Microalgae cultivation
A. platensis (NIES-39) and C. reinhardtii (NIES-2235) were cultured using SOT medium (Ogawa and Terui 1970) and C medium (Ichimura, 1971), respectively (Table 1). Both A. platensis and C. reinhardtii were grown in 1-L flasks under a photoperiod of 12D:12L with light intensity of ca. 440 µmol m− 2 s− 1 at 25 ºC and ca. 180 µmol m− 2 s− 1 at 20 ºC, respectively. The algae were harvested at the exponential growth phase and concentrated through centrifugation (4200 g). An aliquot of the algal suspension was collected from the well-mixed algal culture and the optical density (OD) of the algal suspension was measured with a spectrophotometer (SP-300, OPTIMA®, Tokyo, Japan) at a wavelength of 680 nm. The carbon concentration of the algal suspension was estimated from a regression equation between OD and the carbon mass of the cells established before the experiment. A new inoculation for the microalgal culture was made every 5 days for A. platensis and every 7 days for C. reinhardtii to provide a constant supply of fresh algal food throughout the experiment.
Table 1
Chemical compositions of SOT and C media
SOT- Medium | C- Medium |
Chemicals | Concentration (mg/L) | Chemicals | Concentration (mg/L) |
NaHCO3 | 16800 | Ca(NO3)2・4H2O | 150 |
K2HPO4 | 500 | KNO3 | 100 |
NaNO3 | 2,500 | β-Na2glycerophosphate・5H2O | 50 |
K2SO4 | 1000 | MgSO4・7H2O | 40 |
NaCl | 1000 | Vitamin B12 | 0.0001 |
MgSO4·7H2O | 200 | Biotin | 0.0001 |
CaCl2 | 40 | Thiamine HCl | 1 |
FeSO4·7H2O | 10 | Tris (hydroxymethyl) aminomethane | 500 |
Na2EDTA | 80 | Na2 EDTA·2H2O | 1000 |
H3BO3 | 2.86 | FeCl3·6H2O | 196 |
MnSO4·7H2O | 2.5 | MnCl2·4H2O | 36 |
ZnSO4·7H2O | 0.222 | ZnSO4·7H2O | 10.4 |
CuSO4·5H2O | 0.079 | CoCl2·6H2O | 4 |
Na2MoO4·2H2O | 0.021 | Na2MoO4·2H2O | 2.5 |
Fragmentation of A. platensis trichome
The size of untreated A. platensis trichomes ranges from 50 µm to 500 µm (Wan et al. 2021), which is too large for daphnids to ingest (Geller and Müller 1981; Wenzel et al. 2012; Bednarska et al. 2014). To be easily ingested by the daphnids, A. platensis trichomes were fragmented with a bead crusher (µT-01, TAITEC). A 2-ml conical screw cap microtube (1392-200-C, WATSON) and zirconia beads (CZS0060, AS ONE Corporation) with a diameter of 0.4–0.6 mm were used to fragment the trichomes. Approximately one-third of the volume of the microtube was filled with zirconia beads and the remaining volume was filled with an A. platensis suspension with concentration of 0.5–0.8 mg C mL− 1. The shaking speed of the bead crusher was set at 4600 rpm for 5–25 s. The fragmented A. platensis cells were checked by a microscope every 5 s and the sizes of the trichomes were measured with a digital micrometer (Wraycam, NF500) under a dissecting microscope (Olympus, IX70) at a magnification of 20×.
Growth experiment in D. magna
A stock culture of D. magna was maintained in aged and filtered tap water (ATW) with a glass fiber filter (Whatman, GF/F, diameter: 47 mm) at 20 ºC, photoperiod of 12L:12D and light intensity of ca. 180 µmol m− 2 s− 1 with sufficient amounts of C. reinhardtii (> 105 cells mL− 1). For each food treatment, five neonates born within 12 h were arbitrarily sorted from a stock culture and then transferred to a 200-ml glass jar filled with ATW and an algal food suspension. The food treatments were conducted at five different ratios of fragmented A. platensis: C. reinhardtii (100:0, 80:20, 50:50, 20:80 and 0:100) at a food concentration of 4 mg C L− 1 after fragmentation of the trichomes of A. platensis for 25 s with the bead crusher (Table 2). Control treatments exclusively using C. reinhardtii as the food alga were conducted at 20%, 50% and 80% of 4 mg C L− 1 (Table 2). The algal food suspension and ATW were changed daily.
The survival of experimental animals was monitored daily when the algal food suspension was changed. All treatments were made in triplicate and lasted for 6 days. At the end of the experiment, the surviving animals from each treatment were collected from the jar, washed with ATW, transferred to a tin capsule, dried in an oven (DV-600, YAMATO) for 24 h and then weighed on a digital balance with a minimum weighing value of 1 µg (AD-4212D-32, A&D Instruments Ltd). In addition, 30 neonates born within 12 h were arbitrarily sorted from a stock culture and the same procedure described above was carried out to determine the initial dry weight of the animals. Somatic growth rates (g) of the animals were calculated as the increment of the dry mass between newborn (M0) and 6-day old animals (M6) using Eq. (1):
\(\text{g}=\frac{lnM6-lnM0}{6}\) . (Eq. 1)
Table 2
Food treatments at different supply ratios of Arthrospira platensis to Chlamydomonas reinhardtii (AP:CR) for Daphnia magna growth experiments at food concentration of 4 mg C L− 1
Food treatment (AP:CR) | Carbon mass of food (mg C L− 1) |
A. platensis alone (100:0) | 4 |
80% A. platensis + 20% C. reinhardtii (80:20) | 3.2 + 0.8 |
50% A. platensis + 50% C. reinhardtii (50:50) | 2 + 2 |
20% A. platensis + 80% C. reinhardtii (20:80) | 0.8 + 3.2 |
C. reinhardtii alone (0:100) | 4 |
C. reinhardtii alone (0:80) | 3.2 |
C. reinhardtii alone (0:50) | 2 |
C. reinhardtii alone (0:20) | 0.8 |
No food | 0 |
Elemental analysis of microalgae
To determine the elemental composition of C, nitrogen (N) and P of the microalgae, we took an aliquot of approximately 10–15 ml of the fresh algal suspension, equivalent to a dry weight of 9.1 ± 0.4 mg for A. platensis and 2.9 ± 0.16 mg for C. reinhardtii, and filtered with a pre-combusted and pre-weighed glass fiber filter (GF/F, diameter: 47 mm, Whatman). The filtered microalgae were dried in an oven (DV-600, YAMATO) at 60 ºC for 24 h and weighed on an electronic balance (AW220, SHIMADZU). C and N contents of the algal cells were determined with an NC analyzer (NCH-22A, SUMIGRAPH). The P contents were determined with an autoanalyzer (Bran + Luebbe, AACS II, Norderstedt, Germany) after degrading the sample with potassium peroxodisulfate at a maximum absorbance wavelength of 880 nm, based on the phosphomolybdenum blue method (Murphy and Riley 1962).
Statistical analysis
Differences in the somatic growth rates of daphnids among the food treatments were tested with one-way analysis of variance (ANOVA). Treatments with no surviving animal per beaker were excluded from the ANOVA. When the ANOVA showed a significant difference, Tukey’s honestly significant difference (HSD) test was conducted at p < 0.05. All statistical analyses were made using R statistical software (R Core Team 2022).