Macroalgal Cultivation Research Articles

Modeling carbon dioxide removal via sinking of particulate organic carbon from macroalgae cultivation

Macroalgae cultivation is receiving growing attention as a potential carbon dioxide removal (CDR) strategy. Macroalgae biomass harvesting and/or intentional sinking have been the main focus of research efforts. A significant amount of biomass is naturally lost through erosion and breakage of cultivated or naturally growing seaweed, but the contribution of the resulting particulates to carbon sequestration is relatively unexplored. Here, we use a fully coupled kelp-biogeochemistry model forced by idealized parameters in a closed system to estimate the potential of macroalgal-derived particulate organic carbon (POC) sinking as a CDR pathway. Our model indicates that at a kelp density of 1.1 fronds m−3, macroalgal POC sinking can export 7.4 times more carbon to the deep sea (depths > 500m) and remove 5.2 times more carbon from the atmosphere (equivalent to an additional 336.0 gC m−2 yr−1) compared to the natural biological pump without kelp in our idealized closed system. The results suggest that CDR associated with POC sinking should be explored as a possible benefit of seaweed farming and point to the need for further study on organic carbon partitioning and its bioavailability to quantify the effectiveness and impacts of macroalgal cultivation as a CDR strategy.

Interactive Effects of Blue Light and Water Turbulence on the Growth of the Green Macroalga Ulva australis (Chlorophyta).

Macroalgal growth and yield are key to sustainable aquaculture. Although light and water turbulence are two important factors that affect algal productivity, research on their interaction is limited. Therefore, in this study, we investigated the effects of different wavelengths of light and the presence or absence of water turbulence on the growth of the green macroalga Ulva australis. Water turbulence was found to enhance the growth of U. australis irrespective of photosynthetic performance, but only in blue light cultures. The quantum dose of blue light required to induce 50% growth promotion was 1.02 mol m-2, which is comparable to the reported values for cryptochrome-mediated effects in other macroalgae. The combined effect of blue light and water turbulence led to the accumulation of photosynthesis-related proteins that support plastid differentiation and facilitate efficient photosynthesis and growth. Our findings thus highlight the potential of harnessing blue light and water turbulence to maximise macroalgal cultivation for sustainable and profitable algal aquaculture.

Synergistic effects of warming and low salinity enhanced the production of Gracilariopsis lemaneiformis in the warming drainage zone of Tianwan Nuclear Power Plant, China

Tianwan Nuclear Power Plant in China is the world's largest coastal nuclear power plant in terms of its total installed capacity both in operation and under construction. Its operations create a warming drainage zone that is potentially beneficial to the large-scale cultivation of macroalgae. Moreover, natural freshwater river flow and ocean acidification caused by increasing levels of atmospheric CO2 bring new uncertainties to the growth and photophysiology of commercial macroalgae. In this study, two CO2 concentrations (LC, 400 μatm; HC, 1000 μatm), two salinity levels (LS, 20 ppt; HS, 30 ppt), and two temperatures (LT, 20 °C; HT, 26 °C) were tested to examine the synergistic effects of warming and salinity on the economically important macroalga Gracilariopsis lemaneiformis under ocean acidification. The results showed that the synergistic effect of LS and HT treatments significantly promoted the growth of G. lemaneiformis under LC treatment, but this promotion effect was more obvious under HC treatment. In contrast, HSLT conditions inhibited G. lemaneiformis growth under both LC and HC conditions. Under HC treatment, warming and low salinity together increased the maximum photosynthetic rate and the maximum electron transport rate of G. lemaneiformis. Photosynthetic pigment analysis revealed that LS had a significantly negative effect on the contents of chlorophyll a (Chl a) and carotenoids (Car.), and HT had an obviously adverse influence on the contents of Chl a, Car., phycoerythrin, and phycocyanin. Moreover, the dual limitation of LSHT conditions significantly reduced the contents of Chl a and Car. under HC and LC conditions. These findings suggested that warming and low salinity synergically improved the growth and photosynthetic performance of G. lemaneiformis, especially under ocean acidification. The results provide a scientific basis for macroalgal cultivation as part of environmental restoration around nuclear power plants generally and year-round cultivation of G. lemaneiformis in northern China specifically.

