White paper: Deep sea carbon storage using macroalgae
For millions of years the Earth’s climate has cycled in and out of glacial and interglacial periods, triggered by orbital wobbles, plate tectonics and other planetary factors. However, the current increase in our planet’s temperature is very different. The industrial revolution has driven a frightening rate of change in atmospheric carbon dioxide (CO2) levels from 277 parts per million (ppm) in 1750 to 414.7 ± 0.1 ppm in 20211. It’s clear what we’re facing is an anthropogenic induced climate emergency.
The world we’ve built since the industrial era has been powered by fossil fuels. Our reliance on coal, gas, and oil has meant significant increases in the amount of CO2 we’re releasing into the atmosphere. Over a period of 50 years human emissions of CO2 went from 11 Gt of carbon/yr in the 1960s, to 35 Gt of carbon dioxide/yr in the 2010s1. The Earth’s natural carbon cycle can’t process such huge increases, and the consequences are rising global temperatures and the acidification of our oceans.
We have a collective global challenge to limit the Earth’s rising temperatures to between 1.5 and 2 degrees celsius in order to avoid the environmental, social and economic carnage wreaked by the changing climate.
Limiting climate change means reducing the amount of carbon dioxide we emit, and removing at least some of the excess that we’ve already added. It’s hard to overstate the scale of this challenge. The IPCC’s 2022 report highlighted that current emission trends would see unavoidable climate impacts between now and 20402. Decarbonising our way of life is essential - it will mean making changes and compromises to what we eat, how we travel, and how we build and maintain infrastructure.
But this alone isn’t enough. The extraction and sequestering of carbon is paramount if we’re to reduce our atmospheric CO2 levels from their current high. Global rates of Carbon Dioxide Removal (CDR) stand at just 2 GtCO2/yr3. This is currently achieved through conventional means such as land afforestation. It will be extremely difficult to increase these approaches to the levels needed. Latest reporting states that removals will need to reach 10 GtCO2/yr by 20504 if we’re to limit a global temperature rise to no more than 1.5 degrees.
The need to develop economically viable, environmentally-sustainable and scalable removal solutions is now.
Ocean based carbon dioxide removal pathways
There are multiple pathways to carbon dioxide removal but our oceans will play an important role.
The oceans are continually exchanging gases, including CO2 and other carbon containing gases like methane and chlorofluorocarbons (CFCs), with the atmosphere. There are about 875 gigatonnes of carbon (GtC) in the atmosphere, the bulk of which is in CO2 form5. There’s a similar amount in the surface ocean waters: about 900 GtC.
The CO2 in the surface waters is constantly moving into the atmosphere, and the CO2 in the atmosphere is constantly moving into the surface waters. The scale of this movement is vast: about 80 GtC (that’s 293.6 GtCO2; to convert between C and CO2 you simply multiply by 3.67) per year, but with an overall net movement of CO2 into the ocean. To put that into perspective, our industrial emissions of CO2 are about 36 GtCO2 per year. With that rate of flux and exchange, removing CO2 from the surface ocean is just as good as removing it from the atmosphere. They are two sides of the same coin. Atmospheric or Surface Ocean… It’s all the same CO2 pool, the CO2 is moving freely between the two.
The deep ocean though, that’s another story. Containing 37,000 GtC it is the planet’s true carbon sink and holds the key to achieving the IPCC’s removal goal of 10 GtCO2/yr6 and limiting warming to below 1.5 degrees.
The carbon cycle and the Earth’s biological pump
The global carbon cycle actually consists of two cycles:
The fast surface cycle - where carbon shuttles rapidly and dynamically between the living components of the terrestrial, atmospheric and surface ocean environments on a minute, hourly, daily, weekly, annual and decadal timescale.
The non-living, slow carbon cycle of mineralisation, deposition and sedimentation which occurs passively, primarily in the deep ocean, over centuries, millennia and eon scales. The exchange between the surface and the deep ocean is slow, a bottleneck that limits the rate at which carbon we add to the fast surface cycle can transfer into the slow cycle.
