Influence of the Calcium Carbonate Shell of Coccolithophores on Ingestion and Growth of a Dinoflagellate Predator

Coccolithophores are an important group of ∼200 marine phytoplankton species which cover themselves with a calcium carbonate shell called “coccosphere.” Coccolithophores are ecologically and biogeochemically important but the reason why they calcify remains elusive. One key function may be that the coccosphere offers protection against microzooplankton predation, which is one of the main causes of phytoplankton death in the ocean. Here, we investigated the effect of the coccosphere on ingestion and growth of the heterotrophic dinoflagellate Oxyrrhis marina. Calcified and decalcified cells of the coccolithophore species Emiliania huxleyi, Pleurochrysis carterae, and Gephyrocapsa oceanica were offered separately to the predator as well as in an initial ∼1:1 mixture. The decrease of the prey concentrations and predator abundances were monitored over a period of 48–72 h. We found that O. marina did not actively select against calcified cells, but rather showed a size selective feeding behavior. Thus, the coccosphere does not provide a direct protection against grazing by O. marina. However, O. marina showed slower growth when calcified coccolithophores were fed. This could be due to reduced digestion rates of calcified cells and/or increased swimming efforts when ballasted with heavy calcium carbonate. Furthermore, we show that the coccosphere reduces the ingestion capacity simply by occupying much of the intracellular space of the predator. We speculate that the slower growth of the grazer when feeding on calcified cells is of limited benefit to the coccolithophore population because other co-occurring phytoplankton species within the community that do not invest energy in the formation of a calcite shell could also benefit from the reduced growth of the predators. Altogether, these new insights constitute a step forward in our understanding of the ecological relevance of calcification in coccolithophores.

Pleurochrysis carterae Hot-Water Extract Inhibits Melanogenesis in Murine Melanoma Cells

In this study, we examined the effect of a hot-water extract of coccolithophore Pleurochrysis carterae on melanogenesis in B16F1 and B16F10 melanoma cells. P. carterae extract inhibited the α-melanocyte-stimulating hormone (α-MSH)-enhanced melanin synthesis in B16F1 melanoma cells. P. carterae also inhibited unstimulated melanin synthesis in B16F10 melanoma cells. Western blotting showed that the P. carterae extract inhibited tyrosinase and microphthalmia-associated transcription factor (MITF) in a dose-dependent manner. The reporter assay also revealed a decline in the tyrosinase promoter activity in the presence of P. carterae extract. Furthermore, quantitative real-time RT-PCR analysis showed that P. carterae extract downregulated the mRNA levels of tyrosinase and MITF. Finally, our study demonstrated that the hot-water extract of P. carterae inhibits melanin synthesis via the down-regulation of MITF mRNA level. Our findings indicate that P. carterae extract could be a possible cosmetic ingredient.

Native-state imaging of calcifying and noncalcifying microalgae reveals similarities in their calcium storage organelles

Calcium storage organelles are common to all eukaryotic organisms and play a pivotal role in calcium signaling and cellular calcium homeostasis. In most organelles, the intraorganellar calcium concentrations rarely exceed micromolar levels. Acidic organelles called acidocalcisomes, which concentrate calcium into dense phases together with polyphosphates, are an exception. These organelles have been identified in diverse organisms, but, to date, only in cells that do not form calcium biominerals. Recently, a compartment storing molar levels of calcium together with phosphorous was discovered in an intracellularly calcifying alga, the coccolithophoreEmiliania huxleyi, raising a possible connection between calcium storage organelles and calcite biomineralization. Here we used cryoimaging and cryospectroscopy techniques to investigate the anatomy and chemical composition of calcium storage organelles in their native state and at nanometer-scale resolution. We show that the dense calcium phase inside the calcium storage compartment of the calcifying coccolithophorePleurochrysis carteraeand the calcium phase stored in acidocalcisomes of the noncalcifying algaChlamydomonas reinhardtiihave common features. Our observations suggest that this strategy for concentrating calcium is a widespread trait and has been adapted for coccolith formation. The link we describe between acidocalcisomal calcium storage and calcium storage in coccolithophores implies that our physiological and molecular genetic understanding of acidocalcisomes could have relevance to the calcium pathway underlying coccolithophore calcification, offering a fresh entry point for mechanistic investigations on the adaptability of this process to changing oceanic conditions.