Giant duckweed (Common Duckweed, Spirodela polyrhiza) is a fast-growing aquatic plant found all over the world. It produces a mat of greenery on the surface of slow-moving water bodies. Common Duckweed has begun to be used for creating biofuels, for bioremediation projects, and in carbon cycling systems due to its quick life cycle and small genome.
I. Appearance and Characteristics
Spirodela polyrhiza (orth. var. S. polyrrhiza) is a species of duckweed known by the common names common duckmeat, greater duckweed, great duckmeat, common duckweed, and duckmeat. It can be found nearly worldwide in many types of freshwater habitat.
Spirodela polyrhiza is found worldwide, namely in North America, Asia, more rarely in Central and South America, but also in Central Europe. It grows in tropical and temperate climates. It is not prevalent in New Zealand and only rarely in Australia.
Spirodela polyrhiza is a perennial aquatic plant usually growing in dense colonies, forming a mat on the water surface. Each plant is a smooth, round, flat disc 0.5 to 1.0 cm wide. Its upper surface is mostly green, sometimes red, while the lower surface is dark red. It produces several minute roots and a pouch containing male and female flowers. The top part dies in the fall and the plant often overwinters as a turion. The turion sinks to the bottom of the water body and stays in a dormant phase, until water temperature reaches 15 °C. The turions then germinate on the bottom of the water body and start a new life cycle. As this species lives in ponds and slow-moving water bodies, differs developmentally from terrestrial plants in morphology and physiology. It undergoes mainly vegetative growth in spring and summer, forming new fronds. Spirodela polyrhiza rarely flowers. In fall and winter it switches into a dormant phase represented by the turions due to nutrition starvation and freezing temperatures.
Because of its fast growth, direct contact with media and small genome size (~150 Mb), S. polyrhiza is an ideal system for biofuels, bioremediation, and carbon cycling. A comprehensive genomic study of S. polyrhiza was published in February 2014. The results provide insights into how this organism is adapted to rapid growth and an aquatic lifestyle.
II. How to Grow and Care
Sunlight
Giant duckweed thrives best under full sun conditions, which promotes vigorous growth and optimal health. The preferred intensity is bright, unfiltered daylight for maximum photosynthesis. While giant duckweed can tolerate some level of light variability, prolonged exposure to low light may impede growth and reduce vitality. In outdoor environments, giant duckweed should be positioned in open areas where direct sunlight is abundant. Adaptive traits such as a high growth rate allow giant duckweed to quickly exploit favorable light conditions, used effectively in waterbodies with minimal shading from surrounding vegetation.
Watering
Originating from calm freshwater habitats, giant duckweed thrives in environments with ample water availability. This species exhibits a preference for consistent moisture and is well-adapted to floating life, indicative of its resilience to fluctuating water levels. It requires watering every week to maintain its aquatic lifestyle. With an innate capacity for rapid surface coverage, giant duckweed can significantly influence the microclimate of its immediate surroundings by altering light penetration and water temperature. Typically found outdoors, giant duckweed plays a pivotal role in the ecosystem as it provides shelter and oxygenation to aquatic life, showcasing a symbiotic relationship where water is the central element.
Fertilizing
Fertilize giant duckweed biweekly with balanced nutrition liquid fertilizers to boost growth, vitality, and reproduction. Apply evenly across the water surface, adhering to specified dosages to avoid nutrient imbalance. For young giant duckweed, fertilization may encourage rapid development. During winter or dormant periods, reduce frequency. Prior to application, ensure fertilizer dissolves fully in water for safety and effectiveness. Monitor response and adjust as necessary for optimum plant health.
Propagation
Giant duckweed is a free-floating aquatic plant that naturally propagates primarily through vegetative means. Propagation is as simple as separating mature fronds; each fragment will develop into a new individual. This efficient process typically results in rapid coverage of available water surfaces, needing only water and light to thrive. For successful propagation, ensure the water quality is appropriate, as giant duckweed prefers nutrient-rich, still, or slow-moving waters. Minimal intervention is needed—simply distribute fragments in a suitable aquatic setting.
