The Response Of Different Ecotypes Synechococcus To Ocean Acidification Under Iron Limited Conditions

Since the industrial revolution,a large amount of anthropogenic carbon dioxide has contributed to global climate change,of which nearly one-third has been absorbed by the ocean.It resulted in a dramatic change in the carbonate chemical composition of seawater,accompanied by an increase in pCO2 and a decrease in pH,known as ocean acidification.Iron(Fe)is an essential element that limits primary productivity in many parts of the ocean such as the high-nutrient low-chlorophyll(HNLC)regions.Previous studies have shown that ocean acidification can affect Fe availability and requirement of phytoplankton and thus will likely affect Fe-limited phytoplankton in particular.Synechococcus widely distribute in the global oceans.They are not only limited by some nutrients like nitrogen(N),but also by iron,especially in the HNLC area where iron is limited.Not only are open-ocean Synechococcus susceptible to iron restriction,but coastal Synechococcus are observed to be limited by iron in the boundary upwelling when large amounts of nutrient-rich but iron-deficient water upwells.Therefore,both ecotypes of marine may be limited by iron and have different strategies to deal with iron stress.Under iron restriction and ocean acidification,the response will be different.At present,there have been some studies on the effect of iron limitation or acidification single factor on Synechococcus,but there are few studies on the effect of iron and acidification coupling on Synechococcus.To investigate how coastal and oceanic Synechococcus will respond to ocean acidification under Fe limitation,we chose the coastal strain Synechococcus sp.CCMP 1333 and the oceanic strain Synechococcus sp.CCMP 2370 to compare their growth,elements component and key genes expression involved in carbon fixation and iron utilization.Since photosynthesis is an iron-intensive biological process,previous studies have found that most iron is allocated to photosynthesis in iron-limited cells.In addition,the respiratory electron transfer chain of cyanobacteria also contains a lot of Fe,sharing many parts with the photosynthetic electron transfer chain,which may affect each other.Therefore,in our study the key iron-contained proteins involved in iron utilization is mainly focused on the photosynthetic electron transfer chain and the respiratory electron transfer chain.The results showed that,compared with the iron-sufficient condition,the growth of both strains was significantly inhibited under the iron-limited conditions.But ocean acidification significantly promoted the growth of both strains of Synechococcus under iron limitations.While there were no effects under iron-sufficient conditions.The two strains had different mechanisms for coping with iron stress,but they were mainly reflected in the changes of photosynthesis.Open-ocean Synechococcus sp.CCMP 2370 produced ATP in a more iron-efficient manner by upregulation of the ratio of PsbA:PsaC.The coastal one CCMP 1333 increased iron efficiency by allocating more iron to photosynthesis.The traditional explanation for ocean acidification promoting the growth of Synechococcus is that Synechococcus can save energy by downregulating CCMs to promote growth.It can also be concluded from this experiment that this conclusion is not applicable to all Synechococcus because of the affinity of CCMs.Although acidification significantly increased the growth rate of the two strains of Synechococcus under iron limitation,the response mechanisms were also different,mainly reflected in the changes of iron distribution and energy metabolism.For iron distribution,CCMP 2370 improved iron utilization efficiency by reducing iron distribution in respiration and increasing photosynthesis.CCMP 1333 maybe increased the rate of iron absorption and absorbed more iron actively for photosynthesis.At the same time,the upregulation of PsbA:PsaC improved the iron utilization efficiency.In terms of energy metabolism,on the one hand,acidification led to a decrease in respiration of CCMP 2370,which may reduce the energy required to maintain cell pH homeostasis.On the other hand,acidification down-regulated the CCMs of CCMP 2370,which saved the energy required for its operation and allocated it to photosynthesis and growth.For coastal CCMP 1333,the photosynthetic electron transfer chain and the respiratory electron transfer chain were up-regulated simultaneously to increase the production of cellular ATP,so as to promote the up-regulation of CCMs to obtain more inorganic carbon and promote the carbon fixation reaction.No matter by which means,acidification promoted the growth of both ecotypes of Synechococcus under iron limitation,which alleviated iron restriction to some extent.As ocean acidification intensifies in the future,widespread marine Synechococcus communities in iron-limited areas,such as the HNLC,may benefit from the effects of ocean acidification.Open-ocean Synechococcus may benefit more from ocean acidification both in terms of growth and in response to CCMs.Coastal and oceanic Synechococcus may thus play different roles in Fe and carbon biogeochemical cycles in the future oceans.