Eddy-wind interactions stimulate extraordinary mid-ocean plankton bloomsOne-sentence summary: Mid-ocean eddies, together with wind-forced motions, cause episodic bursts of nutrient supply to the upper ocean, changes in plankton community structure, and export of organic material to the deep sea. Understanding the controls on primary production in the upper ocean is of fundamental importance for two main reasons. First, primary productivity sets a firstorder constraint on the energy available to sustain oceanic ecosystems. Second, fixation and subsequent sinking of organic particles removes carbon from the surface ocean (the so-called "biological pump"), which plays a key role in partitioning of carbon dioxide between the ocean and atmosphere. Geochemical estimates of new production (1) surpass the apparent rate of nutrient supply by vertical mixing by a factor of two or more in subtropical oceans (2-6), which constitute some of the largest biomes on earth. Two possible mechanisms to supply the "missing" nutrient locally include nitrogen fixation by cyanobacteria (7-10), and intermittent upwelling by mesoscale eddies and submesoscale processes (11-21).

Although iron is the fourth most abundant element in the Earth's crust, its concentration in the aquatic ecosystems-particularly the open oceans-is sufficiently low to limit photosynthetic activity and phytoplankton growth. Cyanobacteria, a major class of phytoplankton, respond to iron deficiency by expressing the 'iron-stress-induced' gene, isiA(ref. 3). The protein encoded by this gene has an amino-acid sequence that shows significant homology with one of the chlorophyll a-binding proteins (CP43) of photosystem II (PSII). The precise function of the CP43-like protein, here called CP43', has not been elucidated, although there have been many suggestions. Here we show that CP43' associates with photosystem I (PSI) to form a complex that consists of a ring of 18 CP43' molecules around a PSI trimer. This significantly increases the size of the light-harvesting system of PSI. The utilization of a PSII-like protein as an extra antenna for PSI emphasises the flexibility of cyanobacterial light-harvesting systems, and seems to be a strategy which compensates for the lowering of phycobilisome and PSI levels in response to iron deficiency.

Mesoscale eddies may play a critical role in ocean biogeochemistry by increasing nutrient supply, primary production, and efficiency of the biological pump, that is, the ratio of carbon export to primary production in otherwise nutrient-deficient waters. We examined a diatom bloom within a cold-core cyclonic eddy off Hawaii. Eddy primary production, community biomass, and size composition were markedly enhanced but had little effect on the carbon export ratio. Instead, the system functioned as a selective silica pump. Strong trophic coupling and inefficient organic export may be general characteristics of community perturbation responses in the warm waters of the Pacific Ocean.