The biological pump acts as a natural, one way highway between the two cycles. It shuttles carbon out of the fast cycle and into the slow cycle through the sinking of dead and decaying biological material from the surface of the oceans into the cold, dark depths, where it becomes remineralised or eventually buried in sediment. This transfer of organic matter creates a vertical gradient in dissolved inorganic carbon, enhancing the ocean’s ability to absorb even more atmospheric CO2 at its surface.
Carbon Dioxide Removal (CDR) in the deep ocean
The deep ocean carbon cycle touches on the surface carbon cycle, but is substantially and remotely disconnected from it. It is slower, and significantly larger than the terrestrial environment and contains an estimated 37,000 gigatonnes of carbon7. All human emissions since industrial times (around 1.5 trillion tonnes of CO28), if added to the deep ocean (below 1000m), would be an equivalent addition of just 4%.
Ocean based CDR is so effective that around 80% of any carbon dioxide entering the deep oceans9 from the atmosphere is stored for over 100 years at least10. The remaining 20% will leak back into the atmosphere at a rate determined by location and depth. The deeper it moves, the longer the time to return. Carbon that is buried into the seabed is locked away to a near permanent state11, for 1000’s of years.
With average depths of 3800m, our oceans offer greater durability for carbon removal compared to their terrestrial counterparts. For example, afforestation, a promising strategy for carbon removal, has significant limitations. Carbon is sequestered for decades not centuries, and can be released back into the atmosphere due to disturbances such as wildfires or other extreme weather events.
The role of macroalgae
The Azolla event can provide some hints to help us understand how a biomass based CDR solution in our oceans could work. This biogeological event occurring in the middle Eocene saw atmospheric CO2 content drop from up to 3500 ppm (parts per million) to just over half this figure12. Over an 800,000 year period, freshwater Azolla ferns grew in abundance, absorbing CO2. As the biomass naturally died and sank, the carbon captured was sequestered to the Arctic sea floor. It can be argued that this process was a contributing factor that essentially saw the Earth shift from a greenhouse climate, to the much cooler global temperatures that we see today.
Deep sea carbon storage that looks to mimic and speed up the impacts of the Azolla event, could also be viable through the sinking of macroalgae.
Macroalgae grows photosynthetically, utilising the CO2 in the ocean. As it grows the pCO2 of the water around it lowers, causing more CO2 to enter the sea from the atmosphere. Seaweed grows without the need for artificial fertilisers or freshwater, it increases biodiversity (when grown sustainably), and can absorb CO2 faster than terrestrial plants.
Just as we saw in the Azolla event, dead macroalgae naturally sinks and ends up in deep sea sediments. An estimated 130,000 t/yr of giant kelp (Macrocystis spp.) is exported to the deep sea down the canyons of the Monterey Peninsula.
If seaweed is sent to the deep ocean seabed (at depths of 1000m or more), the carbon it has naturally absorbed is essentially removed from the surface carbon cycle for at least 100 years.
Choosing the right macroalgae species for CDR
Seaweeds are fascinating organisms and the more we study them the more potential we see in them. They are globally distributed, between the intertidal seashore to depths of 120 metres, are the key primary producers in coastal waters, and support valuable ocean ecosystems. Current estimates suggest that macroalgae cover approximately 6.1 to 7.2 million km2 globally (just under the size of Australia), with a global carbon net uptake of 1.2 Gt/yr13.
There are 11,000 documented species of seaweed, and they come in 3 distinct groups - reds (Phylum Rhodophyta), browns (Phylum Heterokontophyta, Class Phaeophyceae) and greens (Phylum Chlorophyta). Each of these groups is more different from the other than humans are to trees - they have been evolving for an extra 500 million years, so they’ve had quite some time to diversify.
Many seaweed species are a fast growing biomass with potential to be used in many key areas where fossil fuels currently dominate. They include, but are by no means limited to:
- Animal feed and supplements
- Human food
However, some species of seaweed are far more opportunistic than others, and in certain areas of the globe these opportunists are proving problematic.
