Irradiance exposure experiments:  Experiments to test the light stress response consisted of exposing dinoflagellates to natural sunlight and monitoring a range of physiological and biochemical responses. Sunlight exposure took place in unshaded outdoor Plexiglas tanks (without lids), supplied with a constant flow of seawater to maintain incubation bottles approximate growth temperatures of 15-16°C for the duration of each experimental exposure period. A Li-Cor LI-1400 data logger with a 2π sensor collected incident PAR data; irradiance received during incubation period is presented as total dose (mol photons m-2) and (for Fv/Fm data) as the maximum instantaneous irradiance recorded during the exposure period (µmol photons m-2 s-1). To initiate experiments, dinoflagellate cultures were combined and divided into 250-mL polycarbonate bottles, which block penetration of UVR. Bottles were returned to the growth incubator for approximately 1 h acclimation before the experiment began. After acclimation, “pre-exposure” samples were collected from each bottle for determination of morphological, biochemical, and physiological characteristics of dinoflagellate cells. Bottles were then covered in treatment-specific neutral density screening as needed to achieve desired irradiances, and placed in the outdoor tank to begin the light exposure period. Control bottles were covered with enough screen layers to approximate growth incubator irradiances. Experiments included four replicates each of either two or three treatments: all experiments included “high PAR” and “control” treatments, while A2 and H2 also included “moderate PAR”. Exposure duration varied slightly by experiment but was approximately 1.5 h. Photosynthetic efficiency Fv/Fm (unitless) was measured at 15, 25, or 30 min time intervals during the exposure period, depending on the experiment. After the exposure period, all bottles were taken indoors for a second round of sampling (henceforth referred to as “post-exposure”). Bottles were then rid of any screening and returned to the growth incubator for the recovery period (1.5-2 h depending on experiment). After recovery a third round of samples (termed “post-recovery”) was collected.

Photosynthetic efficiency Fv/Fm:  Samples (2 mL) were taken from each bottle at regular intervals (see above) during the light treatment exposure and subsequent recovery period; samples were then dark-incubated at 15°C for 20 min. After dark acclimation, Fv/Fm was measured using a Walz Water-PAM pulse amplitude-modulated fluorometer.