Ocean dynamics and biological feedbacks limit the potential of macroalgae carbon dioxide removal

In combination with drastic emission reduction cuts, limiting global warming below 1.5 °C or 2 °C requires atmospheric carbon dioxide removal (CDR) of up to 16 GtCO2 yr−1 by 2050. Among CDR solutions, ocean afforestation through macroalgae cultivation is considered promising due to high rates of productivity and environmental co-benefits. We modify a high-resolution ocean biogeochemical model to simulate the consumption of dissolved inorganic carbon and macronutrients by idealised macroalgal cultivation in Exclusive Economic Zones. Under imposed macroalgal production of 0.5 PgC yr−1 with no nutrient feedbacks, physicochemical processes are found to limit the enhancement in the ocean carbon sink to 0.39 PgC yr−1 (1.43 GtCO2 yr−1), corresponding to CDR efficiency of 79%. Only 0.22 PgC yr−1 (56%) of this air–sea carbon flux occurs in the regions of macroalgae cultivation, posing potential issues for measurement, reporting, and verification. When additional macronutrient limitations and feedbacks are simulated, the realised macroalgal production rate drops to 0.37 PgC yr−1 and the enhancement in the air–sea carbon flux to 0.21 PgC yr−1 (0.79 GtCO yr−1), or 58% of the macroalgal net production. This decrease in CDR efficiency is a consequence of a deepening in the optimum depth of macroalgal production and a reduction in phytoplankton production due to reduced nitrate and phosphate availability. At regional scales, the decrease of phytoplankton productivity can even cause a net reduction in the oceanic carbon sink. Although additional modelling efforts are required, Eastern boundary upwelling systems and regions of the Northeast Pacific and the Southern Ocean are revealed as potentially promising locations for efficient macroalgae-based CDR. Despite the CDR potential of ocean afforestation, our simulations indicate potential negative impacts on marine food webs with reductions in phytoplankton primary production of up to −40 gC m−2 yr−1 in the eastern tropical Pacific.

Composite materials for innovative urban farming of alternative food sources (macroalgae and crickets)

Facing an inexorable growth of the human population along with substantial environmental changes, the assurance of food security is a major challenge of the present century. To ensure responsible food consumption and production (SDG 12), new approaches in the food system are required. Thus, environmentally controlled, sustainable production of alternative food sources are of key interest for both urban agriculture and food research. To face the current challenge of integrating food production systems within existing structures, multidisciplinary discourses are required. Here, we bring together novel technologies and indoor farming techniques with the aim of supporting the development of sustainable food production systems. For this purpose, we investigated the feasibility of 10 composite materials for their innovative use as structural support in macroalgal cultivation (settlement substrates) and cricket rearing (housing). Considering material resistance, rigidity, and direct material-organism interactions, the bio-based composite polylactic acid (PLA) was identified as a suitable material for joint farming. For macroalgae cultivation, PLA sustained the corrosive cultivation conditions and provided a suitable substrate without affecting the macroalgal physiology or nutritional composition (carotenoids and chlorophylls). For cricket rearing, PLA provided a suitable and recyclable shelter, which was quickly accepted by the animals without any observed harm. In contrast, other common composite components like phenolic resin or aramid were found to be unsuitable due to being harmful for the cultivated organisms or instable toward the applied sterilization procedure. This multidisciplinary study not only provides profound insights in the developing field of urban indoor food production from a new perspective, but also bridges material science and farming approaches to develop new sustainable and resilient food production systems.

Responsible supply systems for macroalgae: Upscaling seaweed cultivation in Ireland

To address societal challenges and attend to the increasing demand for seaweed while avoiding the over-exploitation of wild resources, the future expansion of the Irish seaweed industry depends on upscaling macroalgal cultivation. Through an interdisciplinary research approach, this study aimed to conduct an in-depth investigation of the prospects of upscaling seaweed aquaculture in Ireland through the lens of sustainability management. The identification of responsibilities of emerging enterprises was aligned with socio-ecological dimensions of sustainability and found to be related to: 1) the natural environment, 2) the development of rural coastal communities, 3) the engagement with local actors and 4) the customer segment. Moreover, seaweed cultivation still faces many uncertainties and bottlenecks from the operational up to institutional levels. Distinct from seaweed collected from the wild, cultivated seaweed has more phases added to the supply chain cycle. This includes, for example, the optimal site selection, the granting of an aquaculture licence and potentially social licence to operate, seed supply, and optimal timing for cultivation or harvest depending on the species, processing capacity and end-uses. The results contribute to an enhanced and holistic understanding of macroalgal cultivation in Ireland, as well as the social responsibility of emerging enterprises over socio-ecological systems and for the establishment of new supply chains based on seaweed. This can help to guide better decision-making of prospective growers, certification bodies, and policymakers in Ireland and beyond.

