Imgine yourself in a light forest looking upwards, seeing in your mind's eye only the chlorophyll-bearing cells other canopy floating in mid-air, free from the attachment of leaves, twigs, branches antrunks. Now forget the forest an the trees, and see only blurred clouds of tiny green cells obscuring the blue sky beyond. You are looking at a phy- to plankton bloom of a density typical of lakes and coastal oceans. Forests and algal blooms fix about the same amount of carbon-a few grams per square metre per day-because both are based on essentially the same photo- synthetic machinery, fuelled by chlorophyll a in chloroplasts, the descendants of free-living cyanobacteria that have since evolved into plant organelles by endosymbiosis.
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