Marine Electrogenic Organisms

Question 1

What are the types and distribution of electric fish?

Answer 1

Species of electric fish can be found in freshwater rivers of Africa, South America as well as in Marine Environments.

Strongly electric fish use electricity to stun their prey whilst weakly electric fish use electricity for sensing and communication.

Question 2

Give a brief history of microbial fuel cells

Answer 2

First observation Michael Potter 1911 at Newcastle University (then part of Durham Univ.)

Showed that E. coli can release electricity when grown on glucose

1931 Cohen built fuel cells connected in series to produce over 35 volts (but small current)

Key work by Suzuki (1970s) and Benetto (1980s)

Question 3

What are the basic operating principles of a fuel cell?

Answer 3

How do Microbial Fuel Cells Work?

Microbial fuel cells work by allowing bacteria to do what they do best, oxidize and reduce organic molecules. Bacterial respiration is basically one big redox reaction in which electrons are being moved around. Whenever you have moving electrons, the potential exists for harnessing an electromotive force to perform useful work. A MFC consists of an anode and a cathode separated by a cation specific membrane. Microbes at the anode oxidize the organic fuel generating protons which pass through the membrane to the cathode, and electrons which pass through the anode to an external circuit to generate a current. The trick, of course, is collecting the electrons released by bacteria as they respire. This leads to two types of MFCs: mediator and mediatorless.

Without the electrode, the electrons would be cycled through an electron transport chain. In effect stealing the electrons before they can pass through this process.

Question 4

Mediator Microbial Fuel Cell’s

(not in lecture)

Answer 4

Prior to 1999, most MFCs required a mediator chemical to transfer electrons from the bacterial cells to the electrode. Mediators like neutral red, humic acid, thionine, methyl blue, and methyl viologen were expensive and often toxic, making the technology difficult to commercialize.

Question 5

Mediatorless Microbial Fuel Cell’s

(not in lecture)

Answer 5

Research performed by B. H. Kim et al in 1999 led to the development of a new type of MFC’s mediatorless MFCs. The Fe (III) reducer Shewanella putrefaciens, unlike most MFC bacteria at the time, were electrochemically active. This bacteria had the ability to respire directly into the electrode under certain conditions by using the anode as an electron acceptor as part of its normal metabolic process. Bacteria that can transfer electrons extracellularly, are called exoelectrogens.

Question 6

What conditions are produced in the Anode?

Answer 6

MFC Anode

When bacteria consume an organic substrate like sugar under aerobic conditions, the products of cellular respiration are carbon dioxide and water. However, when placed in an environment void of oxygen, cellular respiration will instead produce carbon dioxide, protons and electrons. It is therefore necessary to impart an anaerobic environment in the anode chamber of the MFC.

In mediator based MFC’s, an inorganic mediator takes the place of oxygen in the bacterial electron transport chain. The mediator crosses through the bacterial outer membrane and accepts electrons that would normally be accepted by oxygen or other solubles. Once the mediator has been “reduced” it exits the cell full of electrons which it transfers to the anode.

In mediatorless MFC’s the exoelectrogen sticks to the surface of the anode and uses an oxidoreductase pathway to directly transfer electrons through a specialized protein into the surface of the anode. Electron transfer mechanism may involve conductive pili, direct contact through a conductive biofilm, and/or shuttling via excreted mediator enzymes.

Can add either single species, or use a mixed species inoculum (greater power densities

Question 7

What are the conditions at the cathode?

Answer 7

MFC Cathode

The positively charged half of the cell, the cathode chamber consists of an electrode subjected to a catholyte flow consisting of an oxidizing agent in solution. The oxidizing agent is reduced as it receives electrons that funnel into the cathode through a wire originating from the cathode.

Question 8

How is the power generated by the microbes?

Question 9

MFC’s and Wastewater Treatment

Answer 9

Now that you understand how MFC’s work, let’s take a look at the role they play in the energy industry. The most immediately foreseeable application of an MFC is in wastewater treatment. Microbes love sewage, and the conditions of a wastewater treatment plant are ideal for the types of bacteria that can be used in an MFC. Exoelectrogens are more than happy to breakdown and metabolize the carbon-rich sewage of a wastewater stream to produce electrons that can stream into a cheap conductive carbon cloth anode. The electricity generated from the MFC also offsets the energy cost of operating the plant. As an added bonus, the bacteria eat a lot of the sludge normally present in wastewater. The company Emefcy in Israel claims to be able to cut sludge down by 80% in their wastewater treatment processes, which saves them time and money from having to transport sludge to a landfill or wasteland.

