Rebecca J. Lawton

Overcoming low frequencies of transition from the gametophyte to the sporophyte life history phase is vital to enable efficient hatchery production of the kelp Ecklonia radiata (Phaeophyceae) for aquaculture, restoration and conservation. Therefore, we investigated the effects of nutrient strength (full to 1/20 strength Provasoli enriched seawater (PES)), germanium dioxide strength (full and half strength) and application period (no application, 9-16, 9-30, 16-23 and 16-30 days post seeding), and seeding density (2, 4, 12 and 18 spores mm^-2) on sporophyte production in E. radiata in three laboratory experiments. Nutrient strength and seeding density significantly affected sporophyte transition frequency, sporophyte formation frequency, and sporophyte length, with all three metrics consistently decreasing with decreasing nutrient strength and increasing seeding density. Sporophyte transition frequencies were more than four times higher in the full-strength nutrient treatment (58.2 %) compared to the 1/8 and 1/20 strength treatments (12.5 and 3.0 %, respectively), and 18% higher in the 2 spores mm^-2 seeding density treatment (100.0 %) compared to the 18 spores mm^-2 treatment (84.3 %). Germanium dioxide application period but not strength significantly impacted sporophyte transition frequency and sporophyte formation frequency. Transition frequencies were more than two-thirds lower when GeO₂ was applied from day 9 onwards (75 %). Based on these results, we recommended using full strength PES, half strength GeO₂ from day 16 onwards only if diatom contamination is an issue and seeding at a density of ≤10 spores mm^-2 to maximise sporophyte production.

Algal bioremediation using macroalgae is a promising approach to wastewater treatment. This study compared the productivity and bioremediation performance of the freshwater filamentous algal cultivars; Klebsormidium flaccidum, Oedogonium calcareum, and Oedogonium sp., in primary municipal wastewater in outdoor high-rate filamentous algal pond mesocosms. K. flaccidum had the highest biomass productivity (3.09 g dry weight m−2 day−1 ± 0.20 SE) and bioremediation performance, reducing total ammoniacal-N by 51% to 14.80 mg L−1 (± 0.81 SE), nitrate-N by 59% to 0.30 mg L−1 (± 0.02 SE), and dissolved reactive phosphorous by 15% to 3.52 mg L−1 (± 0.07 SE). This cultivar achieved the greatest reductions in total suspended solids (54%), carbonaceous biochemical oxygen demand (93%), and chemical oxygen demand (74%). K. flaccidum and Oedogonium sp. reduced Escherichia coli by 98%. Competitive dominance of K. flaccidum and Oedogonium sp. was assessed in bicultures at three stocking densities. By day 12, K. flaccidum's proportion increased from 50 to 64% (± 6.1 SE) and 73% (± 5.0 SE) at a stocking density of 0.25 g and 0.5 g FW L−1, respectively. Based on superior biomass productivity, bioremediation performance, and competitive dominance, K. flaccidum was identified as a target cultivar for bioremediation of primary municipal wastewater.

In-situ macroalgal bioremediation could help prevent and reduce estuarine eutrophication. However, estuaries are dynamic ecosystems characterized by fluctuating abiotic conditions. Therefore, target macroalgal species for in-situ estuarine bioremediation must be able to maintain productivity under a range of challenging abiotic conditions. The aim of this study was to assess the tolerance of the novel bioremediation target Gracilaria transtasmanica to ambient and extreme levels of salinity, air-exposure, and light limitation that occur in estuarine environments. Three separate experiments were conducted to assess tolerance to each factor and photosynthetic functioning and growth were used to quantify the tolerance range of G. transtasmanica in each experiment. Specific Growth Rate (SGR) was significantly affected by salinity, air-exposure, and light limitation. Gracilaria transtasmanica was able to grow in salinities of 5 to 35 ppt, but growth rates decreased with decreasing salinity. Air-exposure periods of up to 9 h were tolerated, but growth rates decreased as air-exposure period increased. Gracilaria transtasmanica was able to maintain growth with a loss of up to 75% of ambient light and was also able to tolerate short periods (48 h) of continuous darkness. Photosynthetic function was unaffected by salinity, air-exposure, or light limitation. These results demonstrate the high tolerance of G. transtasmanica to light limitation, air-exposure and a broad range of salinities. Consequently, this species could be cultivated in a range of habitat types within estuaries. However, the optimal habitats for cultivation will be submerged subtidal channels and lower intertidal mudflats where the impacts of freshwater inflows and air-exposure are reduced.

