Abstract
The biomass composition, namely the fatty composition, of microalgae used in aqua feed is of great importance for the nutritional value of the biomass. Day:night cycles of light and temperature could influence the growth and biomass composition of microalgae. To study the effect of these cycles on Rhodomonas sp. the algae were grown under 16:8 day:night cycles in lab-scale photobioreactors running as turbidostat. Different temperature and light intensities were applied during the light phase. Synchronized cell cycles were observed for Rhodomonas sp. under day:night conditions with oscillating cell size, cell number, biomass concentration and fatty acid content and composition. Cells increased in size during the light phase, storing energy, with cell division scheduled in the dark phase.The introduction of a 16:8 day:night cycle did not affect the biomass yield on light, when operating at optimal conditions of light (150 μmol m−2 s−1) and temperature (21 °C). However, under high light (600 μmol m−2 s−1) or temperature (25 °C), an increased biomass yield on light of up to 22% was found under day:night cycles in comparison to continuous conditions under equal light and temperature levels. Implementation of a day:night cycle increased the maximum daytime temperature for Rhodomonas sp. from 25 °C to 30 °C. The fatty acid content and composition was influence by the implementation of day:night cycles. Daily fluctuations in total fatty acid content from 76 ± 2 mg gDW−1 at the end of the light phase to 94 ± 2 mg gDW−1 in the first hours of the light phase are found. The eicosapentaenoic acid and docosahexaenoic acid content fluctuated by 30% (12.1–16.1 mgEPA+DHA gDW−1) on a daily basis. These daily fluctuations can be exploited in aqua feed applications by selecting a specific time of the day to harvest algae.
Highlights
Rhodomonas sp. is an important microalga for aquaculture as it is often used in live feed applications, for copepods [1,2]
The biomass yield on light of the full day:night cycle experiment (FBL-FBT) shows a slight but not significant increase from 0.87 ± 0.03 g mol−1 to 0.91 ± 0.07 g mol−1 (P > 0.05). This increase is not significant as a result of the relatively large standard deviation found on the experimental results. The results of these four experiments performed under base light (BL = 150 μmol m−2 s−1) and temperature (BT = 21 °C) conditions show no effect of a day:night cycle when operating at the optimum conditions for biomass yield on light
It is unlikely that the baseline with continuous light conditions for the two strains tested (Acutodesmus obliquus and Neochloris oleoabundance) in literature were performed at the optimized growth conditions for maximum biomass yield on light of the applied strains, as done in this study
Summary
Rhodomonas sp. is an important microalga for aquaculture as it is often used in live feed applications, for copepods [1,2]. A higher development index of the copepods is reached when feeding Rhodomonas sp. On copepod development is mainly due to the requirements of fatty acid content in copepods during transformation and egg-production. Is used as feed source in copepod production processes because they contain high levels of the essential fatty acids for copepod development; both eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) [4,5,6,7]. These fatty acids are essential for fecundity, eggproduction and copepod transformation [8]. Rhodomonas sp. shows a high content for both EPA and DHA whereas other strains contain only EPA or Abbreviations: Cx, Biomass concentration (g l−1); D, Daily dilution rate (d−1); DHA, docosahexaenoic acid; EPA, eicosapentaenoic acid; Iph, Incident light intensity (μmolph m−2 s−1); MUFA, Mono unsaturated fatty acid; PUFA, Poly unsaturated fatty acid; rph, Volumetric photon supply rate (molph l−1 d−1); SFA, Saturated fatty acids; TFA, Total fatty acid content of the biomass; Vdilution, Daily dilution volume (l); Vreactor, Total reactor volume (l); Yx,ph, Biomass yield on light (gx molph−1)
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