Studies On Photophysiology Of Two Diatoms

The underwater light climate changes with time and water depth.It is also closely related to suspended matters in the water column.In order to better estimate the primary productivity in the ocean,it is of great importance to understand the photophysiological responses of phytoplankton to changing light.Phytoplankton in coastal waters experiences large fluctuations with variations of light.Ice algae communities are always shade-adapted and can experience very high light intensities due to ice thinning and melting in spring.In this work,the physiological responses of two marine diatoms to different light intensities were studied.They are the large centric diatom Coscinodiscus granii isolated from Wuyuan Bay in Xiamen,and the pennate diatom Nitzschia sp.,an ice alga isolated from Saroma Lagoon in Hokkaido.Cells with three different sizes of C.granii were grown at four light intensities(70,125,225,450 ?mol photons m-2 s-1)to investigate their light acclimation strategies.The photophysiology of different-sized cells were different.Up to several folds of variations in cellular chlorophyll a(Chl a)content were employed to ensure a coherent light absorbing efficiency.This pigment-related light absorbing modulation was necessarily effective based on the fact that similar light absorbing capacities were found in all treatments.Higher light absorbing efficiency was found in low light.Although the Chl a content per unit volume in large cells was lower,the light absorbing efficiency was relatively high owning to the less severe package effect.Divergent relationships between the amount of photosynthetically absorbed light and the oxygen evolution in high light were more obvious in large cells,indicating the activity of energy-sinking alternative electron flows.The growth rate showed an upward trend with increased incubation irradiance levels except for the large cells in high light,indicating a less effective carbon accumulation in large cells.These results demonstrate that cells achieved the balance between growth and photo-protection by increasing oxygen consumption when exposed to high light.Despite the large gap between the carbon demand and the flux through diffusion,carbon fixation was light-limited but not carbon-limited,indicating a highly effective carbon concentrating mechanism(CCM)in this species.Cells of Nitzschia sp.were grown in batch cultures at different temperatures(2,10 ?)and light intensities(30,100 ?mol photons m-2 s-1)for approximately one week to test their tolerance.They grew faster at the higher temperature and light intensity in the mid-exponential stage.This indicates that the in situ environmental factors,such as water temperature and light intensity,may limit their growth.The results of saturating irradiances(Ik)in the P-I curve showed that cells could acclimate to high light at both temperatures.The maximum Ik was above 400 ?mol photons m-2 s-1 in 10 ? grown cells,indicating that higher temperature had a positive effect on light acclimation in this species.At 10 ?,the photochemical efficiency of PSII(YII)decayed more slowly while non-photochemical quenching(NPQ)increased more quickly during the two-hour exposure of 450 ?mol photons m-2 s-1.The ratio of repair to damage rate(r/k)was also larger,mainly due to the low damage rates at this temperature.These results demonstrate that a highly effective energy dissipation at high temperature hindered the potential photo-damage in PSII of this species in the exposure.Larger NPQ in low-light acclimated cells suggest that the high light stress in these cells was more severe.The results of the induction curve showed that dark-adaption might be necessary for the quick onset of NPQ when exposed to high light.These two studies show that diatoms are able to regulate light absorption by pigment modulating strategies.Photochemical efficiency and NPQ could be affected by light and temperature conditions in these two species.The lower Ik for growth in large cells of C.granii indicate that diatoms with different sizes might have different energy allocating strategies.Regulated by the redox status of the electron transport chain,NPQ dissipated the excessive excitation energy as heat to ensure an efficient electron transport under high light stress.Oxygen consumptions are likely to be larger in high light,probably due to the activity of alternative electron pathways.The energy costs in carbon assimilation might also played a part in photo-protection.The highly effective photo-acclimation and photo-protection strategies contributed to the dominance of diatoms in marine phytoplankton communities.