Sargassum is a genus of brown algae that grows from temperate to tropical regions. There are 400 species of Sargassum and two species are free floating, S. fluitans and S. natans. Mats of it drift around the ocean especially in the Caribbean and West Coast of Africa, held afloat by gas-filled bladders that look like tiny grapes.
The Great Atlantic Sargassum Belt14 is a recent phenomena (since around 2009) and is most likely a reaction to excessive nutrient and soil runoff into the oceans. Record amounts are now washing up on the shores of the West Coast of Africa and the Caribbean with detrimental environmental effects. This year saw a record breaking 5,000 mile belt of Sargassum track towards Florida making international headlines15.
Sargassum material in the Great Atlantic Sargassum Belt is conservatively estimated to number in excess of a 20 million tonne16 standing stock. It grows rapidly throughout the summer months in particular, and makes landfall in substantial amounts over a 6-8 month period. Tens of millions of tonnes (up to 100 million tonnes) of problematic Sargassum inundates the Caribbean every year. These influxes cause environmental degradation, loss of tourism and health threats17.
Sargassum material which is not intercepted offshore, rots when it becomes beached and releases the CO2 that it has previously absorbed back into the atmosphere and surface waters.
Under anaerobic conditions (which occur when oxygen cannot penetrate, both on land and in water), methane18 gas will be produced. This will occur on the beaches, as well as in landfill sites where Sargassum is most commonly disposed of. Methane is even more potent than CO2 as a greenhouse gas.
While it’s clear that many species of seaweed are ideally placed for the development of useful commodities, Sargassum, and the limitations that are associated with a free floating, uncontrollable, seasonable biomass is not conducive to sustainable industrial manufacturing processes.
Sargassum, when dried, contains between 27.41% - 29.23% carbon19. Conservatively, 6.861 tonnes of wet Sargassum has absorbed 1 tonne of CO2 from ocean waters. With allowances for fluctuations and Life cycle Assessments (LCAs) of our operations, we estimate a 10:1 ratio of biomass to CO2. Sargassum that naturally dies and sinks, has already been reported in deep sea trenches around Japan, in the guts of deep sea crustaceans and is abundant on the seafloor in the Atlantic20. It is therefore an ideal candidate for carbon sequestration in the deep ocean.
Seaweed Generation’s first focus will be on CDR through Sargassum due its abundance and problematic nature. But in tandem we are developing an approach to cultivation of other, more useful, seaweeds that focuses on automation in seeding, monitoring, cultivation and harvesting in order to achieve a scalable and cost effective future proof solution. Robotics and automation allow us to increase control, reduce costs, increase safety and increase our ability to respond to or prepare for extreme events.
Seaweed Generation’s approach
Seaweed Generation will deliver real, long-lasting, scalable CDR using solar driven robotics. The AlgaRay is a simple, solar powered system, designed with automation and rapid scalability in mind. Travelling at around 3knts, it takes less than 1 minute to fill with Sargassum at the surface of the sea offshore. The AlgaRay takes the biomass to around 200m deep and releases it (unbound, unbaled and free to disperse) where it spreads out and passively falls to the deep seabed under the force of gravity.
The AlgaRays are hydrodynamically shaped to cut through the water despite their size. At 135m water pressure has crushed the hundreds and thousands of Sargassum air sacs making it negatively buoyant. The Sargassum will reach 1000m in approximately 1.5 hours, travelling downwards at a rate of 10m a minute. There’s a trade off between going deeper and the associated speed and spread of sinking: it takes more time and energy to go deeper, but the modelling suggests the seaweed will sink faster the deeper it is released and spread out over a smaller area. As the Sargassum falls away to the sea floor, the empty AlgaRay returns to the surface ready to start collecting again. The whole sequence is captured on video and bundled with biomass measurement, time date and GPS data for reporting purposes.