Question 10

MFC and Methane Production

Answer 10

One company takes the MFC’s marriage to wastewater a step further by producing useful hydrocarbons from wastewater streams. Cambrian Innovation’s flagship product, EcoVolt uses an MFC in tandem with a secondary set of electrodes to convert carbon-rich wastewater streams into near pipeline quality methane gas. First, the EcoVolt takes a wastewater stream and screens it for larger particles and solids. Then the waste stream is transferred to a large equalization tank to even out fluctuations in concentration and density, before being processed and passed through Cambrians’ patented EcoVolt units. Inside the unit, an anode coated in one type of bacteria performs the standard oxidation reaction converting dirty water into clean water while producing electricity. The electrons travel to the cathode where electrodes coated with a different type of bacteria convert electricity, hydrogen, and carbon dioxide into pure methane fuel in a process called electro methanogenesis. The methane can be routed back to the plant to provide clean heat and energy.

The process of electromethanogenesis was discovered in 2008 and subsequently commercialized by Cambrian Innovation Inc.

https://cambrianinnovation.com/wp-content/uploads/2012/11/EcoVolt-brochure-updated3.pdf

Question 11

MFC Biosensor

Answer 11

MFC’s don’t only have to be used for power generation, they can also be used as a convenient biosensor for wastewater streams. Wastewater is evaluated based on the amount of dissolved oxygen required by aerobic bacteria to break down the organic contaminants present in a body of water. This value is called the biochemical oxygen demand value (BOD) and correlates with the amount of organic solute in a solution. The richer the wastewater stream is, the greater the current an MFC can provide, design control engineers can take advantage of this direct relationship to measure real-time BOD values in a wastewater stream. As an added bonus, the MFC biosensors power themselves from the wastewater stream.

Question 12

MFC’s in Space

Answer 12

The Naval Research Laboratory (NRL) has a very different idea of how remotely operated vehicles could be powered in space, they have begun work on a prototype rover that is powered by the bacteria Geobacter sulfurreducens, an exoelectrogen with a pentient for breaking down metals. This bacteria was selected for its high energy density compared to lithium ion power sources, and the overall resilience, ruggedness and longevity of the MFC it supports. The NRL’s Dr. Gregory P. Scott plans to use a hybrid MFC/battery system to power a smaller 1 kg hopping rover. The MFC would only be able to power low load devices such as the rover’s electronics, sensors and control system. The battery or capacitor would be used for higher power loads, like locomotion or operation of a more power-intensive scientific instrument. Since a rover spends a large amount of time stationary analysing samples, the MFC could be used to recharge the batteries or supercapacitors for the next heavy load.

Question 13

Give two examples of bacteria with nanowires.

Answer 13

1. Geobacter sulfurreducens
2. Shewanella oneidensis Mr-­‐1

Question 14

Examples of bacteria producing electricity

Answer 14

—— Known as exoelectrogens

—— Geobacter

—— Bacillus

—— Clostridium

—— Different strains produce differing amounts of electricity

Question 15

What are the limiting factors for bacterial energy generation using biofilms?

Answer 15

Supply of energy source (nutrients)

—— Treatment of waste water for example from a brewery

—— Surface area

—— Use carbon cloth electrodes, folded conformations

Question 16

How can a marine microbial fuel cell be powered?

Answer 16

(Dumas et al 2007)

Marine microbial fuel cell: Use of stainless steel electrodes as anode and cathode materials

The marine microbial fuel cell device described in Fig. 1 was set-up in the Mediterranean sea (Italy).

• • Used stainless steel for the first time (2007)
• Anode embedded in sediment
• Cathode in overlying seawater
• Max power = 4 mWm-­‐2
• Lower than graphite electrodes
• Implementation in the sea was a problem
• Lab version much better 140 mWm-­‐2
• Prof of concept shows that this is possible - but as a first version it is expensive, unreliable and does not produce feasible quantities
  *

Question 17

Grant mentions a study by Logan et al 2005, what were the key points from his study?

Answer 17

He looked at electricity generation from a microbe called cysteine from a microbial fuel cell, looking at bacteria producing electricity.

Found that species of Shewanella was one of the most dominant species present.

They didn’t culture and isolate bacteria or demonstrate how my energy was produced by isolated species.

Question 18

How do you analyse bacteria without growing them?

