Contrasting effects of high-intensity photosynthetically active radiation on two bloom-forming dinoflagellates.

While light limitation can inhibit bloom formation in dinoflagellates, the potential for high-intensity photosynthetically active radiation (PAR) to inhibit blooms by causing stress or damage has not been well-studied. We measured the effects of high-intensity PAR on the bloom-forming dinoflagellates Alexandrium fundyense and Heterocapsa rotundata. Various physiological parameters (photosynthetic efficiency Fv /Fm , cell permeability, dimethylsulfoniopropionate [DMSP], cell volume, and chlorophyll-a content) were measured before and after exposure to high-intensity natural sunlight in short-term light stress experiments. In addition, photosynthesis-irradiance (P-E) responses were compared for cells grown at different light levels to assess the capacity for photophysiological acclimation in each species. Experiments revealed distinct species-specific responses to high PAR. While high light decreased Fv /Fm in both species, A. fundyense showed little additional evidence of light stress in short-term experiments, although increased membrane permeability and intracellular DMSP indicated a response to handling. P-E responses further indicated a high light-adapted species with Chl-a inversely proportional to growth irradiance and no evidence of photoinhibition; reduced maximum per-cell photosynthesis rates suggest a trade-off between photoprotection and C fixation in high light-acclimated cells. Heterocapsa rotundata cells, in contrast, swelled in response to high light and sometimes lysed in short-term experiments, releasing DMSP. P-E responses confirmed a low light-adapted species with high photosynthetic efficiencies associated with trade-offs in the form of substantial photoinhibition and a lack of plasticity in Chl-a content. These contrasting responses illustrate that high light constrains dinoflagellate community composition through species-specific stress effects, with consequences for bloom formation and ecological interactions within the plankton.

Broad Salinity Tolerance as a Refuge from Predation in the Harmful Raphidophyte Alga Heterosigma akashiwo (Raphidophyceae).

The ability of harmful algal species to form dense, nearly monospecific blooms remains an ecological and evolutionary puzzle. We hypothesized that predation interacts with estuarine salinity gradients to promote blooms of Heterosigma akashiwo (Y. Hada) Y. Hada ex Y. Hara et M. Chihara, a cosmopolitan toxic raphidophyte. Specifically, H. akashiwo's broad salinity tolerance appears to provide a refuge from predation that enhances the net growth of H. akashiwo populations through several mechanisms. (1) Contrasting salinity tolerance of predators and prey. Estuarine H. akashiwo isolates from the west coast of North America grew rapidly at salinities as low as six, and distributed throughout experimental salinity gradients to salinities as low as three. In contrast, survival of most protistan predator species was restricted to salinities >15. (2) H. akashiwo physiological and behavioral plasticity. Acclimation to low salinity enhanced H. akashiwo's ability to accumulate and grow in low salinity waters. In addition, the presence of a ciliate predator altered H. akashiwo swimming behavior, promoting accumulation in low-salinity surface layers inhospitable to the ciliate. (3) Negative effects of low salinity on predation processes. Ciliate predation rates decreased sharply at salinities <25 and, for one species, H. akashiwo toxicity increased at low salinities. Taken together, these behaviors and responses imply that blooms can readily initiate in low salinity waters where H. akashiwo would experience decreased predation pressure while maintaining near-maximal growth rates. The salinity structure of a typical estuary would provide this HAB species a unique refuge from predation. Broad salinity tolerance in raphidophytes may have evolved in part as a response to selective pressures associated with predation.

A giant cell surface protein in Synechococcus WH8102 inhibits feeding by a dinoflagellate predator.

Diverse strains of the marine planktonic cyanobacterium Synechococcus sp. show consistent differences in their susceptibility to predation. We used mutants of Sargasso Sea strain WH8102 (clade III) to test the hypothesis that cell surface proteins play a role in defence against predation by protists. Predation rates by the heterotrophic dinoflagellate Oxyrrhis marina on mutants lacking the giant SwmB protein were always higher (by 1.6 to 3.9×) than those on wild-type WH8102 cells, and equalled predation rates on a clade I strain (CC9311). In contrast, absence of the SwmA protein, which comprises the S-layer (surface layer of the cell envelope that is external to the outer membrane), had no effect on predation by O. marina. Reductions in predation rate were not due to dissolved substances in Synechococcus cultures, and could not be accounted for by variations in cell hydrophobicity. We hypothesize that SwmB defends Synechococcus WH8102 by interfering with attachment of dinoflagellate prey capture organelles or cell surface receptors. Giant proteins are predicted in the genomes of multiple Synechococcus isolates, suggesting that this defence strategy may be more general. Strategies for resisting predation will contribute to the differential competitive success of different Synechococcus groups, and to the diversity of natural picophytoplankton assemblages.

INTERSTRAIN VARIABILITY IN PHYSIOLOGY AND GENETICS OF HETEROSIGMA AKASHIWO (RAPHIDOPHYCEAE) FROM THE WEST COAST OF NORTH AMERICA(1).

High levels of intraspecific variability are often associated with HAB species, and this variability is likely an important factor in their competitive success. Heterosigma akashiwo (Hada) Hada ex Y. Hara et M. Chihara is an ichthyotoxic raphidophyte capable of forming dense surface-water blooms in temperate coastal regions throughout the world. We isolated four strains of H. akashiwo from fish-killing northern Puget Sound blooms in 2006 and 2007. By assessing numerous aspects of biochemistry, physiology, and toxicity, we were able to describe distinct ecotypes that may be related to isolation location, source population, or bloom timing. Contrasting elements among strains were cell size, maximum growth and photosynthesis rates, tolerance of low salinities, amino acid use, and toxicity to the ciliate grazer Strombidinopsis acuminatum (Fauré-Fremiet). In addition, the rDNA sequences and chloroplast genome of each isolate were examined, and while all rDNA sequences were identical, the chloroplast genome identified differences among the strains that tracked differences in ecotype. H. akashiwo strain 07A, which was isolated from an unusual spring bloom, had a significantly higher maximum potential photosynthesis rate (28.7 pg C · cell(-1) · h(-1) ) and consistently exhibited the highest growth rates. Strains 06A and 06B were not genetically distinct from one another and were able to grow on the amino acids glutamine and alanine, while the other two strains could not. Strain 07B, which is genetically distinct from the other three strains, exhibited the only nontoxic effect. Thus, molecular tools may support identification, tracking, and prediction of strains and/or ecotypes using distinctive chloroplast gene signatures.