Solar radiation is the major factor shaping the environment where algae, cyanobacteria and aquatic macrophytes can thrive. Light is the energy source for photosynthesis and growth and can control algal net primary production. Changes in light intensity occur on orders of magnitude within timescales from milliseconds to hours, seasons and even eons. In water, light is exponentially attenuated with depth, with attenuation being wavelength dependent. As a result, the intensity and spectrum of available light can be very dynamic and unpredictable. To cope with such a challenging environment, an array of sensing and feedback-loop mechanisms has evolved in aquatic phototrophs. The structural and functional plasticity of light harvesting and photoprotection mechanisms is also extremely high. This has allowed algae to occur, thrive and evolve in all niches where light is available.

Active nonequilibrium processes are characterized by the coupling of an ionic current or a mechanical motion to a chemical reaction. This coupling induces energy transduction, satisfying Onsager reciprocal relations in the linear regime close to equilibrium and the bivariate fluctuation relation in regimes farther away from equilibrium. These considerations concern, in particular, molecular motors as well as active colloidal particles that are self-propelled by chemical reactions catalyzed at their surface and diffusiophoresis. These active processes can be described by stochastic processes obeying bivariate fluctuation relations for the coupled currents. The mechanochemical coupling can be characterized in terms of the linear and nonlinear response coefficients, as well as the efficiencies defined in the different regimes of energy transduction, i.e., propulsion by the chemical reaction on the one hand, and the synthesis of fuel from products on the other hand.