Answer 18

Cell morphology was examined (in the paper by logan) using fluorescence microscopy after staining the cells with 40, 6 diamidino-­‐2-­‐ phenylindole (DAPI; final concentration 3:3mg=mL)

Question 19

ammonium recovery and power generation from urine: Water research 46 (2012) 2627-­‐2636

Answer 19

Fixed nitrogen (fertilisers) are important but costly to produce

paper in water research 2012

—— Wastewater treatment of waters which have too much nitrate is also expensive and catalyses the opposite function, conversion of fixed nitrogen back to nitrogen gas

—— This work aimed to recover fixed nitrogen (ammonia) from a waste product, human urine

Set up in a two-compartment system, with a fuel cell anode on the left covered in bacteria. A membrane allows gas diffusion and air and ammonia passes through into the cathode system and is removed from the system - drivem by the electrical microbial fuel cell.

The applied microbial fuel cell used a gas diffusion cathode. The ammonium transport to the cathode occurred due to migration of ammonium and diffusion of ammonia. In the cathode chamber ionic ammonium was converted to volatile ammonia due to the high pH. Ammonia was recovered from the liquid–gas boundary via volatilization and subsequent absorption into an acid solution.

Question 20

Conclusions

Answer 20

• Over 100 years since the discovery of electricity production by bacteria at Newcastle
• Good potential for electricity production combined with water purification (eg Australia), marine sensors.
• Significant technical challenges remain

Question 21

Exoelectrogenic bacteria that power microbial fuel cells

What power densities have microbial fuel cells created so far?

Answer 21

Exoelectrogenic bacteria that power microbial fuel cells

(Logan, 2009)

Enriched anodic biofilms have generated power densities as high as 6.9 W per m2 (projected anode area) which are approaching theoretical limits (2009)

Question 22

Exoelectrogenic bacteria that power microbial fuel cells

(Logan, 2009)

Define air cathode

Answer 22

Air cathode - a cathode that is exposed to air on one side and water on the other side.

Question 23

Exoelectrogenic bacteria that power microbial fuel cells

(Logan, 2009

What is the anode potential?

Answer 23

Anode potential - the potential of the anode relative to a reference electrode (usually a standard hydrogen electrode).

Question 24

Exoelectrogenic bacteria that power microbial fuel cells

(Logan, 2009)

What is the catholyte?

Answer 24

Catholyte - a chemical that accepts electrons at the cathode.

Question 25

Exoelectrogenic bacteria that power microbial fuel cells

(Logan, 2009)

Define dissimilatory metal-reducing bacterium.

Answer 25

Dissimilatory metal-reducing bacterium - a bacterium that is capable of using metals as a terminal electron acceptor for respiration.

Question 26

Exoelectrogenic bacteria that power microbial fuel cells

(Logan, 2009)

Define exocellular.

Answer 26

Exocellular - Occurring outside the cell membrane (equivalent to extracellular) in a cell surface or non-cell-associated process.

Question 27

Exoelectrogenic bacteria that power microbial fuel cells

(Logan, 2009)

Define exoelectrogenic.

Answer 27

Exoelectrogenic - Describes the ability of certain microorganisms to generate and transfer electrons exocellularly.

Question 28

Exoelectrogenic bacteria that power microbial fuel cells

(Logan, 2009)

Define a nanowire.

Answer 28

Nanowire - An electrically conductive appendage produced by a bacterium that is proposed to conduct electrons from the cell to surfaces such as metal oxides or electrodes.

Question 29

Exoelectrogenic bacteria that power microbial fuel cells

(Logan, 2009)

Define a potentiostat.

Answer 29

Potentiostat - A device that can be used to set a specific potential for an electrode.

Question 30

Exoelectrogenic bacteria that power microbial fuel cells

(Logan, 2009)

Define Quorumsignal

Answer 30

Quorum signal - A small molecule that is used as a signal for specialized responses within a bacterial community.

Question 31

Exoelectrogenic bacteria that power microbial fuel cells

(Logan, 2009)

Define redox potential

Answer 31

Redox potential - a relative measure of the potential (in volts) for a chemical to gain or lose electrons.

Question 32

Exoelectrogenic bacteria that power microbial fuel cells

(Logan, 2009)

What classes of proteobacteria has the current generation be shown in?

Answer 32

• Electrical current generation has been shown for four of the five classes of Proteobacteria, as well as the Firmicutes and Acidobacteria phyla.

Question 33

How are microbial fuel cells unique?

Answer 33

• Microbial fuel cells (MFCs) are unique in that they do not require the use of metal catalysts at the anode. Instead, they use microorganisms that biologically oxidize organic matter and transfer electrons to the anode. These electrons flow through a circuit to the cathode, where they combine with protons and a chemical catholyte, such as oxygen.

Question 34

Exoelectrogenic bacteria that power microbial fuel cells

(Logan, 2009)

What are the advantages of microbial fuel cells?

