Biochar in Kelp and Seaweed Habitat Restoration

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Ocean Warming and Ocean Acidification are two representative challenges of Kelp Aquaculture.(Zi-min Hu et al., 2021). Even when propagule supply is not limiting, kelp may fail to establish because of localized abiotic or biotic processes affecting settlement or post settlement survival of recruits.(Morris et. al., 2020).  The Structural complexity of artificial substrates can greatly influence kelp recruitment (Perkol-Finkel et al. 2012), artificial structures could be ecologically engineered (Chapman and Underwood 2011) to incorporate suitable structural complexity.

A potential solution to overcome heat stress effectively while maintaining growth of kelp species is to use a solution of commercial seaweed extracts (biostimulants), to dip juvenile sporophytes before cultivation in the open sea (Umanzor et al., 2019). Alternatively, providing sufficient nitrogen (i.e. NO3−) can help kelp to ameliorate temperature-dependent responses such as growth and photosynthesis, thus enhancing its tolerance and acclimation ability to thermal stress (Fernández et al. 2020).   A study on P. hecatensis (pacific red seaweed) demonstrated that treatment with Sargassum horneri extract (Seaweed Extract)can significantly enhance the species thermal tolerance, growth and biochemical resilience under high temperatures, which is especially important as rising seawater temperatures due to climate change threaten seaweed aquaculture (Kim et al.2024). 

Surface Area

An important consideration is the functional surface area of the biochar. The greater the porosity or surface area, the more nutrients can be retained. The surface area is highly dependent on feedstock and operating conditions, and typically an increase in pyrolysis temperature as well as a high feedstock lignin content is associated with high surface areas. Depending on these factors, biochar can have a surface area of between 20 and 130 m2/g (Tomczyk,  Sokolowska, & Boguta, 2020).The surface area can be enhanced by methods such as steam activation. This process involves exposing the biochar to high-temperature steam at temperatures between 750˚C and 850˚C and can double the biochar surface area (Shim, Yoo, Ryu, Park, &  Jung, 2015). The flame cap kiln method requires quenching with water to stop the combustion process so this method of biochar production has the added benefit of introducing introduce steam to the char.

Algal biomass contains little or no lignin, which enables easy hydrolysis of the biomass for subsequent hydrogen fermentation (Ao Xia a et al., 2015). The addition of Biochar in dark fermentation has increased rate and yield of hydrogen production from 26 % and 41 %, (Tianru Lou et al,. 2024) The mechanisms that boost hydrogen in dark fermentation shed light on advantages biochar can provide as a supplemental material for cultivating kelp gametophytes and sporophytes. Those mechanisms include moderating pH, stimulate the growth of functional microorganisms, moderation of mitigating factors of hydrogen production (biotic and abiotic) and facilitation of electron transfer (Tianru Lou et al,. 2024).

Biochar without post-treatment typically contains only small amounts of nutrients like nitrogen, phosphorous and potassium. Kelp growth requires dissolved Nitrogen in bioavailable forms such as nitrate and ammonium, as well as Phosphorous, Potassium, Calcium, Magnesium, Sulphur, Iron, Iodine, Copper, Boron and Molybdenum (Zhang, et al., 2021).Nutrient impregnation is a common method of improving the effectiveness of biochar as a soil amendment in agriculture, and the same method can be used for kelp cultivation (Osman, et al., 2022).

The critical aspect in question is whether it is more effective to impregnate the biochar with the required bio-stimulants in the hatchery and allowing nutrients to release gradually, or to apply the biochar without impregnation in the marine environment and assess its ability to capture and retain nutrients in the substrate.  Studies on this have never been done so to explore this so a Biochar Aquarium was set up at the Vancouver Island Sea Salt Company.

A biochar aquarium was assembled with the four plastic trays placed in two rows, one above the other on a wood frame above a 55 gallon reservoir barrel. Water was pumped from the barrel, though an aquarium chiller and split between the top two trays. Water flowed from an inlet from one end of the tray to an outlet on the opposite side of the tray where it drained into the tray below where the process was repeated on the lower level. The water then drained into the reservoir barrel where mixing of saltwater ingredients and marine nutrients occurred.  

Levels of nutrient absorption will very depending on where and when biochar is placed in the marine environment. Can this passive functional activation of biochar be enough to induce thermal tolerance in gametophytes and sporophytes retained on the supplemental material?  Can gametophytes and sporophytes even access the nutrient contained in the char?  To address these questions a study was sponsored at North Island College where two kinds of biochar were tested, one that had been functionally activated with marine nutrients (see image to left) and one that had been left empty.  The functionally activated biochar had been soaked in a low grade nutrient solution for approximately six months and a brown film was observed to have formed within the capillary structure of the biochar. 

The study at North Island College found that "Nutrient Tanks had slightly higher initial growth rates: the sporophytes established quicker, and gametophytes grew to larger sizes faster." (Zienhart et al 2025) This demonstrates gametophyte response to the weak bio stimulant contained in the char.    Important research questions to consider is can this seaweed extract pre-treatment mitigation strategy be enhanced using biochar to induce thermal tolerance in gametophytes?  What other kelp and seaweed species respond to this pretreatment?  How much seaweed extract is required to induce adequate heat tolerance?

Based on these trials biochar appears to be a suitable substate for kelp growth. However buoyancy is a concern with the light weight of biochar as a use of green gravel.  This concern is exacerbated when coupled with the positive buoyancy of kelp. (Zienhart et al 2025) This is where biochar's small and variable size can be used as an advantage as it is not necessarily used an an anchor but a vector material to get thermally adapted gametophytes on the bottom with the goal that they will quickly outgrow their small piece of biochar and attach themselves to a more substantial rock on the sea floor.    Kelp reefs play an important role in coastal flood mitigation through their ability to mitigate wave energy but also by depositing copious amounts of organic material on the beach.  This nourishes the shoreline and helps retain sediment higher up on land to armour the shoreline in high wind weather events.  Utilizing biochar in a green gravel program has the potential of not only establishing heat tolerant individuals to augment the kelp reef population but the pieces that do grow out and float away will still benefit ecology and shoreline resilience.

 A sample of washed up seaweed was taken from the Saratoga Beach with a total of 30 specimens observed that day.  The holdfasts on all these seaweed on rock samples were on average 2.5 cm2.  This suggests using pieces of biochar <2.5 cm3 will be small enough for the seaweed holdfast to outgrow it's supplemental biochar.

The smaller the piece of biochar the quicker the sporophyte holdfast is likely to establish on the seafloor and neighboring rocks.   The NIC Study demonstrated the small .2.5-6cm3 pieces of biochar had 25+ gametophytes while the large rocks recruited hundreds of gametophytes and in a green gravel program, only one of those sporophytes would ever survive.  Using much smaller pieces of biochar (as low as 2.5mm) should be explored to increase Gametophyte utilization.  Fish and shellfish lay millions of eggs because even in perfect conditions survival is difficult so exponentially increasing the gametophyte utilization of green gravel activities raises the individual piece count available for restoration.  Just comparing the piece count in the North Island College there are only 5 viable pieces in the picture frames with rocks compared to hundreds of viable pieces in picture frames with biochar.
 

 Can the large surface area of biochar provide a reservoir for the seaweed microbiome to augment halobiont recovery from heat stress events?