Modeling and Analysis of a Novel Offshore Binary Species Free-Floating Longline Macroalgal Farming System

Abstract The investigation of innovative macroalgal cultivation is important and needed to optimize farming operations, increase biomass production, reduce the impact on the ecosystem, and lower system and operational costs. However, most macroalgal farming systems (MFSs) are stationary, which need to occupy a substantial coastal area, require extensive investment in farm infrastructure, and cost high fertilizer and anchoring expenses. This study aims to model, analyze, and support a novel binary species free-floating longline macroalgal cultivation concept. The expected outcomes could provide a basis for the design and application of the novel MFS to improve biomass production, decrease costs, and reduce the impact on the local ecosystem. In this paper, Saccharina latissima and Nereocystis luetkeana were modeled and validated, and coupled with longline to simulate the binary species MFS free float in various growth periods and associated locations along the US west coast. The numerical predictions indicated the possibility of failure on the longline and breakage at the kelp holdfasts is low. However, the large forces due to an instantaneous change in dynamic loads caused by loss of hydrostatic buoyancy when the longline stretches out of the water would damage the kelps. Buoy-longline contact interactions could damage the buoy, resulting in the loss of the system by sinking. Furthermore, the kelp-longline and kelp-kelp entanglements could potentially cause kelp damage.

Assessing the potential for sea-based macroalgae cultivation and its application for nutrient removal in the Baltic Sea

Marine eutrophication is a pervasive and growing threat to global sustainability. Macroalgal cultivation is a promising circular economy solution to achieve nutrient reduction and food security. However, the location of production hotspots is not well known. In this paper the production potential of macroalgae of high commercial value was predicted across the Baltic Sea region. In addition, the nutrient limitation within and adjacent to macroalgal farms was investigated to suggest optimal site-specific configuration of farms. The production potential of Saccharina latissima was largely driven by salinity and the highest production yields are expected in the westernmost Baltic Sea areas where salinity is >23. The direct and interactive effects of light availability, temperature, salinity and nutrient concentrations regulated the predicted changes in the production of Ulva intestinalis and Fucus vesiculosus. The western and southern Baltic Sea exhibited the highest farming potential for these species, with promising areas also in the eastern Baltic Sea. Macroalgal farming did not induce significant nutrient limitation. The expected spatial propagation of nutrient limitation caused by macroalgal farming was less than 100–250 m. Higher propagation distances were found in areas of low nutrient and low water exchange (e.g. offshore areas in the Baltic Proper) and smaller distances in areas of high nutrient and high water exchange (e.g. western Baltic Sea and Gulf of Riga). The generated maps provide the most sought-after input to support blue growth initiatives that foster the sustainable development of macroalgal cultivation and reduction of in situ nutrient loads in the Baltic Sea.

A Macroalgal Cultivation Modeling System (MACMODS): Evaluating the Role of Physical-Biological Coupling on Nutrients and Farm Yield

Offshore aquaculture has the potential to expand the macroalgal industry. However, moving into deeper waters requires suspended structures that will present novel farm-environment interactions. Here, we present a computational modeling framework, the Macroalgal Cultivation Modeling System (MACMODS), to explore within-farm modifications to light, seawater flow, and nutrient fields across time and space scales relevant to macroalgae. A regional ocean model informs the site-specific setting, the Santa Barbara Channel in the Southern California Bight. A fine-scale hydrodynamic model predicts modified flows and turbulent mixing within the farm. A spatially resolved macroalgal growth model, parameterized for giant kelp, Macrocystis pyrifera, predicts kelp biomass. Key findings from model integration are that regional ocean conditions set overall farm performance, while fine-scale within-farm circulation and nutrient delivery are important to resolve variation in within-farm macroalgal performance. Therefore, we conclude that models resolving within-farm dynamics can provide benefit to farmers with insight on how farm design and regional ocean conditions interact to influence overall yield. Here, the presence of repeating longlines aligned with the mean current generate flow diversions around the farm as well as attached Langmuir circulations and increased turbulence intensity. These flow-induced phenomena lead to less biomass in the interior portion of the farm relative to the edges. We also find that there is an effluent “footprint” that extends as much as 20 km beyond the farm. In this regard, MACMODS can be used to not only evaluate farm design and cultivation practices that maximize yield but also explore interactions between the farm and ecosystem in order to minimize impacts.

