K. RICHARDSON, J. BEARDALL, J. A. RAVEN
New Phytologist, Volume 93, Issue 2, February 1983, Pages 157–191
[…]problems surrounding the measurement of light have complicated the algal physiologist’s understanding of light adaptation by phytoplankton. In recent years, the convention of describing the light environment in units of illumination (lux, footcandles) has gradually been replaced by expressing light either in units of energy (joules, ergs, gram calories) or in quanta (Einsteins).
I learned that scientists switched from measuring in lux (the amount of illumination) and started measuring light power only since the 1980’s! Today, light power is mostly expressed as PAR, but some authors still seem to refer to the precursor, µE’s. (An obvious salute to Einstein.)
As photosynthesis at its most basic level is controlled by the absorption of photons, the expression of light in quanta (einsteins) would seem particularly suited to investigations into phytoplankton photoadaptation. Knowledge of the spectral distribution of the quanta received then allows calculation of the energy input.
Here’s why PAR is important to scientists. It enables much deeper levels of analysis by enabling us to know the amount of energy input. (It’s also why hobbyists don’t really need it for many of our activities.)
[…]reports of photoinhibition of growth or photosynthesis occur more often for dinoflagellates than for diatoms and in general, the onset of photoinhibition is at lower photon flux densities for dinoflagellates than for diatoms (see Table 3).
This is one big reason why we see dino’s “mat up” or form slime structures in our brightly lit reef tanks – they’re protecting themselves from exposure. It’s not unlike the adaptations of trees and corals to minimize their exposure. It also explains why dino’s appear to be such close associates of macroalgae – those algae already take an advantageous form that limits exposure.
In the dinoflagellate, A. carterae, photoinhibition has been identified both by impairment of photosynthetic characteristics and by the loss of variable fluorescence associated with PS II (Samuelsson and Richardson, 1982). This same species has been shown to concentrate at approximately 30 µE m2s2 when exposed to a photon flux density gradient from 0 to 120 µE m2s2 (Richardson et al., submitted for publication).