Aquaculture & seafood
Microalgae under cultivation
Why algal growth is light-limited and self-shading, what open ponds and photobioreactors each trade away, and why keeping a monoculture alive against grazers is the central battle of the whole sector.
In the sea, microalgae are the base of every food web, and aquaculture inherits that role: larval fish and shrimp start life eating living algal cells, bivalves eat nothing else, and the omega-3 in farmed salmon is algal in origin two steps removed. Growing the cells on purpose, however, means holding a photosynthetic monoculture in warm fertilized water — an artificial condition the organisms themselves did not evolve to tolerate.
Growth is a light-dilution problem
Each algal cell saturates photosynthetically at a modest irradiance; brighter light does not make it grow faster, it makes the surplus destructive through photoinhibition. But a dense culture is also opaque: the top few centimetres absorb most of the incoming light and the rest of the column sits in darkness, so what limits production is not light at the surface but how light is distributed through the volume. Cultures are therefore mixed, to cycle cells between bright and dark zones, and productivity scales with the light the whole surface intercepts per day rather than with cell performance. This single constraint explains the economics: reactors must be shallow or thin to admit light, which forces large land areas and low volumetric densities, which in turn makes harvesting dilute broth the dominant cost.
Ponds versus photobioreactors
An open raceway is a shallow channel stirred by a paddlewheel — cheap, durable, and completely open to the sky. A photobioreactor is a closed loop of transparent tubes or panels: it buys higher cell density, better light-path control and protection from the outside biota at a multiple of the capital cost per square metre, and it adds its own problems of oxygen buildup, cooling and wall fouling. The choice follows from the product: bulk biomass such as spirulina tolerates open ponds, while any product sold on purity — food-grade omega-3, carotenoid oleoresins — pushes toward closed systems or out of photosynthesis altogether.
Contamination is the central battle
A microalgal culture is, to everything else in the pond, a rich medium: warm, fertilized, sunlit water full of edible cells. Grazers — rotifers, ciliates, amoebae — can strip a tank in days, and faster-growing contaminant algae simply outcompete the crop for nutrients and light. There is no antibiotic for this; the defenses are ecological. Some industrial species survive only in conditions hostile to competitors: spirulina at high alkalinity, Dunaliella at saturated salinity. Others are defended by enclosure, sterile technique and constant monitoring, and even then a crash is a normal operating event, not an anomaly. This is why the sector bifurcated: where light is essential to the product’s story — pigment induction, autotrophic claims — contamination is managed at high cost; where the product is only a molecule, growers abandon sunlight for sugar-fed fermentation in sterile stainless steel, trading the photon problem for a feedstock bill. The same omega-3 oils, grown in the dark, are the clearest proof of what limits the light-fed route.