economic aspects pointed out. A team of multidisciplinary researchers between organic electronics and biology, from the University of Bath, TU Delft in The Netherlands, Instituto Gulbenkian de Ciência in Portugal and the Portuguese Institute for Sea and Atmosphere, gathered to develop an innovative sensor for environmental monitoring and novel forms of exploitation of marine resources, microalgae and cyanobacteria. Different expertise such as advance microscopy, protein labelling, microalgae isolation and culture, organic electronics, were used to develop new tools or devices, aiming to improve water management by devising a low cost early diagnostic tool to enable rapid intervention and to avoid the hurdle of toxin contamination caused by algae consumption or death. To tackle the problem a cell culture of the diatom Pseudo-nitzschia fraudulenta, was chosen as research model because of their ecological importance as a HAB forming species.
“a collaboration has taken place to better understand algae signalling and to create novel sensing technologies to detect the onset of bloom formation”
‘Talking’ algae
Although diatoms do not possess a sensorial neural network capable of reacting as humans do, they can react and cooperatively manifest to changes in their environment. Mechanisms of chemoperception, cell defence and, especially, the ability to collectively bloom are a few evidences that algae do ‘talk’. Yet, accurate and real-time translational systems for algae signalling are scarce.
A process well documented is microalgae growth, a physiological process often limited by temperature, nutrients or light, that by binary fission results in an increase in the number of individual cells. This increase can be measured as a function of time to obtain a growth curve.
Four distinct growth phases can be observed within a growth curve obtained from a batch culture:
- The lag phase during which there is practically no growth, an adaptation to the environment phase
- The log or exponential phase during which the population increase is exponential and the generation time is constant
- The stationary phase where the cell number does not change due to the equilibrium between cellular division and death, as induced by nutrient depletion and waste accumulation
- The phase of decline, or the death phase
Characteristics of stress-induced signals in Microalgae – diatoms
Diatoms revealed to have electrical activity and a signal that was observed to be directly related to changes in the available light and temperature increase. The experimental work has shown that under light, a photosynthesis-inducing photon flux of 120 μmol m-2s-1, the base line current was reproduced. That is, under continuous illumination the diatoms were electrically silent, and no signal could be recorded. Yet, the authors then measured the electrical activity of diatoms in complete darkness. With time, the signals evolve from silent and sporadic into strong electrical spikes approximating nervous cells such as neurons in our brains. The authors measure a population of around 1,000 cells in the electrode. In the first hours the signals among the population were not synchronised, suggesting that no communication between cells was present. Yet, after extended periods of complete darkness the electrical signalling in diatoms was extremely notorious. By monitoring their signalling characteristics, the authors could identify a ‘speech’ speed of about 130 μm/s. For the first time, a bloom forming species has been recorded at their crucial growth stage. The measured signal is then the sum of all individual cell contributions. When the activity of the cells is not coordinated, the overall signal of the whole population appears as uncorrelated noise. However, when the cells operate cooperatively, the signal appears as synchronised electrical spikes.
Isolating variables
To ascertain that algae do signal in their growth stage and when under stress, the authors have isolated others possible variables. The thermal conditions of cells were varied from 19°C to 35°C and back again to 19°C across a total period of three hours. Cells were kept alive and surprisingly their signalling properties drastically sped-up and increased with temperature; thereby showing that cell-to-cell signalling is a feedback mechanism that counteracts changes in their physicochemical environment.
State-of-the-art sensor arrays for algae monitoring
As a comprehensive note to the reader, electrophysiology in marine algae cells has surprisingly received little attention. The first study goes back to the early 20th century, yet a population of diatoms has never been investigated until the author’s recent study. The research team was able to “listen” and record the microalgae electrical interactions and manifestations extracellularly, using sensitive and low resistance planar sensor arrays.
The electrical detection of cells is commonly recorded using Micro – Electrode Arrays (MEAs) that record the extracellular field potentials. Common MEA technology, however, are unable to detect low frequency signals. Low frequency biological oscillations are filtered out; their detection is impaired or even inhibited. The underlying reason is that these technologies focus on recording high frequency neuronal potentials with recording electrodes comprising dimensions similar to the diameter of a human hair. Hence, the electronic design filters low frequency information out, and the small electrode size increases the electric noise and impairs the ability to record large cell populations. To circumvent these constraints, it used large area electrodes, with a size similar to the iris of a human eye. Their low impedance allows low frequency measurements with improved signal-to-noise ratio.
“water management agencies, fishermen, policymakers, stakeholders and the public, will benefit from technology and instruments to detect the onset of algae blooms by algae signals”
The electrical detection of cells has been made using planar electrodes on a substrate in close contact with cells in a culture medium as MEAs do; these conducting electrodes record the extracellular field potential, which consists of a superposition of voltage gated, or ligand gated ion channels, and intrinsic membrane fluctuations. The translation focuses on decoding the ion gradients in the cell’s membrane into electrical signals. Collective phenomena show up when the signals of the thousands discrete cells appear synchronised in time. When the cells operate cooperatively, the signal appears as synchronised electrical spikes. The authors have recorded the electrical response of the whole population of cells adhered to an outsized electrode. The recordings, taken over a period of hours, showed that diatom communication is cooperative and synchronised through the whole measured population. In a time span of hours, their response evolves from weak, sporadic, uncorrelated events of single diatoms, to strong and quasi-periodic oscillations synchronised by the whole diatom population.
“exploiting algae signalling in aquatic environments is broadly exciting and can be used for improved water analysis and more sustainable water management”
Environmental sensing opportunities
This interdisciplinary work has strong scientific and technological implications for probing living algae and testing for ecological and physiological stress adaptations. It is anticipated that water management agencies, fishermen, policymakers, stakeholders and the public in general will benefit from a technology and instruments capable to predict and impair harmful and toxic algae blooms by early detection of the onset of algae signals. An opportunity to exploit algae signalling in aquatic environments is broadly exciting and can be used for improved water analysis and more sustainable water management. Hence, the authors are seeking industrial collaborators to improve and bring this technology to the fore. The vision is to create a high sensitive prime interface between artificial devices and microalgae in the environment, without disrupting or manipulating the cells.
This present integration between living organisms and technology consists of the ability to perform efficient translations between electrical and biological signals, as recently published in Nature Scientific Reports entitled: Collective electrical oscillations of a diatom population induced by dark stress, https://rdcu.be/2cKX