The native kelp Ecklonia radiata is a target species for the emerging seaweed aquaculture industry in New Zealand, however, the effect of line configuration on survival and growth of farmed seaweed is unknown. In this study, we outplanted nine seaweed growth lines (with 20 individuals each) in three line configurations (vertical, horizontal 2 m depth, and horizontal 4 m depth) in northern New Zealand for grow‐out between August 2021 and April 2022. Initial survival rates were similar between line configurations, but only individuals growing on vertical lines survived after four months (38% ± 12 SD). Similarly, on vertical lines, average juvenile growth by elongation was 85% higher (0.11 cm day ⁻¹ ± 0.03 SD) and maximum average weight of individuals was 77% higher (106 g WW ± 73 SD), compared to horizontal lines. Unusually warm water temperatures were likely a central driver for the overall high mortality of E. radiata observed from January to April, while a combination of temperature and light stress was likely a central driver for higher mortality observed on vertical lines at shallow depths compared to deeper depths. In conclusion, this study supports farming E. radiata on vertical dropper lines rather than horizontal lines.

The red alga Asparagopsis armata is an emerging aquaculture-target species due to its application as an antimethanogenic feed ingredient in ruminants, yet information on A. armata reproduction and cultivation is currently lacking. We therefore quantified the effects of temperature, irradiance, nutrients, and photoperiod, and addition of plant growth regulators (PGRs; indole-3-acetic acid, abscisic acid, 1-aminocyclopropane-1-carboxylic acid) on tetrasporogenesis in domesticated A. armata that had been maintained under controlled conditions (18 °C, 12 h light:12 h dark photoperiod) for 18 months prior to experimentation. Tetrasporogenesis was only induced at 5 and 15 µmol photons m⁻² s⁻¹ under an 8 h light:16 h dark photoperiod with 3.5 mg nitrogen L⁻¹ and tetraspore release was 28-fold greater at 18 °C compared to 15 °C after 28 days of exposure. After 29 days, tetraspore release and germination rate both declined. All PGR treatments prevented tetrasporogenesis. This study is the first to provide the detail and framework necessary to enable A. armata hatchery development. We conclude that tetrasporogenesis was most likely induced in response to a significant reduction in photoperiod rather than as a result of replicating seasonal environmental conditions, and that temperature played a key role in determining reproductive output. With overall higher tetraspore release and a consistent germination rate of > 90%, we recommend exposing tetrasporophyte biomass to 18 °C, 15 µmol photons m⁻² s⁻¹ and 3.5 mg nitrogen L⁻¹ under an 8 h L:16 h D photoperiod for up to 29 days to obtain a reliable supply of tetraspores for seeding onto ropes for transfer to the hatchery phase.

The biomass composition of kelp varies within species both spatially and temporally. However, this variation in biomass quality has not yet been investigated for the native kelpEcklonia radiatawithin New Zealand, where the kelp is a target for the emerging seaweed aquaculture industry. In this study we quantified spatial and temporal variation in the composition ofE. radiatabiomass, collected from 12 sites around the North Island of New Zealand and from 12 months across a full year at a single site (n = 138). High spatial variation was detected for most components, including alginate (range: 16.6 – 22.7% DW,n = 12), fucoidan (range: 1.2 – 1.6% DW,n = 12), phlorotannins (range: 4.8 – 9.3% DW,n = 72), and glucose (range: 9.3 – 22.6% DW,n = 12). The biomass composition ofE. radiatavaried significantly among sites but with no clear patterns among regions, indicating that geographic differences were mostly local rather than regional, possibly due to site-specific environmental conditions. Significant temporal variation (measured by positive autocorrelation between months) was detected in the content of lipids, proteins, glucose, guluronic acid, nitrogen, phosphorous, iodine, arsenic, and mercury, and for the mannuronic to guluronic acid (M:G) ratio. Overall,E. radiatahad comparable biomass composition to that of commercially grown northern hemisphere species but with substantially higher phlorotannin content. These results demonstrate thatE. radiatacould be a viable southern hemisphere alternative for a broad range of commercial applications.