While a surface tethered vehicle is our starting point, we are already developing the large-scale underwater glide version of the AlgaRay that will carry out the same task but with additional capabilities and independence. It will ultimately become fully autonomous allowing us to continue to operate irrespective of adverse weather conditions.
Seaweed Generation is conducting a 2023 pilot launch of the AlgaRay in the Caribbean. We are selecting sinking sites that are within close proximity to the coast (allowing us to intercept Sargassum before it washes up onto shore) and in depths of >4,000m with no upwelling currents.
Durability of deep sea carbon storage using macroalgae
With time, the biomass deposited on the seafloor will either be sedimented or remineralised (i.e. dissolved into the deep sea water), and therefore difficult to monitor. However, the long term durability of deep ocean carbon is well documented.
First, because of the high pressure, low temperatures and lack of oxygen, decomposition of material is extremely slow at depth so there is a high likelihood that the biomass will become sedimented. (See Figure 3 in Hain et al21).
Second, assuming carbon is nevertheless remineralised into the surrounding water and not sedimented, the cycling of deep sea water into the upper layers is understood to be in excess of 100 years. It then takes several hundred more to reach the surface.
It is therefore generally accepted that for seaweed that reaches the sea floor at depths of more than 1000m and becomes sedimented, the removal of the absorbed CO2 in that biomass is indefinite - on geological timescales. For remineralised material it becomes a depth and time factor. Below 300m - 0.6% could return within 50 years. Movement of material to depths below 2000m (way below the bottom of the thermocline, where the oceans become stably stratified by temperature), enhances longevity by reducing mixing. A study published in 202222 indicated that below 2000m 94% of particulate organic carbon that is remineralized would not reach the mixed layer and interact with the atmosphere for over 100 years. Below 3000m - just 0.2% returns within 1000 years. Seaweed Generation will always sink at >4000m so durability and permanence will be even longer.
Diel vertical migration of zooplankton and nekton occurs largely in the euphotic zone (upper 400m of ocean) and is therefore also unlikely to impact on deep ocean material. Unplanned disturbance will only happen due to a natural disaster such as an earthquake or deep sea volcanic activity. Some minor, but planned, localised disturbance will occur through Seaweed Generation’s monitoring program as we remove samples to ensure that the environmental and ecological consequences (or lack of) are measured appropriately.
We’re working with local governments to guarantee that the areas will remain undisturbed long term (which is also highly likely even without a guarantee, given the depths).
While climate change is a global issue there are particular communities around the world experiencing outsized impacts of the climate emergency. Tens of millions of tonnes (up to 100 million tonnes) of problematic Sargassum inundates the Caribbean every year. These influxes cause environmental degradation, ecological disruption, loss of tourism and health threats to these small island nations who don’t have the finances or resources to deal with this ever increasing burden of biomass.
In 2022 the Mexican Navy used 11 Sargassum gathering vessels, 23 boats and five air units in their attempts to clear Sargassum from their oceans and beaches. However, most locations affected by Sargassum do not have such government resources and support to help clear the accumulation. The cleanup cost in the Caribbean was $120M in 2018 alone, a devastating economic bill in addition to the loss of revenue from tourism.
There’s been significant media coverage of the Sargassum around Florida this year. It is the affluence of the region that has ensured the issue is being highlighted, but for many communities who have been suffering for years there’s been little interest in their continued battle.
By intercepting and sinking this problematic biomass before it hits shorelines we are trying to redress the outsized impacts faced by these nations. We secure agreements with local governments to not only ensure access to waters, but also agree a percentage of our revenue will go to local communities. By working with local communities (in knowledge sharing and resources) we can create climate positive jobs. We are turning a massive environmental negative into a climatic positive, and generating wealth and prosperity for local communities at the same time.
Monitoring, reporting and verification (MRV)
At time of writing there are no established monitoring, reporting, and verification (MRV) protocols for assessing carbon capture and sequestration by macroalgae. To fully recognise the potential of CDR through this method it is important to understand both the quantity of carbon captured (“uptake”) and the duration that the carbon will be sequestered (“permanence”).
