Effect of irradiances up to 2000 μE m -2 s -1 on marine Synechococcus WH7803—II. Photosynthetic responses and mechanisms
Abstract
We investigated the photosynthetic behavior of Synechococcus WH7803 when grown over the irradiance range of 30-2000 μE m -2 s -1 in nutrient-replete, continuous light, pre-adapted batch cultures. For each of 8 growth irradiances investigated, we found a unique photosynthesis vs irradiance ( P vs I) relationship. Cell- and carbon-specific photosynthetic light harvesting efficiencies (α (cell) and α (C)) decreased 10-fold from the lowest to highest growth irradiances. Chlorophyll and phycocyanin-specific efficiencies also decreased but to a lesser extent. Phycoerythrin-specific efficiencies increased ca. 2-fold with increasing irradiances. Photosynthetic capacity ( Pmax) approximated the in situ rate of photosynthesis ( Pi) only at growth irradiances which saturated growth rate; at light limiting irradiances, Pmax increased to 2.9 times Pi. Photoinhibition of photosynthesis was only observed in light-limited cells. The mechanism of photoadaptation which resulted in the observed growth rate vs irradiance (μ vs I) response involved regulation of α (cell) by changes in phycobilisome size over saturating irradiances and phycobilisome numbers over limiting irradiances. Our results are consistent with previously reported field observations of α (Chl), α (cell) and Pmax increasing with depth, and of no photoinhibition of near-surface collected assemblages, suggesting that high irradiances in near-surface waters can support maximal growth rates of Synechococcus. These results raise important questions regarding the interpretation of parameters of P vs I measurement from field studies. Our results do not conform to the previously reported hypothesis that a significant percentage of phycoerythrin serves as a non-photosynthetic nitrogen storage compound in these cells.
- Publication:
-
Deep Sea Research A
- Pub Date:
- April 1987
- DOI:
- 10.1016/0198-0149(87)90002-1
- Bibcode:
- 1987DSRA...34..497K