From the light to the darkness: thriving at the light extremes in the oceans

Light-acclimation processes are central to allowing photosynthesis in aquatic ecosystems to span from high light conditions, that are 10-fold higher than the light levels required to saturate photosynthesis, to the deep sea with extremely low light levels. In dim light systems, nutrient levels are often high, and cells maximize the absorption of light by increasing the cellular pool of pigments. The upper limits of light absorption are constrained by the package effect, which ultimately restricts the benefit of the light absorption associated with an increase in cellular pigmentation, thus decreasing the cost/benefit ratio relative to the metabolic cost of manufacturing cellular light-harvesting pigments. At extremely low light levels in the deep sea, chloroplasts are sequestered in numerous organisms; however, these species are not obligate autotrophs and supplement a heterotrophic/mixotrophic existence with opportunistic autotrophy. While low light acclimation is based on maximizing light absorption, photosynthetic systems under high light, in addition to decreased light-harvesting cross sections, rely on energy-dissipation processes to avoid light-induced damage to the photosynthetic apparatus and other free radical susceptible cell structures. Dissipation of excess light energy represents the largest sink of the absorbed light in high light environments; however, these processes remain largely unstudied and are rarely quantified. Cells supplement their energy-dissipation processes through increasing the capacity to remove high-light-generated radicals and/or inducing vertical movement. Improved understanding of strategies remains central for the understanding of algal distributions in nature and has broad industrial implications.