Bottom-up controls on a mixed-species HAB assemblage: A comparison of sympatric Chattonella subsalsa and Heterosigma akashiwo (Raphidophyceae) isolates from the Delaware Inland Bays, USA

Recent novel mixed blooms of several species of toxic raphidophytes have caused fish kills and raised health concerns in the highly eutrophic Inland Bays of Delaware, USA. The factors that control their growth and dominance are not clear, including how these multi-species HAB events can persist without competitive exclusion occurring. We compared and contrasted the relative environmental niches of sympatric Chattonella subsalsa and Heterosigma akashiwo isolates from the bays using classic Monod-type experiments. C. subsalsa grew over a temperature range from 10 to 30 °C and a salinity range of 5–30 psu, with optimal growth occurring from 20 to 30 °C and 15 to 25 psu. H. akashiwo had similar upper temperature and salinity tolerances but also lower limits, with growth occurring from 4 to 30 °C and 5 to 30 psu and optimal growth between 16 and 30 °C and 10 and 30 psu. These culture results were confirmed by field observations of bloom occurrences in the Inland Bays. Maximum nutrient-saturated growth rates (μ_max) for C. subsalsa were 0.6 d⁻¹ and half-saturation concentrations for growth (K_s) were 9 μM for nitrate, 1.5 μM for ammonium, and 0.8 μM for phosphate. μ_max of H. akashiwo (0.7 d⁻¹) was slightly higher than C. subsalsa, but K_s values were nearly an order of magnitude lower at 0.3 μM for nitrate, 0.3 μM for ammonium, and 0.2 μM for phosphate. H. akashiwo is able to grow on urea but C. subsalsa cannot, while both can use glutamic acid. Cell yield experiments at environmentally relevant levels suggested an apparent preference by C. subsalsa for ammonium as a nitrogen source, while H. akashiwo produced more biomass on nitrate. Light intensity affected both species similarly, with the same growth responses for each over a range from 100 to 600 μmol photons m⁻² s⁻¹. Factors not examined here may allow C. subsalsa to persist during multi-species blooms in the bays, despite being competitively inferior to H. akashiwo under most conditions of nutrient availability, temperature, and salinity.     

Evidence for inhibition of bacterial luminescence by allelochemicals from <Emphasis Type="Italic">Fibrocapsa japonica</Emphasis> (Raphidophyceae), and the role of light and microalgal growth rate

The marine microalga Fibrocapsa japonica Toriumi and Takano (Raphidophyceae) produces haemolysins, neurotoxins and reactive oxygen species (ROS). To quantify potential effects of such bioactive compounds on surrounding organisms the marine bacterium Vibrio fischeri was exposed to F. japonica culture samples. Inhibition of V. fischeri ‘s natural luminescence, indicative of impaired metabolism, was related to the number of F. japonica cells added. The effect was fast, within 15 min. It was caused by one, possibly several, excreted substances that were less active after heating. Freezing of culture supernatant partly inactivated these substances, but ROS-scavenging enzymes had no effect. Light enhanced the V. fischeri luminescence inhibition in two ways. The direct effect of light on the action of F. japonica luminescence inhibiter(s) could be described by a saturation curve with maximum effect above 20 μmol photons m⁻² s⁻¹. Light also had an indirect effect: biomass production, dependent on light availability, was closely related to the amount of inhibiting compound(s) produced by the alga. Algal growth rate, rather than its cell density, determined the bacterial luminescence inhibition per F. japonica cell, resulting in a 5-fold stronger inhibition at maximum growth rates compared to cells that barely grew during the stationary growth phase. The bioassay with F. japonica and V. fischeri has allowed quantification of the negative effects on bacteria in the microalgal microenvironment. The results presented here suggest that at favourable growth conditions F. japonica releases bioactive compounds that improve its competitive abilities.     

Differences in photoprotective pigment production between Japanese and Australian strains of Chattonella marina (Raphidophyceae)

Previous studies have shown that isolates of Chattonella marina from Australia and Japan exhibit differences in tolerance to high intensities of visible light. Here we show that the Australian strain of C. marina produces around five times more UV-absorbing mycosporine amino acids (MAAs) than the Japanese strain. This corresponds with 66% increased growth by the Australian strain under UVB exposure compared to no UV exposure. The MAA mycosporine-glycine, which reportedly acts as an antioxidant, was found in high quantity (110 fg cell^-1) in the Australian but was absent in the Japanese strain. In contrast, changes in the concentration of violaxanthin and zeaxanthin per cell were 4.7-4.8 times greater in the Japanese relative to the Australian strain suggesting that the Japanese strain uses a xanthophyll cycle to moderate inhibition by high photosynthetically active radiation (PAR) irradiance. Increased MAA production under high irradiance was also observed in other Australian strains of Chattonella, but not noted in other Japanese strains suggesting ecophenotypic adaptation due to differing environmental conditions.     

Chemotaxonomic survey of sterols and fatty acids in six marine raphidophyte algae

Fatty acid and sterol profiles allowed for clear discrimination betweentheraphidophyte genera Chattonella,Heterosigma, Fibrocapsa andOlisthodiscus, but exhibited little differentiation forindividual Chattonella species(C.marina, C. antiqua and C.subsalsa). Sterol and fatty acid profiles do not support theseparation of Chattonella antiqua and C.marina as distinct species. Ecophenotypic variations in lipidprofiles were also observed between Chattonella strainsfromdifferent geographic locations. Sterol signatures which may be useful aschemotaxonomic markers were: the absence of C₂₇ sterols (cholesteroland 24-dihydrozymosterol) in Heterosigma akashiwo; thepresence of isofucosterol in Chattonella; and theoccurrence of brassicasterol, poriferasterol and fucosterol inOlisthodiscus luteus. High levels of eicosapentaenoic acid(EPA; 17-27% of fatty acids) were present in all raphidophyte species. Lipidcomposition correlated more closely to recent molecular classification ofraphidophytes than carotenoid pigments.     

Sublethal effects of the toxic alga Heterosigma akashiwo on the southeastern oyster (Crassostrea virginica)

Over the last three years, several blooms of Heterosigma akashiwo (Raphidophyceae) were documented in South Carolina (SC) brackish waters, including areas containing extensive oyster (Crassostrea virginica) beds. This study examined the sublethal effects of H. akashiwo on C. virginica, based on cellular biomarker responses after exposure to laboratory cultures of H. akashiwo isolated from SC waters, and to water collected from two SC H. akashiwo blooms. Exposure to laboratory cultures or blooms of H. akashiwo significantly increased oyster hepatopancreas lysosomal destabilization rates, but had little effect on gill p-glycoprotein (p-gp) expression. Lysosomal destabilization in oysters continued to increase even after a 7-day recovery period in clean seawater, suggesting that H. akashiwo toxin or other cellular byproducts continued to damage the hepatopancreas. These results suggest that even short-term exposures of oysters to high cell densities of H. akashiwo could have long-term adverse physiological effects, and imply that oyster health may be compromised in areas where repetitive H. akashiwo blooms occur.