The Effect Of Nitrogen And Phosphorus Availability And Ocean Acidification On Picocyanobacteria In West Pacific And South China Sea

Marine picocyanobacteria make significant contributions to marine primary production,particularly in the oligotrophic subtropical and tropical regions.With climate change,greenhouse warming of the surface ocean results in enhanced stratification and shoaling of upper mixed layers in subtropical and tropical regions,exposing photosynthetic organisms dwelling there to a decreased nutrient supply.However,little is known about how picocyanobacteria,the major photosynthetic organisms there,response to the changes of nutrient availability.At the same time,there is a growing consensus that ocean acidification(OA)caused by increasing atmospheric CO2 concentration will lead to a variety of effects on marine phytoplankton.But little is known about how natural picocyanobacteria populations will respond to OA.In addition,we also wants to investigate whether the effect of OA would affected by the changes of nutrient supply.Therefore,We conducted onboard incubation experiments with surface waters at three stations in the west Pacific and South China Sea where phytoplankton community was dominated by the picocyanobacteria Prochlorococcus and Synechococcus.Our results revealed that nitrogen was the primary factor in modulating the biomass,phytoplankton community structure and primary production in South China Sea and West Pacific.In addition,the growth of and picoeukaryotes not only limited by nitrogen,but also influenced by phosphate to some extent,compared to Prochlorococcus was limited by nitrogen only.What's more,the abundance of Synechococcus and picoeukaryotes increased by 283%±230%and 135%±72%under nutrient replete conditions respectively,which were greater than Prochlorococcus increased by 79%±51%only.At the same time,we found that under nutrient replete conditions,the phytoplankton community was dominated by Synechococcus and picoeukaryotes instead of Prochlorococcus which dominated under oligotrophic conditions.So the decreased nutrient supply may be benefit for Prochlorococcus,which means Prochlorococcus may have hold the high ground in oligotrophic areas.However,we calculated the relative primary production among Prochlorococcus,Synechococcus and picoeukaryotes,and found that Prochlorococcus made less contribution to primary production than Synechococcus and picoeukaryotes,which may be bad for primary production of community dominated by Prochlorococcus.The OA result revealed that the abundance of Prochlorococcus and Synechococcus increased under elevated pCO2 and the proportion of Synechococcus in community increased by 2-16%.In contrast,the OA had negative effect on picoeukaryotes dominated by Haptophytes regardless of the growth or competitiveness in community.Particularly in West Pacific,where Haptophytes are the major picoeukaryotes,the abundance of Haptophytes decreased by 36%±9%under ocean acidification.Meanwhile,we found that OA effect influenced by nutrient availability.In station C02,K8 and K11,the positive OA effect on Prochlorococcus was weakened under nutrient replete.However,nutrient enrichment with nitrate only or with nitrate plus phosphate resulted in amplifying the positive OA effect on Synechococcus.And the negative OA effect on Synechococcus under nutrient limitation was reversed meanwhile.In contrast,in South China Sea,the negative OA effect on picoeukaryotes was strengthened and the positive effect disappeared under nutrient replete conditions.While in West Pacific the negative OA effect on picoeukaryotes was weakened under nutrient replete.Overall,we couldn't ignore the roles of nutrients playing in the response of phytoplankton to ocean acidification.Besides,we measured the changes of primary production under ocean acidification.The result showed that,the primary production in oligotrophic oceanic areas decreased under ocean acidification.Particularly in West Pacific,where Haptophytes are the major picoeukaryotes,the primary production decreased by 41%.The results may indicate the significant contributions picoeukaryotes make to the primary production.What's more,in station A19,C02 and E1,nutrient enrichment with nitrate only or with nitrate plus phosphate resulted in strengthening the negative OA effect on primary production in South China Sea,which may be due to the negative OA effect on picoeukaryotes strengthened under nutrient replete conditions.