Introduction
The production of the macroalga Macrocystis pyrifera is mainly associated with the demand for alginate extraction (V��squez et al., 2014), driving advances in cultivation techniques to improve its growth. The availability of nitrogen is essential for tissue formation, energy conversion , and storage in vacuoles and chloroplasts (Harrison and Hurd, 2001). Rapid growth during early stages of production is essential for the viability and productivity of the culture. Consequently, the addition of commercial fertilizers to culture media can influence production costs. As an alternative to external inputs, Integrated Multitrophic Aquaculture (IMTA) recycles nutrients from the main species cultured into biomass of low-trophic-level organisms, such as macroalgae (Neori et al., 2004). Therefore, this study evaluated whether increasing nitrogen availability in a continuous-flow system with turbot (Scophthalmus maximus) affects the growth of two M. pyrifera morphotypes (few- and multiple-frond seedlings) in outdoor tanks and whether these effects persist after transfer to suspended longline culture at sea.
Materials and methods
In the winter of 2026, a two-part experiment was set. The first part lasted 21 days and consisted of two treatments with four replicates each: natural water (NW) and integrated aquaculture (F). The NW treatment used 1,000-L tanks with constant aeration and seawater recirculation, whereas the F treatment integrated 1,000-L macroalgae tanks with 1,500-L turbot tanks, with continuous water flow from the fish to the macroalgae tanks. Sixty juvenile M. pyrifera sporophytes per replicate, representing two morphotypes (few- and many-frond) (Buschmann et al., 2020), were used. Ten sporophytes of each morphotype were tagged for weekly weight monitoring. In the second-part experiment, the 20 tagged sporophytes from each tank were transferred to a longline at 1.5 m depth, and cultivated for 30 days. Forty sporophytes were collected and weighed after 15 days, and the remaining individuals after 30 days. Average weight gain (g) and SGR (% day-1) were calculated. Water quality (ammonia, nitrite, nitrate, and phosphate) was monitored weekly during the experiment. Statistical analyses were performed using PAST v4.03. Data normality and homoscedasticity were assessed using the Shapiro–Wilk and Levene tests, respectively. One- or two-way ANOVA followed by Tukey's test was used when assumptions were met, with p ≤ 0.05.
Results
Ammonia concentrations differed significantly between treatments on days 14 (NW 26.6 ± 1.7 b; F 34.0 ± 1.3 a µmol L-1) and day 21 (NW 25.3 ± 1.7 b; F 31.8 ± 1.8 a µmol L-1) in the tank culture. Nitrate concentrations differed significantly between treatments at the beginning of the cultivation ( NW13.2 ± 2.8 b; F 18.9 ± 1.8 a µmol L-1). For phosphorus, significant differences were found on Day 0 (NW 2.4 ± 0.0 a; F 1.0 ± 0.5 b µmol L-1) and on Day 21 (NW 2.3 ± 0.8 b; F 3.1 ± 0.1 a µmol L-1). Weight gain did not differ significantly between treatments during the 21-day tank culture (Table 1). In contrast, during longline culture, the few-frond morphotype showed higher weight gain and specific growth rate in the F treatment than in the NW treatment (Table 1).
Table 1. Performance of Macrocystis juveniles in natural seawater (NW) and fish tank water (F) during the 21 days they were cultivated in land tanks and 30 days on a longline at sea.
Land-based tanks (21 days)
Longline (30 days)
Few fronds
Seawater - NW
Fish tank - F
Seawater - NW
Fish tank - F
Avarage weight gain (g)
3.81 ± 1.64
3.61 ± 1.24 B
11.26 ± 5.08 B b
26.76 ± 10.61a
SGR (% day-1)
2.19 ± 1.26
2.49 ± 0.94
2.35 ± 1.05 b
4.43 ± 0.49 a
Many fronds
Avarage weight gain (g)
5.21 ± 2.05
6.14 ± 2.13 A
37.00 ± 17.96 A
35.13 ± 7.34
SGR (% day-1)
1.86 ± 0.61
2.19 ± 1.08
3.92 ± 1.60
3.62 ± 0.72
SGR specific growth rate. Different letters in the same line indicate a significant difference between treatments. Capital letters in the same column indicate a significant difference within the same treatment between groups of fronds.
Discussion
The higher ammonia and nitrate concentrations in treatment F resulted from animal excretion and feed leaching (Silva et al., 2013), compared to with seawater. During land-based cultivation, the higher growth of the "many fronds" morphotype in treatment F may indicate a competitive advantage resulting from enhanced light interception due to its higher number of fronds, potentially increasing shading and limiting light availability to the "few fronds" morphotype. The insignificant difference between treatments can be explained by space limitations as macroalgal biomass increased (Xiao et al., 2019). In contrast, under longline cultivation, the higher weight gain in treatment F may be associated with the higher nitrogen retention achieved during the tank cultivation phase, which may have promoted subsequent macroalgal growth (Harrison and Hurd, 2001). With the increased space available, the "few fronds" morphotype showed growth similar to that of the "many fronds" morphotype in the treatmente F.
Acknowledgment
The authors are grateful for the financial support provided by the Co-ordination for the Improvement of Higher- Level Personnel (CAPES), Process number 88881.243199/2025-01 and 88887.138094/2025-00.
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