Responses Of Marine Diatoms To Temperature And Light Environment Changes And Potential Acclimation Mechanisms

Phytoplankton,the base of marine food chains,can support marine ecosystems and provide the preliminary driving force for the marine carbon sequestration and storage.Diatoms are one of the most important marine phytoplankton groups,with the most diversity of species and quantity.Cell size of diatoms varies from few microns to several millimeters in effective diameter,with over 9 orders of magnitude in volume difference.Marine diatoms can contribute to?20%of global primary productivity and transport more than 50%of total organic carbon to deep sea.At present,global warming has become an indisputable fact that temperature on surface earth has been rising irreversibly for a long time due to anthropogenic CO2 emission since the industrial revolution.Ocean warming can shallow the upper mixing layer through enhancing stratification,which can alter the phytoplankton-experienced temperature and light environments(i.e.,light intensity,photoperiod).On the other hand,rising temperature has shifted the niches of phytoplankton polewards,wherein they were also experiencing the changing temperatures and lights.Light drives the photosynthesis of phytoplankton while temperature affects various physiological metabolisms through altering intracellular enzymes activity,both of which affect the growth and interspecific competition of phytoplankton.Therefore,how to respond and adapt to the temperature and light variations caused by global climate changes is the key for diatoms to survive,grow and dominate in changing habitats.To answer this question,we conducted the in situ and mesocosm experiments in the coastal area of Daya Bay where diatoms are dominant groups to explore the physiological and ecological responses of phytoplankton assemblages to diel light and temperature changes.Furthermore,based on these in situ data,we selected two representatively marine diatoms,Thalassiosira pseudonana(?40 ?m3)and T.(?300000?m3)with?4 orders of magnitude difference in biovolume to study the physiological response and adaptation to the changed temperature and light,as well as the underlying mechanisms.The main findings are summarized as follows:Growth rate is a comprehensive reflection of competitiveness,environmental response and adaptation abilities of phytoplankton.Therefore,accurately determining the specific growth rate that characterizes the growth,is the prerequisite for studying the phytoplankton physiology and ecology in the in situ or laboratorial conditions.Through statistically analyzing the randomly searched 200 literatures and comparing the methods therein with 8 microalgae species,we found that 1)the current methods used for measuring specific growth rate mainly include microscope counting,spectrophotometry,chlorophyll measurement,chlorophyll fluorescence,dry weight measurement,organic carbon or nitrogen measurements,flow counting,etc.Different methods have their own advantages and disadvantages under varied conditions,and the application of each method in literature is different;2)Under the same culture condition,different methods showed differences in accuracy,and the differences are species-specific;3)Spectrophotometry method is suitable for measuring the growth rate of T.pseudonana as its advantages of simplicity,repeatability and high accuracy.So,we used it in the subsequent laboratorial studies.As influenced by tides,together with diel change of sunlight,the underwater light in coastal area varies largely throughout a day.Phytoplankton can acclimate to varying lights by adjusting their physiological metabolisms,presenting a diel rhythm of photophysiological characteristics.The tide-induced physical and chemical changes are evidenced of affecting phytoplankton photophysiology,but whether they impact the circadian rhythm is largely unknown.Using the fast rapid repetition fluorometer(FRRf),we comparably explored the diel changes of photosynthetic performance of phytoplankton assemblages from in situ and from two mesocosms in coastal area of the Daya Bay.Results showed that phytoplankton biomass(Chl a)reached as high as 16.7 ?g L-1 during the experimental periods,and diatoms were the dominant species.The water stratification was severe and the diel variations of temperature and salinity were limited,and the solar PAR showed a great diurnal variation.Photosynthetic characteristics of in situ phytoplankton assemblages showed obvious diel rhythm:i.e.,photosynthetic capacity in daytime was significantly higher than that in nighttime and accordingly varied with solar PAR.By comparing the photosynthetic parameters of surface phytoplankton with those from 3.5m depth,we found that phytoplankton at deep layer could acclimate to low lights through reducing saturated irradiance and enhancing light utilization efficiency,to get an optimal growth.The statistical results proved that the photosynthetic parameters of phytoplankton from in situ and from the mesocosm systems showed no significant difference,indicating the effects of tide-induced