Induced Allelopathic Effects of Thalassiosira weissflogii on Colony Formation in Phaeocystis globosa

Comparison of photosynthetic responses between haptophyte Phaeocystis globosa and diatom Skeletonema costatum under phosphorus limitation

The diatom Skeletonema costatum and the haptophyte Phaeocystis globosa often form blooms in the coastal waters of the South China Sea. Skeletonema costatum commonly dominates in nutrient enrichment coastal waters, whereas P. globosa starts flourishing after the diatom blooms when phosphorus (P) is limited. Therefore, P limitation was proposed to be a critical factor affecting diatom-haptophyte transition. To elucidate the tolerance to P limitation in P. globosa compared with S. costatum, the effect of P limitation on their photosystem II (PSII) performance was investigated and their photosynthesis acclimation strategies in response to P limitation were evaluated. P limitation did not affect the growth of P. globosa over 7 days but decreased it for S. costatum. Correspondingly, the PSII activity of S. costatum was significantly inhibited by P limitation. The decline in PSII activity in S. costatum under P limitation was associated with the impairment of the oxygen-evolving complex (the donor side of PSII), the hindrance of electron transport from Q_A ^- to Q_B (the acceptor side of PSII), and the inhibition of electron transport to photosystem I (PSI). The 100% decrease in D1 protein level of S. costatum after P limitation for 6 days and PsbO protein level after 2 days of P limitation were attributed to its enhanced photoinhibition. In contrast, P. globosa maintained its photosynthetic activity with minor impairment of the function of PSII. With accelerated PSII repair and highly increased non-photochemical quenching (NPQ), P. globosa can avoid serious PSII damage under P limitation. On the contrary, S. costatum decreased its D1 restoration under P limitation, and the maximum NPQ value in S. costatum was only one-sixth of that in P. globosa. The present work provides extensive evidence that a close interaction exists between the tolerance to P limitation and photosynthetic responses of S. costatum and P. globosa.

Effects on Cell Growth, Lipid and Biochemical Composition of Thalassiosira weissflogii (Bacillariophyceae) Cultured under Two Nitrogen Sources

The protein and polyunsaturated fatty acid (PUFA) enrichment of microalgae can improve their nutritional value for larvae of various reared organisms. Protein enrichment can be achieved by increasing nitrogen concentration and selecting nitrogen sources that are easy to assimilate during microalga culture. Nitrogen-rich cultures can increase organism growth, biomass, and protein content, but their lipid content tends to stall. Since the diatom Thalassiosira weissflogii is usually provided to feed shrimp larvae, this study evaluated its digestibility and biochemical composition, culturing it with two nitrogen sources (NaNO3 and NH4Cl) at different concentrations (111.25, 222.50, 445 and 890 µM). The cell abundance, dry-weight biomass, Chl a, proteins, carbohydrates, total lipids and essential fatty acids were determined. The cell density and biomass peaked faster (day 12) with treatment < 890 µM than with 890 µM (day 15) in both nitrogen sources. However, the highest cell density, biomass and peak protein yield were not significantly different among treatments, suggesting the need to compare maintenance costs for a given production. After nine days of culture, concentrations ≤ 222.5 µM increased lipid content irrespective of the nitrogen source and decreased by 10–20% afterwards. With higher lipid production, the dominant PUFA were eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). One gram of NH4Cl provides ~60% more nitrogen than 1 g of NaNO3. In conclusion, based on time and growth rate, T. weissflogii cultivated with NH4Cl at 222.50 µM produced EPA and DHA at a better yield–cost ratio for biomass and lipid production. Furthermore, its nutritional value as enriched live-food for the reared larvae of marine organisms suggests potential biotechnological applications for aquaculture.

Pigment characterization of the giant colony-forming haptophyte Phaeocystis globosa in the Beibu Gulf reveals blooms of different origins

The giant colony-forming haptophyte Phaeocystis globosa has caused several large-scale blooms in the Beibu Gulf since 2011, but the distribution and dynamics of the blooms remained largely unknown. In this study, colonies of P. globosa, as well as membrane-concentrated phytoplankton samples, were collected during eight cruises from September 2016 to August 2017 in the Beibu Gulf. Pigments were analyzed by high performance liquid chromatography coupled with a diode-array detector (HPLC-DAD). The pigment 19'-hexanoyloxyfucoxanthin (hex-fuco), generally considered as a diagnostic pigment for Phaeocystis, was not detected in P. globosa colonies in Beibu Gulf, whereas 19'-butanoyloxyfucoxanthin (but-fuco) was found in all colony samples. Moreover, but-fuco in membrane-concentrated phytoplankton samples exhibited a similar distribution pattern to that of P. globosa colonies, suggesting that but-fuco provided the diagnostic pigment for bloom-forming P. globosa in the Beibu Gulf. Based on the distribution of but-fuco in different water masses in the region prior to the formation of intensive blooms, it's suggested that P. globosa blooms in the Beibu Gulf could originate from two different sources. IMPORTANCE Phaeocystis globosa has formed intensive blooms in the South China Sea and even around the world, causing huge social economic losses and environmental damage. However, little is known about the formation mechanism and dynamics of P. globosa blooms. 19'-hexanoyloxyfucoxanthin (hex-fuco) is often used as the pigment proxy to estimate Phaeocystis biomass, while this is challenged by the giant colony-forming P. globosa in the Beibu Gulf which only containing 19'-butanoyloxyfucoxanthin (but-fuco) but not hex-fuco. Using but-fuco as a diagnostic pigment, we traced two different origins of P. globosa bloom in Beibu Gulf. This study clarified the development process of P. globosa blooms in the Beibu Gulf, which provided a basis for the early monitoring and prevention of the bloom.

The effect of Ditylum brightwellii (Bacillariophyceae) on colony development of bloom forming species Phaeocystis globosa (Prymnesiophyceae) under nutrient-replete condition

To improve our knowledge of the factors regulating Phaeocystis globosa colony formation, the effects of the diatom Ditylum brightwellii on P. globosa colony development were investigated using co-culture and cell-free filtrate approaches. The co-culture experiments showed the moderate abundance of D. brightwellii significantly increased the number and size of colonies, whereas a dramatically decreased effect from high abundance of D. brightwellii. The low abundance of D. brightwellii promoted early formation of P. globosa colony. The cell-free filtrate experiments indicated that culture-filtrates from the exponential phase of D. brightwellii were stimulatory for P. globosa colony formation with more and bigger colonies formed, whereas an inhibitory effect from its senescence phase filtrates. D. brightwellii may influence P. globosa colony formation by regulating the growth of P. globosa solitary cells. Our results suggest that D. brightwellii influences P. globosa colony development, but its effects vary according to its concentrations and growth phases.