Quantifying habitat provisioning at macroalgal cultivation sites

AbstractMacroalgal cultivation is expanding rapidly, and promises to contribute significantly towards future food and energy security, sustainable livelihoods, ecosystem services and habitat provisioning for a range of associated organisms globally. Habitat provisioning underpins biodiversity and ecosystem structure and functioning, supports many ecosystem services and has possible benefits to other marine industries, including enhancement of commercial fish stocks. In macroalgal cultivation, however, only recently has habitat provisioning started to be assessed at a local scale (within a farm's footprint) and with a range of different approaches. This review evaluates techniques used to quantify habitat provisioning in and around macroalgal cultivation sites, for species ranging from microorganisms to megafauna, and outlines recommendations to enable a more comprehensive ecological valuation of macroalgal cultivation in the future. The majority of information on biodiversity associated with macroalgal cultivation is associated with quantifying biofouling or pest organisms, rather than the contribution of colonising species to healthy ecosystem functioning. We suggest how better monitoring of macroalgal cultivation could enable an ecosystem approach to aquaculture (EAA) in the future. To achieve this, we highlight the need for standardised and robust methods for quantifying habitat provisioning that will enable assessment and monitoring of macroalgal cultivation sites of varying scales and within different regions and environmental settings. Increased evidence for the potential habitat value of macroalgal cultivation sites will help inform and shape marine legislation, licencing and certification for macroalgal farmers and potentially reduce marine user conflicts, helping the industry to continue to grow sustainably using EAA.

Comparison studies of epiphytic microbial communities on four macroalgae and their rocky substrates

Macroalgae and their rocky substrates both support diverse and abundant microbiota, performing essential ecological functions in marine ecosystem. However, the differences in the epiphytic microbial communities on macroalgae and rocky substrate are still poorly understood. In this study, the epiphytic microbial communities on four macroalgae (Corallina officinalis, Rhodomela confervoides, Sargassum thunbergii, and Ulva linza) and their rocky substrates from Weihai coast zone were characterized using high-throughput sequencing technology. The results showed that the alpha diversity indices were greater in rocky substrates than that in macroalgae. The microbial similarities among macroalgae and rocky substrate groups tended to decrease from the high taxonomic ranks to lower ranks, only 22.69% of the total amplicon sequence variants (ASVs) were shared between them. The functional analysis revealed that the microbiotas were mainly involved in metabolic activities. This study would provide the theoretical foundation for macroalgal cultivation and algal reef applications.

Automation Concepts for Industrial-Scale Production of Seaweed

In order to industrialize macroalgal cultivation in Norway, new automated methods and solutions for seeding, deployment and harvesting need to be developed. Today's solutions are time and resource demanding, still yielding volumes nationally in the range of 100–200 tons per year in total (not including wild harvest), while the potential is in the megaton range. Standardization of equipment and automation can be one way to upscale production. Here we present results from a design study of a module-based solution for industrial cultivation, with specific solutions for spinning of thin seedling strings onto longlines, and a robotic module for interaction with the submerged farm at deployment and harvest. A reduced-scale physical prototype of the farm concept with the robot has been built for testing of deployment and harvesting techniques. The concept has been named SPOKe: Standardized Production of Kelp.

Removal of metals from aqueous solutions using dried Cladophora parriaudii of varying biochemical composition

Macroalgal biosorption has shown promise for the removal of metal ions from wastewaters, whose presence can pose a threat to the aquatic environment. There is a wealth of literature published on macroalgal biosorption, the common thread being that the biosorbent material was collected from the field, under undefined conditions. These studies offer little insight into the impact of prior cultivation or biomass production practices upon the biosorbent material, its adsorptive physico-chemical properties and its subsequent capacity for metal removal. The present study sought to investigate the influence of changes in macroalgal cultivation, specifically nutrient regime, upon biomass properties and the resultant adsorption performance. The macroalga Cladophora parriaudii was cultivated under six different nutrient regimes; 2:1 and 12:1 N:P molar ratios, with nitrogen supplied either as ammonium (NH4+), nitrate (NO3−), or urea (CO(NH2)2). These nutrient regimes were designed to produce biomass of varying biochemical and cell surface profiles. After cultivation, the biomass was rinsed, dried, biochemically analysed and then used for the removal of four individual metals from solution. Metal removal varied considerably between treatments and across initial metal concentrations, with removal values of 46–85%, 9–80%, 8–71%, and 49–94% achieved for Al, Cu, Mn, and Pb, respectively, with initial metal concentrations varying between 0 and 150 mg L−1. The observed variation in metal removal can only be attributed to differences in biochemistry and cell surface properties of the biosorbent induced by nutrient regime, as all other variables were constant. This study demonstrates that prior cultivation conditions influence the biochemistry of a biosorbent material, namely macroalgae Cladophora parriaudii, which has an impact upon metal removal. This aspect should be given due consideration for future biosorption research and when reviewing already published literature.