Answer 34

• The flexibility of microorganisms to use a range of fuels makes the MFC an ideal technology for renewable bioelectricity generation from biomass.
• In the United States, approximately 1.5% of the electricity produced is used for waste-water treatment
• • Through the recovery of energy from waste waters and waste biomass using energy-producing MFCs, it may be possible to ensure the energy sustainability of the water infrastructure. It is estimated that domestic waste water contains 9.3 times as much energy as that currently used to treat the waste water through energy-intensive aeration-based processes. Using an air cathode MFC can eliminate the need for aeration, and therefore much of the existing energy demands, while producing energy

Question 35

Exoelectrogenic bacteria that power microbial fuel cells

(Logan, 2009)

How is the power output of microbial fuel cells affected by pure or mixed cultures

Answer 35

• Although many different types of microorganisms produce electrical current in MFCs, many of these strains exhibit low power densities when grown as pure cultures. It is therefore unclear whether these bacteria exist as exoelectrogenic oligotrophs among faster-growing competitors, or whether a low level of current generation provides some other benefits through interactions.
• What is the role of other bacteria in relation to exoelectrogenic strains in anodic communities, and how do mixtures of communities affect power production? Answering these questions will provide useful insights into the ecology and complex functions within electrogenic biofilms.
• The highest power densities in MFCs are almost always produced by inoculating the anode with a rich and diverse source of bacteria, such as wastewater or sludge. The power densities produced by isolates or mixed cultures are often more dependent on the specific architecture, electrode spacing and solution conductivity of the fuel cell rather than the specific bacterium.

Question 36

How much is the efficiency of microbial fuel cells increasing by?

Answer 36

• Up to 1.55 kW per m3 has been achieved in an air cathode MFC42, yet the limit on volumetric power production is theoretically a function of the growth rate of the microorganism.
• The projected maximum power densities achieved per electrode surface area in an MFC increased by six orders of magnitude to 1.54 W per m2 between 1999 and 2006 (Fig. 2). These increases in power resulted from improvements in architecture and our understanding of how to extract power from bacteria more effectively. For example, various Shewanella strains and mixed cultures produced less than 1 mW per m2 before 2000, but we now routinely extract >2 W per m2 with these inocula owing to improved MFC designs.

Question 37

Exoelectrogenic bacteria that power microbial fuel cells

(Logan, 2009)

What methods would improve microbial fuel cell efficiency?

Answer 37

• Higher power densities should be achievable by using thick exoelectrogenic biofilms owing to the large number of bacteria that would contribute to electrical current generation.
• G. sulfurreducens maintains electrogenic activity across thick biofilms. Transport of substrate to the biofilm, or metabolism products from the biofilm, will ultimately limit maximum power densities3. A power density of 6.9 W per m2 of the anode area was recently achieved using a cathode that was 14 times larger than the anode
• • Further increasing anode surface areas relative to the cathode will have little impact on MFC performance, as anode surface areas in MFCs can already greatly exceed those of the cathode. Cathode performance currently limits current generation in MFCs and will probably continue to do so for some time. This will give microbiologists time to study and perhaps genetically engineer electrogenic bacteria50, so that higher power densities can one day be achieved when cathodic limitations are overcome in MFCs and bacteria become the limiting factor in power generation.

Question 38

Enhanced Electricity Production by use of reconstituted artificial consortia of estuarine bacteria grown as biofilms.

What was the study?

Answer 38

(Zhang et al, 2012)

Grants project –

• Used acetate-fed MFCs inoculated with sediment, with two-chamber bottles and carbon cloth electrodes to deliver a maximum power output of ∼175 mW·m–2 and a stable power output of ∼105 mW·m–2.

Question 39

Enhanced Electricity Production by use of reconstituted artificial consortia of estuarine bacteria grown as biofilms.

Results

Answer 39

​

(Zhang et al, 2012)

• Twenty different species (74 strains) of bacteria were isolated from the consortium under anaerobic conditions and cultured in the laboratory, of which 34% were found to be exoelectrogens in single-species studies.
• Exoelectrogenesis by members of the genera Vibrio, Enterobacter, and Citrobacter and by Bacillus stratosphericus was confirmed, by use of culture-based methods, for the first time.
• An MFC with a natural bacterial consortium showed higher power densities than those obtained with single strains.

Question 40

Enhanced Electricity Production by use of reconstituted artificial consortia of estuarine bacteria grown as biofilms.

What power output did they achieve?

Answer 40

• The maximum power output could be further increased to ∼200 mW·m–2 when an artificial consortium consisting of the best 25 exoelectrogenic isolates was used, demonstrating the potential for increased performance and underlying the importance of artificial biofilms for increasing power output. It may be that the presence of nonexoelectrogenic bacteria can disrupt or reduce the overall electrical conductivity of the biofilm.