Seawater nitrogen concentration and light independently alter performance, growth, and resource allocation in the bloom-forming seaweeds Ulva lactuca and Ulvaria obscura (Chlorophyta)

Enhancement of growth in Ulva prolifera by diurnal temperature difference combined with nitrogen enrichment

Many studies have documented the responses of Ulva prolifera to environmental factors. However, the diurnal temperature differences and interactive effects of eutrophication are usually ignored. In this study, we selected U. prolifera as material to examine the effects of diurnal temperature on growth, photosynthesis and primary metabolites under two nitrogen levels. We cultured U. prolifera seedlings under two temperature conditions (22-22 °C: 22 °C during day and night; 22-18 °C: 22 °C during day and 18 °C at night) and two nitrogen levels (LN: 0.1235 mg L^-1; HN: and 0.6 mg L^-1). The results showed that 1) HN-grown thalli had higher growth rates, the chlorophyll a (Chl a) content, photosynthesis, superoxide dismutase (SOD) activity, soluble sugar, and protein contents under the two temperature conditions; 2) The growth of thalli was enhanced by 22-18 °C condition compared with 22-22 °C, but the increase was only significant under HN condition; 3) 22-18°C-grown thalli had a lower net photosynthetic rate, maximal quantum yield (F_v/F_m), and dark respiration rate (R_d) than those grown at 22-22 °C; 4) No significant effects of diurnal temperature difference were detected on the SOD activity and soluble sugar content under LN and HN conditions, while the soluble protein content was enhanced by 22-18 °C under LN condition; 5) The nitrogen affected metabolite variations in U. prolifera more significantly than the diurnal temperature difference. The metabolite levels in the tricarboxylic acid cycle, amino acid, phospholipids, pyrimidine, and purine metabolism pathways increased under HN condition. The levels of glutamine, γ-aminobutyrate (GABA), 1-aminocyclopropane-1-carboxylate (ACC), glutamic acid, citrulline, glucose, sucrose, stachyose, and maltotriose were enhanced by 22-18 °C, especially under HN condition. These results identify the potential role of the diurnal temperature difference and offer new insight into the molecular mechanisms for U. prolifera responses to eutrophication and temperature.

Carbon limitation in response to nutrient loading in an eelgrass mesocosm: influence of water residence time

Altered primary productivity associated with eutrophication impacts not only ecosystem structure but also the biogeochemical cycling of oxygen and carbon. We conducted laboratory experiments to empirically determine how residence time (1, 3, 10 d) influences eutrophication responses in a simplified Pacific Northwest Zostera marina-green macroalgal community. We expected long-residence time (RT) systems to exhibit eutrophication impairments. Instead, we observed an accumulation of nutrients at all RTs and a shift in the dissolved inorganic carbon speciation away from CO_{2 (aq)} with unexpected consequences for eel grass plant condition, including shoot mortality. Most metrics responded more strongly to temperature treatments than to RT treatments. No dramatic shifts in the relative abundance of Z. marina and green macro algae were detected. Z. marina shoot density proliferated in cool temperatures (12°C) with a modest decline at 20°C. Eelgrass loss was associated with high total scale pH (pH_T) and CO_{2 (aq)} concentrations of <10 μmol kg^-1 CO_{2 (aq)}, but not with high nutrients. Z. marina δ¹³C values support the hypo thesis that carbon availability was greater at short RT. Further, very low leaf sugar concentrations are consistent with extreme photosynthetic CO_{2 (aq)} limitation. We suggest that the effects of extremely low environ mental car bon concentrations (CO_{2 (aq)}) and increased respiration at warm temperatures (20°C) and other physiological processes can lead to internal carbon limitation and shoot mortality. Eutrophication responses to nutrient loading are more nuanced than just light limitation of eelgrass and require additional research on the interaction of the biogeochemical environment and plant physiology to better understand estuarine ecosystem disruption.

Nitrogen enrichment mediates the effects of high temperature on the growth, photosynthesis, and biochemical constituents of Gracilaria blodgettii and Gracilaria lemaneiformis

Gracilaria blodgettii and Gracilaria lemaneiformis are often adopted as tools to purify aquaculture tail water. However, there has been such phenomenon that high temperature in summer restricts the process of aquaculture. To explore the adaptive capacity of G. blodgettii and G. lemaneiformis, we experimented them and cultured for 12 days under three temperatures (20, 25, and 30 °C) and three levels of multiple nitrogen sources (0.12, 0.6 and 4.4 mg L-1). Their growth, photosynthetic characteristics, and biochemical compositions including the contents of pigments and soluble protein were determined to investigate the single and interactive effects of temperatures and nitrogen levels on these two species. The results showed that in terms of G. blodgettii, the higher growth rate and more pigment (chlorophyll a and carotenoids) contents were observed at 25 and 30 °C in comparison to 20 °C, and the pigments showed maximum contents at 25 °C. More nitrogen improved the growth rate, net photosynthetic rate (Pn) at 25 and 30 °C, Fv/Fm at 20 °C, maximal photosynthetic electron transfer rate (ETRm), as well as soluble protein content at 20 and 25 °C. Additionally, the growth rate, Pn, and ETRm of G. lemaneiformis all showed a decline as increasing temperature; analogously high nitrogen concentration increased the growth rate at 25 and 30 °C, Fv/Fm at each temperature, ETRm, and pigments contents at 20 °C, as well as soluble protein content at 20 and 25 °C. Conclusions indicated that high temperature restricted the growth rate, inhibited photosynthetic characteristics, and decreased the soluble protein content of G. lemaneiformis. The reduced photosynthetic performance, pigments, and soluble protein contents of G. blodgettii were noted under similar conditions. However, nitrogen enrichment induced the greater resistant level to high temperature, and G. blodgettii showed better response. These findings suggested that these two Gracilaria species possessed a certain adaptability to tail water from aquaculture at high temperature and G. blodgettii can resist more to. Therefore, it seems to be an alternative and workable scheme to adopt some suitable macroalgae to optimize the solution to present purification of aquaculture wastewater or eutrophic waters under high temperature.

