Abstract
Filamentous species of Ulva are ideal for cultivation because they are robust with high growth rates and maintained across a broad range of environments. Temperate species of filamentous Ulva are commercially cultivated on nets which can be artificially ‘seeded’ under controlled conditions allowing for a high level of control over seeding density and consequently biomass production. This study quantified for the first time the seeding and culture cycle of a tropical species of filamentous Ulva (Ulva sp. 3) and identified seeding density and nursery period as key factors affecting growth and biomass yield. A seeding density of 621,000 swarmers m-1 rope in combination with a nursery period of five days resulted in the highest growth rate and correspondingly the highest biomass yield. A nursery period of five days was optimal with up to six times the biomass yield compared to ropes under either shorter or longer nursery periods. These combined parameters of seeding density and nursery period resulted in a specific growth rate of more than 65% day−1 between 7 and 10 days of outdoor cultivation post-nursery. This was followed by a decrease in growth through to 25 days. This study also demonstrated that the timing of harvest is critical as the maximum biomass yield of 23.0±8.8 g dry weight m−1 (228.7±115.4 g fresh weight m−1) was achieved after 13 days of outdoor cultivation whereas biomass degraded to 15.5±7.3 g dry weight m−1 (120.2±71.8 g fresh weight m−1) over a longer outdoor cultivation period of 25 days. Artificially seeded ropes of Ulva with high biomass yields over short culture cycles may therefore be an alternative to unattached cultivation in integrated pond-based aquaculture systems.
Highlights
The global production of seaweeds of 19 million tonnes is dominated by only five key species and their products are targeted for food and phycocolloid markets [1]
This study demonstrates the importance of seeding density and nursery period on the biomass yield and growth rate of Ulva sp. 3 when artificially seeded onto ropes
Seeding density was a key factor affecting both growth rate and biomass yield of Ulva sp. 3 confirming previous studies, where the biomass and size of seaweeds was related to their settlement density [27,30,34]
Summary
The global production of seaweeds (marine macroalgae) of 19 million tonnes is dominated by only five key species and their products are targeted for food and phycocolloid markets [1]. While less than 0.1% of total production is accounted for by green seaweeds [2], species within the genus Ulva have in many ways the most compelling characteristics for biomass applications in their cosmopolitan distribution [3], very high growth rates [4], high stocking densities [5,6], and wide environmental tolerances for year round production [7,8]. The existing commercial production of Ulva is either focussed on niche, high-value food products in Japan using controlled seeding with small production volumes (‘aonori’; ,1,500 tonnes dry weight per annum; [9]) or on integrated aquaculture in land-based high rate algal ponds in South Africa using vegetative cultivation of biomass as abalone feed (,1,000 tonnes fresh weight per annum; [10]). To ensure a reliable and cost-effective biomass supply of Ulva, it is essential to develop and optimise the closed-life cycle production and yield of biomass, as a fundamental basis for commercial applications and more widespread production throughout the globe
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