Tropical Microalgae Research Articles

Copper and zinc tolerance of two tropical microalgae after copper acclimation

Current toxicity tests with microalgae are often criticized as being overly sensitive to metals because algae are cultured in metal-deficient media. If such bioassays overestimate copper toxicity in surface waters, the relevance of water quality guidelines derived from these tests is questionable. In this study, the effect of acclimation to copper at environmentally relevant concentrations, on the sensitivity of the marine diatom Nitzschia closterium and the freshwater green alga Chlorella sp. to copper and zinc was examined. N. closterium was acclimated in culture medium containing 5 or 25 microg Cu L(-1) for 200 days, while Chlorella sp. was acclimated in medium containing 2 microg Cu L(-1) for 100 days. Changes in algal growth rates and copper and zinc tolerance were monitored using standard growth inhibition toxicity tests in minimal medium over 72 h. Neither of the two acclimated N. closterium cultures had increased zinc or copper tolerance compared with that of the nonacclimated algae, nor were there any changes in control growth rates. Similarly, no changes in copper tolerance or control growth rates were observed for the acclimated Chlorella sp. culture. This was supported by measurements of intracellular and extracellular copper which confirmed that there were no differences in copper accumulation in either acclimated or nonacclimated algae. These results suggest that these algae grown in standard culture media are generally no more sensitive than algae grown in a metal-enriched medium. This supports the continued use of current laboratory bioassays with microalgae, as part of a suite of tests for assessing metal bioavailability, for use in ecological risk assessments and for providing data for the derivation of water quality guidelines.

Use of tropical microalgae as food for larvae of the black-lip pearl oyster Pinctada margaritifera

This study determined the nutritional value of tropical microalgae for black-lip pearl oyster ( Pinctada margaritifera, L.) larvae. One-day old larvae were fed the flagellates Pavlova salina, Pavlova sp., TISO and Micromonas pusilla in binary and ternary combinations in Experiment 1. In a second experiment, umbo-stage larvae were fed the best binary combinations from Experiment 1 with the addition of one diatom species ( C. muelleri, Chaetoceros sp. or Skeletonema sp.) per combination. The best two ternary combinations (flagellates only) from Experiment 1, Pav. salina/ Pavlova sp./TISO and TISO/ M. pusilla/ Pavlova sp., were also assessed in Experiment 2. In Experiment 1, greater growth rate was shown by larvae fed the ternary combination of Pavlova sp./ Pav. salina/TISO followed for the binary combination of Pavlova sp./ M. pusilla; however, larvae fed Pavlova sp. as a mono-specific diet performed as well as those fed the ternary combination of Pavlova sp./ Pav. salina/ M. pusilla ( P = 0.001). The addition of a diatom to microalgae diets composed of flagellates resulted in increased growth rates and survival of umbo-stage P. margaritifera larvae when compared to combinations without a diatom (Experiment 2). The results showed that Pavlova sp. supported a high growth rate of D-stage P. margaritifera larvae which was equivalent to that of larvae fed plurispecific diets. For umbo-stage P. margaritifera larvae, the best growth rate was achieved when the binary combination of Pavlova sp. and C. muelleri was used. Based on these results, Pavlova sp. and Pavlova sp./ C. muelleri are recommended as diets for D-stage and umbo-stage P. margaritifera larvae, respectively.

The nutritional value of seven species of tropical microalgae for black-lip pearl oyster (Pinctada margaritifera, L.) larvae

Recent years have seen major developments in the culture and availability of tropical microalgae as a food source for tropical bivalve species. The nutritional value of seven small (<9 μm) tropical microalgae species: two diatoms (Chaetoceros muelleri and Chaetoceros sp.); three golden-brown flagellates (Isochrysis sp., Pavlova salina and Pavlova sp.) and two green-flagellates (Micromonas pusilla and an unidentified coccoid CS-126), were analysed for carbohydrate, lipid and protein contents as well as fatty acid composition. Each species of microalgae was fed singly to early (D-stage veliger) and later (umbo stage veliger) stage larvae of the black-lip pearl oyster, Pinctada margaritifera. Highest survival of D-stage larvae over the 10-day experiment was recorded for those fed Pavlova sp. (CS-50). Greatest shell growth was shown by D-stage larvae fed the golden-flagellates Pavlova sp. (CS-50) and Pav. salina. Based on growth of D-stage larvae, the microalgae could be divided into three groups: (1) larvae fed Pav. salina and Pavlova sp. showed significantly greater growth than those fed other microalgae; (2) those fed Isochrysis sp., C. muelleri and M. pusilla showed significantly greater growth than unfed larvae; and (3) larvae fed Chaetoceros sp. and CS-126 did not grow at a rate greater than unfed larvae. Growth of D-stage veliger larvae was significantly correlated with carbohydrate, lipid and protein content of microalgae and with levels of dietary polyunsaturated fatty acid, specifically DHA (r=0.829, P=0.021). In a second experiment survival of umbo-stage larvae (including the unfed control) did not differ significantly between treatments (P<0.05) after 8 days of culture. Larvae fed Pavlova sp. and Pav. salina showed the greatest incremental growth increases, but these were not significantly greater than those of larvae fed TISO and C. muelleri (P>0.05). Growth of umbo-stage larvae fed M. pusilla, Chaetoceros sp. and the Prasinophyta sp. (CS-126) did not differ significantly from that of unfed larvae (P<0.05). This study is the first comprehensive assessment of the nutritional value of tropical microalgae species for pearl oyster larvae. The results provide a basis for development of more effective larval culture techniques by identifying microalgae supporting good growth of P. margaritifera larvae of different ages.

