Coordinated response of photosynthesis, carbon assimilation, and triacylglycerol accumulation to nitrogen starvation in the marine microalgae Isochrysis zhangjiangensis (Haptophyta).

The photosynthetic performance, carbon assimilation, and triacylglycerol accumulation of Isochrysis zhangjiangensis under nitrogen-deplete conditions were studied to understand the intrinsic correlations between them. The nitrogen-deplete period was divided into two stages based on the photosynthetic parameters. During the first stage, carbon assimilation was not reduced compared with that under favorable conditions. The marked increase in triacylglycerols and the variation in the fatty acid profile suggested that triacylglycerols were mainly derived from de novo synthesized acyl groups. In the second stage, the triacylglycerol content continued increasing while the carbohydrate content decreased from 44.0% to 26.3%. These results indicated that the intracellular conversion of carbohydrates to triacylglycerols occurred. Thus, we propose that sustainable carbon assimilation and incremental triacylglycerol production can be achieved by supplying appropriate amounts of nitrogen in medium to protect the photosynthetic process from severe damage using the photosynthetic parameters as indicators.

Identification of carbohydrates as the major carbon sink of the marine microalga Isochrysis zhangjiangensis (Haptophyta) and optimization of its productivity by nitrogen manipulation.

Microalgae represent a potential feedstock for biofuel production. During cultivation under nitrogen-depleted conditions, carbohydrates, rather than neutral lipids, were the major carbon sink of the marine microalga Isochrysis zhangjiangensis (Haptophyta). Carbohydrates reached maximum levels of 21.2 pg cell(-1) on day 5, which was an increase of more than 7-fold from day 1, while neutral lipids simultaneously increased 1.9-fold from 4.0 to 7.6 pg cell(-1) during the ten-day nitrogen-depleted cultivation. The carbohydrate productivity of I. zhangjiangensis was improved by optimization of the nitrate supply mode. The maximum carbohydrate concentration was 0.95 g L(-1) under batch cultivation, with an initial nitrogen concentration of 31.0 mg L(-1), which was 2.4-fold greater than that achieved under nitrogen-depleted conditions. High performance liquid chromatography (HPLC) analysis showed that the accumulated carbohydrate in I. zhangjiangensis was composed of glucose. These results show that I. zhangjiangensis represents an ideal carbohydrate-enriched bioresource for biofuel production.

Salinity manipulation as an effective method for enhanced starch production in the marine microalga Tetraselmis subcordiformis.

Microalgal starch is considered a promising feedstock for bioethanol production. The biomass and starch accumulation in the marine microalga Tetraselmis subcordiformis were characterized under different salinities in response to nitrogen repletion (+N) or depletion (-N) at high irradiance (HI) or low irradiance (LI). Under favorable nutritional conditions (HI+N), biomass accumulation was seldom affected under 20% normal salinity, though starch accumulation were somewhat reduced. Increased salinity impaired overall biomass and starch accumulation, though it led to a temporary starch accumulation at initial cultivation phase. Under nitrogen deprivation, decreased salinity strengthened biomass and starch accumulation regardless of irradiance. The highest starch content of 58.2% dry weight and starch productivity of 0.62 g L(-1) d(-1) were obtained under HI-N with 20% normal salinity. Decreased salinity combined with -N generated moderate stress to facilitate starch accumulation. Salinity manipulation can be effectively applied for enhanced starch production in marine microalgae.

Characterization of cell growth and starch production in the marine green microalga Tetraselmis subcordiformis under extracellular phosphorus-deprived and sequentially phosphorus-replete conditions.

Microalgal starch is a potential feedstock for biofuel production. Nutrient stress is widely used to stimulate starch accumulation in microalgae. Cell growth and starch accumulation in the marine green microalga Tetraselmis subcordiformis were evaluated under extracellular phosphorus deprivation with initial cell densities (ICD) of 1.5, 3.0, 6.0, and 9.0×106 cells mL⁻¹. The intracellular stored phosphorus supported cell growth when extracellular phosphorus was absent. The maximum starch content of 44.1% was achieved in the lowest ICD culture, while the maximum biomass productivity of 0.71 g L⁻¹ day⁻¹, starch concentration of 1.6 g L⁻¹, and starch productivity of 0.30 g L⁻¹ day⁻¹ were all obtained in the culture with the ICD of 3.0×106 cells mL⁻¹. Appropriate ICD could be used to regulate the intracellular phosphorus concentration and maintain adequate photosynthetic activity to achieve the highest starch productivity, along with biomass and starch concentration. The recovery of phosphorus-deprived T. subcordiformis in medium containing 0.5, 1.0, or 6.0 mM KH2PO4 was also tested. Cell growth and starch accumulation ability could be recovered completely. A phosphorus pool in T. subcordiformis was shown to manipulate its metabolic activity under different environmental phosphorus availability. Though lower starch productivity and starch content were achieved under phosphorus deprivation compared with nitrogen- or sulfur-deprived conditions, the higher biomass and starch concentration make T. subcordiformis a good candidate for biomass and starch production under extracellular phosphorus deprivation.