Ilto~ hmma ,'t lh~q~h~~i~ a A<tu. i I)%' ( Iq't2 | 2¢~7 2711 ¢ lqqZ t-lscvicr S¢icnve |h.dqishe~s B.V. All Iighi~, it.'scr'vctl (~()tl5-2728/¢~2./$11~,.1~i) 267 IIIIA[II( ) .1.I'~7 t# Partial uncoupling of energy transfer from phycocrythrin in the marine cya:l b ;cterium Synechococcus sp. WH7803 P. Hea'hcote ~', M. Wyman h,t, N.G. Carr h and G.S. Beddard ~ " School ~J" Biological .S'ciem'e~,Queen Matt and 14 i'.~tlh'M ('ollege, Unirer~itv of i.omhm. I.omlon, ~' ih'partmoll of ihohmu'al .~'c,'n¢c~. Ihtit ersity o1" Warwi~'k. ('reentry ~tttd ' Ih'lmrtment of ('lwmt~tn'. I/mt o'~ttv ,/ ,lhmchester, Ahu',he~tcr t l/Kk (Re<tired 4 ()cl~d~e! 1991) Key words: lincrgy transfer; Phye~crylhrm; P, iliprotcm; |'holqmynthc,,i,~;I 0'ht h;,a'vesling; Nllr,~gell slora~!t'; (( 'v,0ol~b;wtt't ilnm) Time-resolved fluorescence spectroscopy of lilt nmrinc cyanobacterium Svn,'cho~,,cu~ sp WII7XlI3, sh~wed Ihat a ,,ignilicanl fraction of the biliprolcin phyeocrylhrin did not tran.,,fcr its excitation t'l|t.rgy to the pllolt~synthclic reactiou ccnlrcs when the organism had bccn cultttrcd in non-limiting light conditions and with excess nitrogen supplied as nitrate. Wc c~m~,idcr thal this partial dccoupling of energy transfer '~hows the capacity that this organism has to use ;t portion N the phycocrythrin as a nitrogen reserve, withou! the corresponding photo-oxidative damage that could ~ccur from having a larger functional :tntclln,3 Ihan is required by photosynthesis. Introduction Picoplanktonic cyanobacteria of less than 2.t) /~m diameter, such as Synechococcus sp., make a substan- tial contribution to primary productivity throughout the world's oceans [1-3]. Most obligatory marine isolates of these small autotrophs synthesize the biliprotc~n phycoerythrin (PE) as a major component of their light-harvesting apparatus [4]. It has been suggested that in the oceanic isolate Synechococcus sp. WH7803, PE not only is a light-harvesting pigment but also a functionally distinct nitrogen store [5]. Evidence sup- porting this was obtaincd from the comparison of Synechococcus cells grown in continuous culture under either nitrogen-sufficient or nitrogen-limited condi- tions, Thc mobilisation of PE accumulated by nitrogen-replete cells was sufficient to maintain the growth rate during short-term periods of nitrogen star- vation; however, interruption of the supply to nitrogen-limited cells resulted in an immediate declinc in growth mate, resulting in a signifi~,antly lower biomass at stationary phasc [5]. The nitrogen-replete cells had a i Present address: Plymouth Marine Laboratot3'. titadel llill. Ply- mouth. UK. Abbreviations: PE. phycoerythrin; PC, phycocyanin. Correspondence: P. i lealhcote. School ol I~liotogic:d Scicncc~,. Queen Mary and Wcstficld College. University of London. Mile End Rd.. lamdon, E! 4NS. LIK. higher PE content than nitrogcn-limitcd cells (213~ tsg/unit of biomass against 16.3 p.g/tmit) and the higher PE : PC ratio of 21.6 to 12.5. Two conlplcmcn- tary observations suggest thai although the nitrogen- sufficient cclls had this higher PE content, a fraction of the light ab,~orbcd by PE was not transferred to the reaction ccntrcs. Firstly, preferential excitation of PIE with a series of 54t) nm flashcs resulted in the tt, rnovcr of a simit,r number of PS I1 reaction centrcs in both high- and low-nitrogen grown cells, although the PE cell concentration was about 4tt(',~ Ilighcr in the nitro- gen-replete cells. Sectmdly. tnml mcasttrcmcIHs m,t only of tt~c relative slcady-slalc llm~rcsccncc of P]( in cells treated with 51V;~- glycerol, which uHc~uplcs en- ergy transfer from PE to chlorophyll and hence the reaction centre [5], but also of untreated cells, a sub- .,,tantial but unquantificd, fraction of the excitation wits lost as fluorescence [5]. This conclusion was questioned by Yeh ct at. [6], who measured PE fluorescence decays in intact cells and a freshwater isolate as a control, in ,~vltechococcus sp. WH7803 Yeh ct al. [6] found that the proportion of PE not coupled to the reaction centrcs did not exceed 2%. They estimated that the results of Wyman ct al. [5] could bc the result of small, about 3%, changes in the cell content of PE uncoupled from energy transfer, although it has been pointed out that they produced their cxpcl imcntal material under different grox~d~ conditions [7]. This paper reports picosecond time-resolved mea- surements of PE fluorescence in intact 3)nechococ~its