Algae as a biostimulant and possible effects on the root microbiome

The year 2023 came to a close on a high note, with our first master's student successfully defending their thesis: Using algae as a biostimulant. In their thesis, the student looked at how powdered algae included in growth media would affect plant growth of two different micro-tomato cultivars: ‘Jochalos’ and ‘Micro Gemma’. Due to low nitrogen content in the algae, and the volume needed to use it as a stand-alone fertiliser, we tested the effect of the algae as a biostimulant, rather than as the sole nutrient source. We tested it in peat, a commercial compost mix, and in a hydroponic system. The results were very interesting!

Algae as a biostimulant means more plant growth

Micro-tomatoes have determinate growth, meaning they grow to a certain height rather than continuously growing. This made the study very interesting, as the algae's effects were clear in how the plants grew, branched, and produced flowers and fruit. As the plants are determinate, the number of sideshoots determines the number of flower clusters and potential for producing fruit. Plants grown using algae as a biostimulant increased the production of sideshoots, thereby increasing the number of leaves, flower clusters, and fruits on these plants. This was most likely because the plants with algae had better nutrient uptake, despite the negligible amount of nutrients provided by the algae. The compost and peat treatments performed equally well, meaning that algae has potential for use as a biostimulant in a more sustainable growth medium – good news for the environment and our goals for sustainability and circular economy.

Interestingly, including algae in the water of a hydroponic system had the opposite effect than in the solid growth media. We gave them three different fertiliser regimes: only mineral fertiliser; mineral fertiliser and algae; only algae. Despite both treatments with mineral fertiliser receiving the same amount and having the same amount of nutrients available, the plants grown with algae in the water were severely inhibited in their growth for several weeks (Picture 1).

Picture 1. Tomato plants – roots and their respective shoots. Grown in different fertiliser regimes, from left to right: Algae only; algae and mineral fertiliser; mineral fertiliser only. All plants were sown 41 days before the pictures were taken. Photos: Sheona Innes.

We are now looking into potential reasons for why the algae may have inhibited tomato growth in the hydroponic system, and algal samples will soon be sent for hormone screening.

Future experimentation

In a further experiment, we are investigating how the inclusion of algae in solid growth media affects the microbiome surrounding the roots by looking at the bacterial and fungal diversity in the algae, peat growth media, and the two combined. In addition, we have included a treatment where we combined the growth media with algal biochar produced by WP3 to investigate whether the effects will differ. Samples were taken at two time points to investigate changes with time. The plants are also growing to maturity to investigate their growth (Picture 2). DNA samples harvested from the growth media are currently being sequenced and will be analysed in March through our collaboration with Flinders University in Adelaide, Australia. Our WP4 post-doc will be visiting Associate Professor Martin Breed – an expert in environmental genomics - at his lab and learning the bioinformatic techniques needed for processing so-called metabarcoding data. This is a fascinating experiment and collaboration, and we look forward to seeing the results!

Picture 2. Side and top views of Crispi lettuce plants from left to right: Plants grown in peat only; peat combined with powdered, freeze-dried algae; peat combined with algal biochar from WP3. Photos: Sheona Innes.