Advice on our MRV plans is being sought from our Science Advisory Board and supported by Ocean Visions. We’re seeking third party verification of all our activities through EcoEngineers (specialists in Eco Auditing to ensure the integrity and accuracy of carbon declarations for the marketplace).
Our approach to MRV is shaped and structured according to ocean depth:
During our pilot our first steps will be to accurately measure the volume and weight of Sargassum collected in each removal event per filled module of the AlgaRay.
AlgaRays generate positional (GPS, time, date) and physical data (weight) which can be used to correlate seasonal biomass variation to carbon composition (time-location-weight).
We have established through published literature that 6.861 tonnes of wet Sargassum has absorbed 1 tonne of CO2. We will look to further quantify Sargassum uptake of CO2 and the impact it has on the surface ocean environment through sensors and samplers. This will allow the continual monitoring of:
- Air Sea equilibration (pCO2) - to be fed into large scientific data sets
- Dissolved oxygen
- Dissolved inorganic carbon (DIC)
- Dissolved organic carbon (DOC)
- Total alkalinity
- Particulate organic carbon (POC, total amount, elemental composition, and size distribution)
- Macronutrients (Nitrate, nitrite, Ammonia, Phosphate)
- Trophic Exchanges - This requires development as it will involve a multi-faceted approach including stable isotope analysis (e.g. δN14, δC13), species composition of samples of Sargassum and stomach content analysis.
Our biomass will be ‘in transit’ through this zone with an estimated time of no more than 16-18 hours. We plan to ‘follow’ regular drops down to 4000m to prove speed of sinking and dispersal dynamics, and to show that the biomass reaches the seabed as expected.
To do this, we are developing a series of camera / light arrays on an ROV that will track the Sargassum as it sinks.
We’ll also use a sonar system that can track the Sargassum to depths of up to 3,000 metres.
Turbidity sensors will be dropped once a month to assess water clarity and potential shifts caused by upwellings. Current meters (such as ADCPs) will be deployed once a month to verify the absence of upwelling and speeds of deeper currents.
We’ll supplement this work with available modelling data that will allow us to further our understanding of how the biomass sinks and establish where it lands on the seafloor.
Direct measurement of durability over time is difficult in the deep ocean. Understanding how best to verify this is hard and will take time to develop. In our early stages, for the sake of extreme simplicity and being overly conservative, we will account for the movement of the carbon as if it remineralises instantly as it hits the seabed. We can then model the rate of mixing directly using known oceanographic modelling to get the lowest possible estimate of the longevity of the removal.
Over time our monitoring will improve and we can extend the time frame on that assumption.
To verify the permanence of our removals and the impact deposited biomass has on deep ocean ecosystems we are developing a visual monitoring system and the AlgaProbe (our own robotic probe) for sampling.
This low cost deep ocean mapping and monitoring system (akin to a diving bell in design) has a small footprint and buoyancy controls allowing for short term and high frequency deployment of multiple probes across a large area.
Our initial probe will include a CDT, pH probe, dissolved oxygen probe and camera system.
The next iteration will include a sediment core and water sampler, using a smaller landing print than traditional lander systems currently use.
We will preserve and filter water samples in situ to generate an accurate snapshot of the genomic, transcriptomic, proteomic and metabolic signatures of the benthic microbial community, providing crucial information on ecosystem function and activity.
Ultimately through a combination of visual monitoring and the AlgaProbes we will measure and record:
- Dissolved Oxygen
- Doppler current meter
- Dissolved inorganic carbon (DIC)
- Particulate organic carbon (total amount, elemental composition, and size distribution)
- Macronutrients (Nitrate, nitrite, Ammonia, Phosphate)
- Demersal and mesopelagic zooplankton
- Microbial communities
- Arsenic concentration in the sediment
- Trophic Exchanges and food webs (using stable isotope - requires development)
- Passive particulate collection
We will use metagenomic analysis to study the prokaryotic and eukaryotic community response to the addition of seaweed biomass, which will provide vital information on both community composition and function. We’ll assess:
- Degradation of the seaweed biomass using hyperspectral imagery
- Trophic interactions using stable isotope analysis of microbes (with the aim of in-situ monitoring using a similar method as used by Amano et. al. 202223)
- Fauna within sediments, benthic, demersal and those organisms found on sunk Sargassum
This, combined with camera footage of any interactions, will allow us to quantify the durability of the carbon sequestered by our project.
