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Phytoplankton Bloom

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    Tiny round cells of the alga Phaeocystis are only a few micrometers in diameter, but they form large colonies of hundreds of individual cells held together with a gel-like substance. Here, in a rare view, are single cells, doublets, and the beginnings of colony formation. Colony-forming cells are getting a start by attaching to two cells of a larger algae species, Thalassiosira.
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    Probably the chain-forming diatom Thalassiosira. Cylindrical cells (seen here on edge) are linked by a central spine that looks like a very faint line. Other long spines protrude from the cells.

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    Two chains of the diatom Chaetoceros socialis curve toward each other. Long spines come from each cell's four corners and intertwine to attach it to its neighbor cells' spines. One spine at each end of a cell is longer than the other, and these intertwine with the long spines of other cells, forming three-dimensional ball-like clusters of chains—hence the name 'socialis', for this 'social' behavior.

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    Not all floating algae cells are round. This is a pennate (wing-shaped) diatom called Pseudonitzschia, each cell about 70 micrometers long. They form chains, but the chains aren't necessarily linear. They were found in brine channels inside the sea ice.

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    Another pennate diatom, Pleurosigma, tapers elegantly at either end. Small clumps of material inside the cell are the chlorophyll-containing chloroplasts, the organelles where photosynthesis takes place.
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    This strawberry-shaped cell, bearing a crown of hair-like cilia, is not part of the phytoplankton, but instead a single-celled predator on other cells in the bloom. It is a ciliate, part of the group sometimes called protozoa.

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    Diatom cells have hard inner and outer covers, like larger and smaller lids. They reproduce by both asexual cell division and a sexual form of reproduction. The Thalassiosira cell on the right has undergone sexual reproduction, leaving behind the "lid" of the parent cell—visible as the thin bracket-shaped object on the far right.

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    Two pairs of Thalassiosira diatoms 'caught in the act' of asexual reproduction. The pair on the left has just finished binary cell division, their usual, asexual form of reproduction. The thin gap between these two daughter cells indicates that division is complete and these are two independent individuals. The pair on the right is still dividing—still building, WHOI biologist Sam Laney says, the new cell walls and "exterior siding" that will separate them. 

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    Something different—a delicate fan-shaped colony of a golden-colored algae, called Dinobryon. The three-dimensional structure means that some cells are in focus, while others are not.
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    This diatom chain, says Laney, is not healthy. The cells are more or less empty shells, not packed with cytoplasm as in the earlier Thalassiosira images. "The shriveled cells here are a clear indicator that environmental conditions have taken a turn for the worse," he said, "presumably at the end of the bloom when the nutrients are all used up."

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