Environmental factors shape the epiphytic bacterial communities of Gracilariopsis lemaneiformis

Macroalgae host various symbionts on their surface, which play a critical role in their growth and development processes. However, there is still incomplete understanding of this epiphytic bacteria-host algae interactions. This study comprehensively analysed variation of the epiphytic bacterial communities (EBC) composition of red macroalga Gracilariopsis lemaneiformis at different geographic locations and environmental factors (i.e., nitrogen and phosphorus), which shape the EBC composition of G. lemaneiformis. The composition and structure of EBC were characterized using high throughput sequencing of the V3-V4 hypervariable region of the 16S rRNA gene. The results revealed that epiphytic bacteria varied significantly among three different geographic locations in China, i.e., Nan'ao Island (NA), Lianjiang County (LJ), and Nanri Island (NR). Redundancy analysis (RDA) showed that the relative abundance of Bacteroidetes, Firmicutes, Verrucomicrobia, and Epsilonbacteraeota at NR were strongly positively correlated with total nitrogen (TN), total phosphorus (TP), nitrate nitrogen (NO₃-N), and dissolved inorganic nitrogen (DIN), but negatively correlated with nitrite nitrogen (NO₂-N). The relative abundance of Cyanobacteria at NA and LJ were strongly positively correlated with NO₂-N, but negatively correlated with TN, TP, NO₃-N, and DIN. Besides, the Mantel test results indicated that the EBC composition was significantly correlated with these environmental factors, which was also confirmed by Spearman correlation analysis. Thus, environmental factors such as NO₃-N and DIN play a key role in the community composition of epiphytic bacteria on G. lemaneiformis. This study provides important baseline knowledge on the community composition of epiphytic bacteria on G. lemaneiformis and shows correlation between different epiphytic bacteria and their surrounding environmental factors.

Darkness and low nighttime temperature modulate the growth and photosynthetic performance of Ulva prolifera under lower salinity

In order to understand how darkness/irradiance and low nighttime temperature might alter physiology of Ulva prolifera under lower salinity conditions, we analyzed the growth rates, water content, superoxide dismutase (SOD) activity, total soluble proteins (SPs) and carbohydrates content at the end of dark and light period under three temperature levels (25-25 °C treatment: 25 °C for day and night; 15-15 °C treatment: 15 °C for day and night; 25-15 °C treatment: 25 °C for day with 15 °C for night) and two salinity conditions (15, 25), meanwhile, the pigment content (chlorophyll a and b), chlorophyll fluorescence and photosynthetic oxygen evolution also were determined during light phase. We found that the U. prolifera showed higher growth rate and SOD activity during dark phase at 25 °C, but this dark-induced increase could not be observed at 15 °C. The reasons for this increase varied, however, maybe not included water content and SPs for no significant difference in water content observed under all the treatments, as well as lower SPs content for dark period aside that at 15 °C and salinity 15. Compared to other two temperature treatments, the thalli grown at 25-15 °C showed higher growth rate and the photosynthetic oxygen evolution rate in light phase under salinity 15 conditions, although the maximum relative electron transport rate (rETR_max) showed higher value under 25 °C treatment. These results indicate that the darkness and the lower nighttime temperature maybe responsible reason for the rapid growth of these green tide algae.

Salinity mediates the effects of nitrogen enrichment on the growth, photosynthesis, and biochemical composition of Ulva prolifera

To study the combined effects of multiple nitrogen (N) sources and salinity on the growth and physiology on macroalgae, we cultured Ulva prolifera under three N levels (N₀, 0.1235 mg L^-1; N₁, 0.6 mg L^-1; and N₂, 4.4 mg L^-1; the ratios were 18:74:8 for NH₄-N, NO₃-N, and NO₂-N, respectively) and three salinity conditions (15, 25, and 35). Then, the growth, pigment content, photosynthetic performance, superoxide dismutase (SOD) activity, and contents of soluble protein and carbohydrates were measured. The results showed the following: (1) Compared to that grown at salinity 25, the growth of U. prolifera decreased under salinity 35, especially under the N₀ and N₂ levels, but there were no significant effects of salinity 15 under any of the N levels. (2) There were no significant effects of salinity on the chlorophyll a (Chla) content, but compared to the content at salinity 25, the chlorophyll b (Chlb) content was enhanced by salinity 15 and 35; lower ratio values between Chla and carotenoids (Car) occurred under the salinity 25 treatment. Under each salinity condition, the pigments were enhanced by a high N level. (3) A relatively higher salinity level decreased the photosynthetic oxygen evolution rate, while a higher N level increased this value. Compared to the rate at salinity 25, the dark respiration rate (R_d) significantly increased at salinity 15 under the N₀ condition. (4) SOD activity was enhanced by a high N level, but no significant effects of salinity were observed. (5) The carbohydrate content was enhanced at salinity 35 under the N₀ and N₁ levels, and under salinity 15, this value increased with increasing N levels. In conclusion, although the growth of U. prolifera decreased at high N levels under high salinity conditions, a high N level induced an increase in photosynthesis, while no significant decrease in growth occurred. These findings indicate that low salinity and high N levels may be nonnegligible reasons why this species thrives, and low salinity was the better choice when this species was used for wastewater treatment.