Simulating the Trajectory and Biomass Growth of Free-Floating Macroalgal Cultivation Platforms along the U.S. West Coast

Trajectory tracking and macroalgal growth models were coupled to support a novel macroalgae-harvesting concept known as the Nautical Off-shore Macroalgal Autonomous Device (NOMAD). The NOMAD consists of 5 km long carbon-fiber longlines that are seeded and free float southward along the U.S. West Coast for approximately 3 months before harvesting off the California coast, taking advantage of favorable environmental conditions. The trajectory and macroalgal growth models were applied to answer planning questions pertinent to the techno-economic analysis such as identifying the preferred release location, approximate pathway, timing until harvest, and estimated growth. Trajectories were determined with the General NOAA Operational Modeling Environment (GNOME) model, using 11 years of current and wind data, determining probabilities by running nearly 40,000 Monte Carlo simulations varying the start time and location. An accompanying macroalgal growth model was used to estimate the growth of macroalgae based on the trajectory tracks and environmental forcing products, including light, temperature and nutrients. Model results show that NOMAD lines transit south in the months of April to September due to seasonal currents, taking approximately 3 months to reach Southern California. During transit, NOMAD lines are dispersed but typically avoid beaching or passing through marine sanctuaries. NOMAD lines can yield up to 30 kg wet weight per meter of cultivation line.

Technical challenges for offshore cultivation of kelp species: lessons learned and future directions

Abstract Traditional kelp farming methods require a high amount of labor and are limited in geographic distribution – occurring mainly in nearshore, sheltered sites. To address growing global demand for sustainable biomass, the continued expansion of kelp cultivation will most likely have to move further offshore. Although many offshore cultivation trials have been done over the last 50 years, few were sufficiently robust to be viable in exposed and deep-water areas. In the North Atlantic Ocean, a Faroese company developed and tested a structural farm design that has survived in open-ocean conditions since 2010. The durable structure has withstood harsh weather events common in the Faroe Islands and thereby presents a potential strategy and method for moving kelp farming further offshore. This paper describes the primary challenges of offshore kelp farming and provides an overview of work previously done. Ultimately, the improved productivity, system survivability and scalability the MacroAlgal Cultivation Rig (Faroe Islands) and the BioArchitecture Lab cultivation grid (Chile) represent state-of-the-art and powerfully transformative strategies to pursue large-scale offshore farming to support mass production of kelp in the near future.

Challenges for marine macroalgal biomass production in Indian coastal waters

Abstract Due to its large, exclusive economic zone, India has considerable potential for implementing large-scale cultivation of macroalgae. However, such cultivation requires the availability of, and access to, sites where technical, legal, governmental, and environmental factors are favorable. This review discusses the challenges that have held back the development of seaweed cultivation in India. The review is based on a literature survey and informal discussions with industry-related personnel. It cites the strong need for clear and definitive policies related to access to and use of coastal waters to enable the Indian seaweed industry to reach its full potential. The main challenges that the expansion of macroalgal cultivation in India face are related to legal and regulatory aspects that can be resolved by focusing the policy issues on providing planning tools toward success. In addition, there is a strong need for an adequate bioeconomy that clearly defines the need for marine macroalgal biomass for food, chemicals, and biofuels. Furthermore, the Indian government needs to allocate sufficient funds for accelerating seaweed R&D in areas of seaweed cultivation, harvesting, processing technologies, and their implementation in the local industry.

Novel species of the oomycete Olpidiopsis potentially threaten European red algal cultivation

The rapid growth of marine macroalgal cultivation amplifies the potential impacts of seaweed diseases. Here, we combine microscopy and molecular analysis to describe two novel European species, Olpidiopsis palmariae and O. muelleri spp. nov., that infect the commercially important red algae Palmaria and Porphyra, respectively. A Scottish variety of Olpidiopsis porphyrae, a devastating pathogen of Pyropia previously thought to be restricted to Japanese seaweed farms, is also described as O. porphyrae var. scotiae. In the light of their destructiveness in Asian farms, together with the global expansion of algal cultivation and pertaining seed trade, Olpidiopsis pathogens should be treated as a serious threat to the sustainability of red algal aquaculture. Our findings call for the documentation of seaweed pathogens and the creation of an international biosecurity framework to limit their spread.

