Photoacclimation by the Antarctic flagellate Pyramimonas sp. (Prasinophyceae) in response to iron limitation

Abstract

In this study we tested the hypothesis that iron limitation suppresses photoacclimation in cultures of the Antarctic flagellate Pyramimonas sp. The cultures were exposed to two different irradiances under iron-rich and iron-poor conditions. Light-harvesting capacity was determined by assessing the pigment composition and measuring in vivo absorption spectra. Light utilization efficiency (α) was determined from photosynthesis versus irradiance curves. The quantum yield of photosynthesis (ϕ_m) was calculated using α and the absorption spectra. Iron limitation led to commonly observed changes in cells of Pyramimonas, that is, a decrease in cellular pigment content and a reduction in cellular carbon and nitrogen quota. A reduction in α^cell followed a decrease in ϕ_m and light-harvesting capacity. Interpretation of the effects of iron limitation was different when considered on a carbon basis. Because iron limitation resulted in a decrease in cellular carbon content, the carbon-specific absorption coefficient was not affected. Consequently, the observed decrease in α^C was mainly due to the decrease in ϕ_m, showing that iron limitation did not control light utilization via pigment synthesis but exerted control on energy transfer. This is supported by the findings that at high irradiance a shift in pigment ratios within the total pool of violaxanthin, antheraxanthin and zeaxanthin towards zeaxanthin, which is indicative of photoacclimation to high irradiance, was observed for iron-replete cells as well as for iron-depleted cells. In contrast to what is generally hypothesized, the effects of iron limitation were not enhanced at low irradiance. Low irradiance led to an increase in the cellular light-harvesting pigment content. This increase was less pronounced in iron-depleted cells than in iron-replete cells. However, looking at the light-harvesting capacity of the cells on a carbon basis, it was found that iron-depleted cells responded similarly to iron-replete cells. We therefore conclude that the light-harvesting capacity was governed by light conditions and not by iron limitation. In addition to the increase in absorption capacity at low irradiance, an increase in light utilization efficiency was measured, again under both iron-rich and iron-poor conditions. Notably, the relative increase in α^C was strongest in iron-depleted cells. Photoacclimation was clearly demonstrated by normalizing α to chl a. For iron-replete cells, α^chl was highest at high irradiance. In contrast, for iron-depleted cells α^chl was highest at low irradiance. We argue that iron-depleted cells can photoacclimate to low irradiance by a reduction in the ‘package effect’ and reducing growth rates.