Ocean and marine impacts
There are important questions that must be answered before large scale CDR can be undertaken using macroalgae.
Our knowledge and understanding of the deep ocean is limited by the challenges it presents in terms of distance to get there, light, depth (i.e. pressure) and temperature. Data collection at significant depths is challenging and expensive, which has curtailed extensive exploration of the seabed.
Understandably (given the impact human activity has already had on our planet), there is a clear desire to tread carefully when it comes to any kind of activity in these lesser known areas of the Earth. We need to be sure that any biomass placed in the deep ocean does not have any negative impacts or implications on the surrounding environment and organisms. We’ve spoken to a lot of scientists about this and we’re also working closely with our Science Advisory Board and Ocean Visions who are leaders in this area.
Our conclusion from these conversations is: we don’t know for sure. Some feel that naturally occurring biomass may stimulate deep ocean life, some worry it will impact it negatively somehow. Often this will depend on the amounts of additional biomass you’re talking about. Scientists studying the oceans often have wildly differing opinions. The reason for that is fairly simple - there just isn’t a lot of data, and there’s a general consensus that we’ve got more questions than answers.
Some of this we will look to answer with our pilot projects in the coming years, others will be the work of large scale modelling and monitoring by scientists in the space24. What we do know for sure is that Sargassum has and continues to destroy and disrupt massive areas of the coastal marine environment. It kills large numbers of animals each year25 in some of the most beautiful islands in the world. This includes impacts on turtle populations, coral reefs, and mangrove ecosystems.
Seaweed Generation is taking active measures in the design and operation of the AlgaRay to mitigate any additional impact on marine life which is already disrupted by the Sargassum inundations. The vessel moves slowly and the scoop action mimics being captured and eaten. The vast majority of marine life will easily swim away. We are commissioning reports on the current impacts of Sargassum and the future impacts of the AlgaRay on various marine species, conducting in situ observational studies on the presence of creatures in Sargassum mats along with our Science Advisor from the Antiguan Environmental Awareness Group, and working on further measures (lights, water disturbance) to evade marine life if our trials highlight a requirement to do so.
By combining cutting edge robotic technology with the power of Mother Nature, at both the surface and bottom of the ocean, Seaweed Generation will ensure that a responsible and sustainable approach is taken to combat the climate crisis, in a way which protects and works in harmony with the natural environment.
The IPCC’s highlights that even with rapid decarbonization the amount of CO2 already emitted, and current emission trends, will mean unavoidable climate impacts in the near term. Scaling up CDR solutions is now an urgent priority.
The State of Carbon Dioxide removal report published in Jan 2023 highlights that ‘virtually all scenarios that limit warming to 1.5°C or 2°C require “novel” CDR.’ Innovative, forward thinking initiatives are going to form a significant part of the Net Zero puzzle. Durable CDR with macroalgae has the potential to develop cost effectively and has a clear pathway to gigatonne scale. We believe it should be considered as part of the multi-faceted solution needed to tackle the climate crisis.
This document was developed by Seaweed Generation with expert advice, commentary and consultation provided from our Science Advisory Board and Eco Engineers. The views, information and opinions expressed are those of Seaweed Generation alone and do not necessarily reflect the views of all members of the SAB.
Baker, C. A., Martin, A. P., Yool, A., & Popova, E. (2022). Biological carbon pump sequestration efficiency in the North Atlantic: A leaky or a long-term sink? Global Biogeochemical Cycles, 36, e2021GB007286 ↩