Evaluation of recently isolated Australian tropical microalgae for the enrichment of the dietary value of brine shrimp, Artemia nauplii

Thirteen species of tropical Australian microalgae (both benthic and planktonic) were compared in terms of survival and growth of Artemia and enrichment of polyunsaturated fatty acids (PUFAs), including eicosapenaenoic acid (EPA; 20:5 n−3) and docosahexaenoic acid (DHA; 22:6 n−3) in Artemia. Artemia nauplii showed enhanced growth beyond the unfed control value after 24 h feeding with all species of microalgae except Isochrysis NT14. However, when the growth was compared against the commercial northern hemisphere species Isochrysis galbana TISO, only Artemia fed Cryptomonas CRFI01 showed better growth (significance at P<0.05). Artemia fed Chaetoceros sp. CS256, Nephroselmis GOC52, Tetraselmis sp. NT18 and Nitzschia palacea NT7 showed similar growth over 24 h to those fed TISO. Artemia consumed Isochrysis NT14 at a very low rate. In 7 day feeding trials, CRFI01, CS256 and GOC52 performed much better than TISO ( P<0.05) as food sources for Artemia. The species Isochrysis sp. PS11, Tetraselmis spp. TEQL01 and NT18, and NT7 showed similar results to TISO. Survival of Artemia at the end of a 7 day feeding with seven species of microalgae (NT7, PS11, CRFI01, NT18, CS256, TEQL01, and GOC52) ranged from 65 to 97%, with NT18 showing a wide range of survival results, due to the crash of some of the cultures. The suitability of these tropical Australian microalgae as a food source for Artemia was related to the fatty acid composition of the microalgae, rather than the taxonomic group, size or proximate chemical composition. Artemia fed CRFI01 had significantly higher levels of EPA and DHA ( P<0.05) than those fed TISO. Artemia fed CS256 and NT7 had highest levels of EPA, however, only Artemia fed CS260 had higher amounts of DHA than Artemia fed TISO. The results showed that one species of tropical Australian microalgae CRFI01 was superior to the commercial northern hemisphere TISO as a food source for Artemia. NT7, PS11, CS256 and GOC52 could be used as a replacement or supplementary feed to TISO.

Assessment of the nutritional value of three species of tropical microalgae, dried Tetraselmis and a yeast-based diet for larvae of the blacklip pearl oyster, Pinctada margaritifera (L.)

This paper reports on two experiments in which three species of tropical microalgae, Isochrysis (T-ISO), Pavlova salina and Chaetoceros simplex, dried Tetraselmis suecica and a commercial yeast-based diet (`L-10', Microfeast®) were assessed for their nutritional value for larvae of the blacklip pearl oyster, Pinctada margaritifera. In the first experiment, T-ISO supported significantly greater growth of D-stage larvae than either Pav. salina, C. simplex or a ternary algal diet (TAD) composed of an equal mixture of the three species. Fifty percent of the mixed algal diet could be replaced with dried Tetraselmis without significantly affecting larval growth rate or survival. Indeed, growth of larvae fed a 1:1 mixture of TAD and dried Tetraselmis was significantly greater than that of larvae fed the three species of microalgae alone or the TAD. In the second experiment, umbo larvae were fed either the TAD, dried Tetraselmis, `L-10' or 1:1 combinations of these diets. Survival of unfed larvae was significantly lower than that of fed larvae and larvae receiving food had significantly greater antero-posterior shell length than unfed controls. Larvae fed dried Tetraselmis were significantly larger than those fed `L-10' but did not differ significantly in size from those fed the TAD. Larvae fed the 1:1 mixture of dried Tetraselmis and `L-10' were significantly larger than those fed `L-10' alone but significantly smaller than those fed dried Tetraselmis alone. The largest larvae at the end of Experiment 2 were those fed a 1:1 mixture of TAD and `L-10' which were significantly larger than larvae fed the TAD.

Effect of temperature on the growth, total lipid content and fatty acid composition of recently isolated tropical microalgae Isochrysis sp., Nitzschia closterium, Nitzschia paleacea, and commercial species Isochrysis sp. (clone T.ISO)

The effect of temperature from 10 °C to 35 °C on the growth, total lipid content, and fatty acid composition of three species of tropical marine microalgae, Isochrysis sp., Nitzschia closterium, N. paleacea (formerly frustulum), and the Tahitian Isochrysis sp. (T.ISO), was investigated. Cultures of N. closterium, Isochrysis sp. and T.ISO grew very slowly at 35 °C, while N. closterium did not grow at temperatures higher than 30 °C or lower than 20 °C. N. paleacea was low-temperature tolerant, with cells growing slowly at 10 °C. N. paleacea produced the highest percentage of lipids at 10 °C, while the other species produced maximum amounts of lipid at 20 °C. None of the species maintained high levels of polyunsaturated fatty acids (PUFAs) at high growth temperature and there was a significant inverse relationship between the percentage of PUFAs and temperature for N. paleacea. A curved relationship was found between temperature and percentage of PUFA for N. closterium and tropical Isochrysis sp., with the maximum production of PUFA at 25 °C and 20 °C, respectively. The two Nitzschia species produced higher levels of the essential fatty acid eicosapentaenoic acid [20:5(n-3)] at lower growth temperatures, but the two Isochrysis species had little change in percentage of 20:5(n-3) with temperature. Only T.ISO had the highest percentage of 22:6(n-3) at lowest growth temperature (11.4% total fatty acids at 10 °C).