Protoplasmic streaming of chloroplasts enables rapid photoacclimation in large diatoms

Abstract

Long-Term (2004-2020) studies showed yearly summer/autumn blooms in the NE Black Sea dominated by large (cell volume > 5000 μm3) diatoms (Pseudosolenia calcar-Avis and Proboscia alata). This phenomenon is characterized by high (>250 W m-2 photosynthetically active radiation, PAR) insolation, and low phosphorus concentrations (to analytical zero). These diatoms contained >100 chloroplasts per cell, which at low irradiance are evenly distributed throughout the cell. As light increases (to 1000 μmol photons m-2 s-1 PAR), chloroplasts aggregate within 20 min, usually to the center of the cell. In consequence, the light absorption coefficient is decreased by >3 fold. At elevated photon flux density (PFD), P. calcar-Avis also shows a "conveyor"of chloroplasts moving from the aggregate to the cell periphery and back. This mechanism enables a continuous fine-Tuning of the cells' ability to absorb light, likely also facilitating photo-damage repair. This rapid photoacclimation mechanism allows large diatoms to minimize photodamage at high PFD and acclimate well to low PFD. We hypothesize that competitive success of large diatoms in conditions of high light gradients is aided by this short-Term rapid photoacclimation enhancing growth rate while minimizing chloroplast repair costs, aided by the ability of large cells to accumulate nutrients for chloroplast synthesis.