Culture of Chondracanthus teedei and Gracilariopsis longissima in a traditional salina from southern Spain

The cultivation of two red macroalgal species, Chondracanthus teedei (Martens ex Roth) and Gracilariopsis longissima (S.G. Gmelin) Steentoft M, L.M. Irvine & W.F. Farnham, was assessed in a traditional salina, a system of earthen ponds used for marine salt extraction taking advantages of solar evaporation and tidal cycle. Vegetative thalli of both species were cultivated in rafts holding polypropylene ropes, from January to June 2015, when lock-gates were opened during the period of no salt production. The effects of three factors in the net growth rate were analysed: seedling density, water motion and seasonality. Water motion and seasonality showed a significant effect in the growth of both species. Seedling density only showed a significant effect in the growth of Gp. longissima, where the growth rates improved at high seedling densities. Values of tissue N were generally lower than critical quotas, suggesting that maximum growth was limited by the concentrations of dissolved nutrients. In addition, the high salinity and temperatures in late spring seemed to condition the values of net growth rate. The study suggested that macroalgal cultivation of these two valuable species could be a promising complementary activity in the integrated management of the salina during winter and early spring, when salinity is lower than 40 PSU, if nutrients in the water are increased with the semi-intensive fish cultivation and the hydrodynamic conditions along the rafts are enhanced.

Production method and cost of commercial-scale offshore cultivation of kelp in the Faroe Islands using multiple partial harvesting

The current work aimed to develop a cultivation method for macroalgae that can be applicable and economically profitable in the Atlantic Ocean. An offshore long-line macroalgal cultivation rig was designed by Ocean Rainforest Sp/f, tested in the Faroe Islands from 2010, and found suitable for cultivation in exposed and deep-water locations (water depth > 50 m). The economic risk related to lost cultivation structures was hereafter considered to be low. Saccharina latissima and Alaria esculenta were cultivated in commercial scale (5 km of growth lines). A high cost of seeding material and cost of deployment was reduced by testing multiple partial harvesting. Four non-destructive harvests were carried out in a two-year growth period without re-seeding of lines. In total, 3.2 t dry weight (dw) biomass was harvested and sold to customers within the food and cosmetic industries. The productivity was 1437.5 kg dw ha−1 yr−1 (including handling space). The 10-meter vertical growth lines had an average yield of 0.29 kg dw m−1 per harvest and four partial harvests were made over a 2-year period. An economic analysis showing the cost structure of important aspects of offshore macroalgae cultivation was conducted. The total cost per kg dw of cultivated S. latissima decreased when the number of possible harvests without re-seeding was increased (from € 36.73 to € 9.27). This work has demonstrated that large-scale kelp cultivation is possible using multiple partial harvesting in the Faroe Islands, and highlighted the need for further innovation to lower the cost per unit macroalgal produced.

Global potential of offshore and shallow waters macroalgal biorefineries to provide for food, chemicals and energy: feasibility and sustainability

Displacing fossil fuels with renewables and increasing sustainable food and chemicals production are among the major challenges facing the world in the coming decades. Integrating climatological oceanographic data with a metabolism and growth rate model of the green marine macroalga from Ulva genus, we analyze the potential of offshore biorefineries to provide for biomass, ethanol, butanol, acetone, methane and protein, globally and in 13 newly defined offshore provinces. We show that for optimum fresh weight stocking density of 4kgm−2 the total potential of offshore cultivated Ulva biomass is of the order of 1011 dry weight (DW) ton year−1, over a surface area of ~108km2. We found that the distance of the offshore cultivation site to the processing facility is limited to 114–689km, depending on cargo moisture content. The near-future technologically deployable areas, associated with up to 100m water installation depth, and 400km distance from the shore, can provide for 109 DW ton year−1, which is equivalent to ~18EJ. This has the potential to displace entirely the use of fossil fuels in the transportation sector or provide for 5–24% of predicted plant proteins demand in 2054. In addition, we modeled the potential production of ethanol, butanol, acetone and methane from the offshore produced biomass. Finally, we analyzed the environmental risks and benefits of large-scale offshore macroalgal cultivation. These results are important as they show for the first time the potential of offshore biomass cultivation to reduce the use fossil fuels and arable land to provide for food, chemicals and fuels required for the society.