Transplanting
For giant duckweed, the prime time to relocate is during the vibrant growth period from late spring until the heart of summer, ensuring robust establishment. Opt for spots with ample water and moderate sunlight. Gentle handling during the move fosters success.
III. Uses and Benefits
- Bioremediation
Because of its capability to hyperaccumulate heavy metals and its high uptake of nutrients from the water, S. polyrhiza is used for bioremediation. The main pollutants it can be used to remediate are arsenic (As) and mercury (Hg) and common wastewater nutrients, like sulphate (SO42-), phosphate (PO43-) and nitrate (NO3−).
Arsenic
Greater duckweed showed accumulation of arsenic in laboratory tests. Arsenic uptake was found to be negatively correlated with phosphate and positively correlated with iron uptake. This indicates that phosphate and arsenic compete for uptake by S. polyrhiza, while arsenic’s absorption is facilitated by iron oxides, because it shows an affinity to the root surface of S. polyrhiza, where it is taken up. Greater duckweed is thought to detoxify the arsenic by reducing As (V) to the less toxic As (III). Difficulties arise with the management of the plants with high As contents. One possible use of the biomass containing As is production of charcoal and gas as a byproduct, which can be used as a fuel. The problems with this approach are low charcoal quality and high investments. Direct burning or burning of the coal is thought to release arsenic into the air, which would pollute the environment. Other options for fuel production would be hydrolysis and fermentation, which are economically not feasible. The biomass would have to be treated with strong acids and heat, which are both capital intensive. Briquetting is considered one of the best options, where the plants are dried and pressed into pellets of briquets. This raises the question of whether the arsenic is released back into the environment during the burning process. The production of biogas is also considered, but again, the redistribution of the As has to be avoided .
Mercury
Spirodela polyrhiza was found to be an efficient bio accumulator of mercuric chloride (HgCl2) in laboratory settings. Its plant biomass showed a 1000 times higher mercuric chloride concentration than its aquatic environment. Spirodela polyrhiza showed the highest accumulation factor compared to Lemna gibba and L. minor, which were also investigated.
- Urban wastewater treatment
dGreater duckweed has been used to remove common pollutants from wastewater. In a laboratory setting, S. polyrhiza showed a maximum of 90% removal efficiency of nitrate, 99.6% of phosphate and 69.8% of sulphate. The efficiency for all three pollutants combined was 85.6%, which makes it an environmentally and economically viable bioremediatory for wastewater treatment.
- Biofuel
Due to space-efficient starch production and good growth in animal wastewater, S. polyrhiza has great potential in bioethanol production. Despite environmental problems associated with production and competition from human and animal feed, corn is the main raw material for bioethanol. Spirodela polyrhiza could produce up to 50% more bioethanol on the same area. At the same time the production of bioethanol from S. polyrhiza is not in competition with human food. The production of bioethanol from S. polyrhiza is still in the development phase.
- Animal feed
In small-scale agriculture S. polyrhiza is used as fish or poultry feed. Due to its fast growth and high protein content, it is an interesting feedstuff. Because of sanitary problems and the risk of heavy metal accumulation, it is not yet used for feeding in larger animal husbandry systems. For rainbow trout, poorer growth rates were found when S. polyrhiza was added to the feed. For tilapia (Oreochromis niloticus L.), greater weight gains were found when 30% of the fish meal in the feed was replaced with S. polyrhiza. A review has also shown that duckweed can be used in cattle, pig and poultry diets. However, the problems of heavy metals and pathogen contamination occur.
- Human nutrition
Although other duckweed species, such as Wolffia arrhiza, are consumed by people in rural areas, S.polyrhiza is not cultivated for human consumption. This is because of high concerns about heavy metal accumulation and possible contamination with Escherichia coli or Clostridium botulinum. In contrast to W. arrhiza, S. polyrhiza contains, like most duckweed species, calcium oxalate crystals which are known to cause kidney stones.