This project aimed to determine the optimal conditions for cultivating M. pyrifera gametophytes using California, Cape Town, and the Falkland Islands populations in the laboratory, with the goal of maximizing sporophyte production for large-scale offshore farming. The study revealed varying outcomes; all populations showed increased sporophyte production at higher temperatures, on the other hand, Cape Town and California experienced reduced production at lower temperatures, while the Falkland Islands had increased sporophyte production at 7.5 ⁰C. Significant differences were observed in the induction success of gametophytes from these populations under varying irradiances (22.5 µmol m− 2 s− 1, 30 µmol m− 2 s− 1, and 37.5 µmol m− 2 s− 1) and stocking densities (0.085 mg cm− 2, 0.114 mg cm− 2, and 0.142 mg cm− 2), rendering all three hypotheses invalid. M. pyrifera gametophytes displayed a broad range of optimal induction temperatures. The optimal induction temperature for M. pyrifera gametophytes may be influenced by various factors, including the prevailing temperature of [21], gametophyte gametogenesis, and germplasm temperature [22].
The observed lack of induction success at 7.5°C for CAL and CAT over the three-week induction period and increased success for the FL population at 7.5°C compared to 10°C and 12°C could be attributed to the carry over effects of the cold water in which the FL population thrives:temperature range of 4°C to 9°C [23], whereas CAT and CAL are found within temperature range of 10°C to 15°C [24]. This suggests that for optimal sporophyte production, M. pyrifera gametophytes could be induced at temperatures within the average range of their source environment. This might explain why there wasviable sporophyte production at 10°C and 12.5°C for CAL and CAT, as these temperatures fall within the average temperature ranges of these populations' locations, supports this. However, the lower limit of the average temperature range for a given strain can be used for gametophyte preservation, particularly for longer durations [25], instead of sporophyte production. This could explain why the 7.5°C was unsuitable for sporophyte production in CAL and CAT, which are adapted to higher temperatures compared to FL. Furthermore, lower temperatures have been found to enhance the health of male gametophytes in some instances [26], whereas higher temperatures can negatively impact male gametophyte health, especially for populations adapted to colder waters [27]. This may explain the higher sporophyte production at 7.5°C than at 10°C and 12.5°C observed in the FL population in this study. Good male and female gamete health is important because it ensures a balanced sex ratio for successful fertilization and the production of quality and healthy sporophytes [28]. This underscores the importance of colder temperatures in supporting male gametophyte health during gametogenesis and preservation [27]. However, in this experiment, an imbalanced sex ratio was observed across all populations, potentially leading to insufficient male structures to fertilize the eggs. The recommended sex ratio for seaweed propagation is 1:1 or 2:1 for female gametophytes [29]. In this study, the sex ratio was not determined.
In this study, gametophytes reared at 10°C showed that approximately 12.5°C was the optimal temperature for sporogenesis across all populations, leading to early reproduction with egg production starting in the first week and viable sporophyte production in the second week of the three-week induction period. Comparatively, other treatments initiated egg production in the second week and sporophyte production in the third week, indicating that induction temperatures could potentially be increased to the upper limit of the natural occurrence range to accelerate production timelines without compromising sporophyte output. An increase of approximately 2°C in gametogenesis/gametophyte preservation, as demonstrated in a study by [26], had a similar effect of accelerating sporophyte production, corroborating the findings of this study. Additionally, research by [22] showed that gametophytes maintained at 8°C produced sporophytes more rapidly at 6°C than those induced at 12°C, suggesting that lower gametogenesis temperatures may lead to lower optimal temperatures for sporogenesis. In another study on Laminaria digitata by [30], inducing gametogenesis at temperatures as low as 5°C resulted in sporophyte production within two weeks, further highlighting the importance of pre-treatment conditions for gametophytes. Overall, the FL population exhibited greater thermal plasticity in this study, with successful induction observed at temperatures ranging from 7.5°C to 12.5°C.