physical and chemical changes were limited on the circadian rhythm of phytoplankton photosynthetic physiology.Our results indicate that phytoplankton has a certain circadian rhythm in photosynthesis,which is light dependent with less effected by the tide-induced physical and chemical changes.Based on the in situ data and considered the fact that phytoplankton cells are simultaneously experiencing the temperature and light environmental changes in global warming scenario,we carried out the laboratorial studies to explore the coupling effects of temperature and light exposure duration(i.e.,photoperiod)on two representatively marne diatoms T.pseudonana(?40 ?m3)and T.punctigera(?300000?m3)with?4 orders of magnitude in biovolume difference,to probe the adaption mechanisms to temperature and photoperiods.By comparably exploring the growth and physiological changes of T.pseudonana and T.punctigera under a matrix of temperatures(12?,15?,18? and 21?)and Light:Dark cycles(L:D 4:20,L:D 8:16,L:D 16:8 and L:D 24:0),we found that 1)Both T.pseudonana and T.punctigera grew faster under the intermediate temperature and longer photoperiods,while the shorter photoperiods weakened their responses to increasing temperatures;2)As the temperature increased,intracellular pigments content in small T.pseudonana increased,while that in large T.punctigera showed no significant changes.Photoperiod showed no obvious effect on the pigments content in former species,while the prolonged light duration significantly reduced the pigments content in latter species.3)As the temperature increased,molar carbon to nitrogen ratio(C:N)increased in small T.pseudonana,but decreased in large T.punctigera.Again,photoperiod showed no significant effect on the C:N of both strains;however,the shortened photoperiod greatly increased the intracellular proteins content in T.pseudonana.4)Prolonged photoperiod significantly reduced the photosynthetic capacity of both strains,and such a decrease trend become more obvious under the low temperature.As the temperature increased,the dark respiration rate increased in small T.pseudonana,but decreased in large T.punctigera.5)With the temperature increased,the intracellular activity of RubisCO in small T.pseudonana increased firstly and then decreased,while that in large T.punctigera gradually decreased;agiain,the photoperiod showed no obvious effect on RubisCO activity.Consistently,results of transcriptome data showed that 1)Smaller T.pseudonana had more differentially expressed genes(DEGs)under longer photoperiod,as compared to other treatments,and the DEGs were significantly enriched in a series metabolic pathways;2)Larger T.punctigera presented more DEGs at low temperature,and the DEGs were significantly enriched in the energy and carbon metabolic pathways including the carbon fixation,glycolysis and pentose phosphate pathways,photosynthesis antenna protein and photosynthesis.In a word,differently cell-sized marine diatoms showed differently physiological responses to ambient temperature and photoperiod changes.To acclimate the changes,they also presented flexibly genetic regulations at transcriptional level.In nature,ocean warming has aggravated the stratification of water column and shallowed the upper mixing layer,making phytoplankton cells expose to the higher solar radiation and temperature.Results of the coupling effects of rising temperature and light on T.pseudonana showed:1)The increased light significantly reduced the cellular light-harvesting pigments content but enhanced protecting pigments content.The increased temperature enhanced the pigments content,and strengthened the increasing light-induced decrease in pigment content.2)The increased light promoted cellular C:N ratio,while rising temperature decreased it.Meanwhile,the increased light and temperature synergistically reduced the intracellular protein content;and at higher temperature,cell membrane was more vulnerable to light-caused damage.3)Under the light-limited or light-inhibited conditions,the photosynthetic parameters of T.pseudonana,such as the maximum photochemical quantum yield of PS ?(Fv/FM),light utilization efficiency(?),saturation irradiance(E?)and maximal relative electron transfer rate(rETRmax),were more affected by the increased temperature.Our results clearly showed that the light and temperature could synergistically alter the physiological processes and biochemical compositions of T.pseudonana and ultimately change the growth.In summary,our results showed that the increasing temperature and varied light environments would affect the growth of phytoplankton through altering their physiologies,biochemical compositions and even gene expression,and phytoplankton with different cell size have differently physiological responses and adaptation mechanisms to such environmental changes.Therefore,the global warming-induced changes in the phytoplankton-experienced temperatures and lights would inevitably alter the community compositions,which probably lead to the change of marine ecosystem and cause the fluctuation of marine organic carbon pool in future.