The ocean's invisible engine
The most abundant photosynthetic organism on Earth is a marine cyanobacterium named Prochlorococcus. Its existence was unknown until 1986, when it was identified by a team of scientists including Sallie "Penny" Chisholm and Robert J. Olson. The official discovery was published in 1988. The organism's tiny size, just 0.5 to 0.7 micrometers in diameter, allowed it to evade detection by all but the most sensitive modern instruments like flow cytometers. This bacterium is responsible for producing up to 20% of the oxygen in the planet's biosphere, which is more than terrestrial rainforests combined.
The global population of Prochlorococcus is immense, estimated at around three octillion (3 x 10^27) individuals. These cells are found throughout the world's oceans in a wide band between 40°N and 40°S latitudes. In the nutrient-poor open oceans where it dominates, a single milliliter of surface seawater can contain over 100,000 cells. It thrives in the sunlit upper layer of the ocean, known as the euphotic zone, down to depths of about 150 to 200 meters. Together with its relative, Synechococcus, it accounts for about half of all marine carbon fixation, forming the base of the ocean's food web.
A streamlined global force
Prochlorococcus has one of the smallest genomes of any free-living photosynthetic organism. High-light adapted varieties have genomes with as few as 1,716 genes, compared to over 10,000 in many eukaryotic algae. This genetic efficiency is a result of streamlining its genome to survive in low-nutrient environments. For example, it has adapted to use sulfolipids instead of phospholipids in its cell membranes, which reduces its need for phosphorus.
Different "ecotypes" of Prochlorococcus are adapted to specific ocean niches. High-light (HL) ecotypes live closer to the surface, while low-light (LL) ecotypes are found deeper in the water column. These varieties possess different ratios of unique pigments, including divinyl derivatives of chlorophyll a and b, which are specialized for absorbing the blue light that penetrates deep into ocean water. This genetic diversity allows the species to colonize a vast range of oceanic environments. Temperature is a major factor controlling its distribution, with different ecotypes showing distinct optima and tolerance ranges.