Understanding the effects of temperature on the early life stages of M. pyrifera is crucial for selecting suitable gametophyte strains for aquaculture [26]. The results of this study suggest that temperatures can be increased (within the strain's acceptable range or by ~ 2 ⁰C above the gametogenesis temperature) to speed up production without affecting the output. For instance, the temperature for producing sporophytes can be raised from 10°C (control) to 12.5°C without reducing sporophyte numbers in all three populations. Studies by [31] and [24] also support these findings, with optimal sporophyte production occurring at approximately 12°C and 11–14°C, respectively. This explains why no reduction in sporophyte output was observed at 12.5°C compared to 10°C in the CAL population. The study's findings also highlight that early sporophyte production leads to quicker out-planting, as observed at 12.5°C, where holdfast formation began in the second week instead of the third week. This fast tracking of outplanting is beneficial for seaweed cultivation, ensuring timely and successive seeding, accelerating offshore farm expansion, and enabling a swift response to market demand. Higher temperatures can positively affect kelp microscopic development stages and recruitment post-out-planting in warmer cultivation environments. Although faster production is valuable, maintaining quality and quantity is crucial. Optimal temperatures should expedite sporophyte production and enhance output; however, close monitoring of induction progress is necessary for sporophyte health.
As sporophytes grow, they require more nutrients and space to meet their physiological requirements. In this study, sporophytes in the highest temperature treatment appeared pale, potentially because of nutrient depletion or reduced photosynthetic activity. Promptly moving sporophytes from enclosed trays once they reach a sufficient size is crucial to avoid fluctuations in water parameters that could exceed acceptable quality limits. While increased temperature accelerates sporophyte production, exceeding the induction threshold could reduce success; thus, it is important to monitor daily environmental conditions during the hatchery phase. Understanding the effects of temperature on M. pyrifera's early life stages of M. pyrifera is key for strain selection and potential thermal priming in aquaculture [26].
Besides temperature, sporophytes also rely on light for growth, with optimal irradiance varying by strain, gametophyte source, and growth medium [32]. This study tested induction success at 22.5 µmol m− 2 s− 1, 30 µmol m-2 s− 1, and 37.5 µmol m− 2 s− 1 for CAL, CAT, and FL populations. FL showed significantly more viable sporophytes at 22.5 µmol m− 2 s− 1, possibly because of adaptation to lower irradiance levels. The gametophyte mortality at 37.5 µmol m− 2 s− 1 across all populations may be due to a sudden increase in irradiance from to 1–5 µmol m− 2 s− 1 without acclimation. This resulted in reduced sporophyte outputs for CAT and FL compared to the control treatment (30 µmol m− 2 s− 1), which is likely linked to a decrease in reproductive cells. [33] found that light saturation varied with temperature; this study only explored one temperature treatment (10 ⁰C) with higher irradiance, limiting insights into temperature-irradiance synergies.
Optimal stocking density is crucial for good yield, as both high and low stocking densities can limit production outputs. The indifference in the number of viable sporophytes produced at a high stocking density (0.142 mg cm− 2) in this study may be due to gametophyte mortality and egg mortality, leading to a reduction in reproductive cells and sporophytes. The observed mortality suggests potential overstocking, with a higher stocking density possibly leading to increased gametophyte presence at the end of the induction period, which could hinder fertilization. High stocking densities often reduce resource conversion efficiency, leading to growth and survival rate reductions, and water quality deterioration. Conversely, low gametophyte stocking densities can lead to dispersed cell distribution, potentially reducing the production efficiency. A spacing that is too large may limit the success of sperms reaching eggs during fertilization. The experiment revealed that a control stocking density of 0.114 mg cm− 2 (dry weight) was optimal for sporophyte production in Macrocystis.
This study aimed to optimize the production of Macrocystis hatchery sporophytes using gametophytes from different regions. Although that these populations belong to the same species, they exhibit variations in the induction outcome. Higher temperatures (12.5°C) accelerated sporophyte production in all three populations, whereas lower temperatures (7.5°C) were less successful for CAT and CAT. High stocking densities (0.142 mg cm− 2) increased mortality, whereas low stocking densities (0.085 mg cm− 2) lowered fertilization rates. Various irradiance levels (22.5–37.5 µmol m− 2 s− 1) were effective, with the control (30 µmol m− 2 s− 1) yielding the most sporophytes per population. Further research is required to understand the effects of elevated temperatures on sporophyte production. Optimizing sex ratios, spore extraction, and gametogenesis processes could also improve gametophyte quality. This study focused on basic environmental parameters, but additional factors such as nutrients, photoperiod, and irradiance quality are also essential. Overall, enhancing hatchery kelp production is crucial for the aquaculture sector, ensuring a reliable supply of high-quality kelp seedlings while reducing the stress on natural kelp ecosystems. Hatchery methods support genetic diversity preservation and selective breeding to adapt to changing environmental conditions, restore depleted wild kelp populations, and promote biodiversity and economic growth.