... Inc.Currently,itisevidentthatmicroorganismsformcomplexmicrobialfoodwebsinallaquaticecosystems,andthattheiractivitiesandmetabolismsoftenaretightlycoupled and/ ormutuallyaffected(132,143,144).Therefore,itisnotsurprisingthatenzymaticpropertiesandactivitiesofdifferentcomponentscreatingthemicrobialfoodwebsinlakeecosystemshavedemonstratedcloserelationships.Severalreportshavedocumentedthestrongdependencyofbacterialsecondaryproductiononectoenzymeactivitiesofaquaticmicroorganisms(2–4,16,17,19,25,28,29,33,36,59).Thereoftenisasignificantcorrelationbetweenphytoplanktonprimaryproductionandactivitiesofdifferentectoenzymesinfreshwaterecosystems(25,28,29,33,52).Ourstudiesinlakesofdifferingdegreesofeutrophicationhaveshownmicrobialesteraseactivitytobepositivelycorrelatedtophytoplanktonprimaryproduction,bacterialsecondaryproduction,andconcentrationofdissolvedorganiccarbon(DOC)(Fig.13).Wehavefoundasignificantnegativerelationshipbetweenenzymeactivityandtheper-centageofphytoplanktonextracellularrelease(PER)ofphotosyntheticorganiccarboninthestudiedlakes.ThisnegativecorrelationbetweenPERandesteraseactivityindicatedthatenzymesynthesiswaspartiallyinhibitedinbacteriabylow-molecular-weightphoto-syntheticproductsofphytoplanktonthatwerereadilyutilizedbythesemicroheterotrophs:i.e.,catabolicrepressionofesterasesynthesiswasfoundinlakescharacterizedbyhighPERofphytoplankton(29,33).VIII.ECTOENZYMEACTIVITYANDLAKEWATEREUTROPHICATIONTheimportanceoforganicmatterasavariableforevaluatingthetrophicstatusoflakeshasbeenrecognizedsincethebeginningofthe20thcentury(145,146).Increasingconcen-trationsoforganicconstituentsinwaterarethedistinctindicatorsofacceleratedeutrophi-cationprocessesinmanylakes(147–149).OurstudiesclearlydemonstratedthatenzymeactivitiesweresignificantlypositivelyproportionaltoDOCcontentoflakes(Fig.13C).Asdescribedearlierinthischapter,severalmicrobialectoenzymesareresponsibleforrapidtransformationanddegradationofbothdissolvedorganicmatterandPOMinfresh-waterecosystems.Therefore,wehypothesizethatan‘‘enzymaticapproach’’canbeveryusefulinthestudiesoflakeeutrophication.Severalreportspointedoutthatmicrobialenzymaticactivitieswerecloselyrelatedtotheindicesofwatereutrophicationand/orthetrophicstatusofaquaticecosystems(25,27,29,31,33,38,52,58,62,78).Ourstudiesalongthetrophicgradientoflakes(fromoligo/mesotrophictohypereutrophiclakes[Fig.14A]supportourhypothesis(andtheassumptionsofothers)thatselectedenzymaticmicrobialactivitiesareverypracticalforarapidrecognitionofthecurrenttrophicstatusoflakes.Activitiesofalkalinephosphatase,esterase,andaminopeptidaseincreasedexponentiallyalongatrophicgradientandcorre-latedsignificantlywiththetrophicstateindexofthestudiedlakes(Fig.14B,C,D).Wealsofoundastrongrelationshipbetweenactivitiesofectoenzymesandphytoplanktonprimaryproductionintheselakes.RapidincreasesinectoenzymeactivitieswereobservedespeciallyinarangeofgraduallyeutrophiclakeswhenthevalueofCarlson’strophicstateindex(TSI)wasabove55 (150 )(Fig.14).Moreover, ... Inc.lakewater.Figures2Band2CshowthatectoenzymesynthesisinDOM-enrichedsampleswasnolongerrepressedwhentheconcentrationofthereadilyutilizablelowmolecular-weightmoleculesfellbelowacriticallevel,andpolymericsubstrateshadtobeusedtosupportthegrowthandmetabolismofbacteria.Similarinsituobservationsduringphyto-planktonbloomdevelopmentandbreakdownwerereportedforβ-glucosidaseactivityineutrophicLakePlußsee(24),forβ-glucosidaseandaminopeptidaseactivitiesinmeso-trophicLakeScho¨hsee(25),andforlipaseactivityineutrophicLakeMikołajskie(40).Despitethewidespreadoccurrenceofcatabolicrepression,withtheexceptionofthoseforentericbacteria,themoleculardetailsoftherepressionarepoorlyunderstood.Somestudieshaveindicatedthatcyclicadenosinemonophosphate(cAMP),togetherwithitsreceptorprotein,mayplayacentralroleincontrolofcatabolicrepression(41,42).Usingtherepressionstrategyforectoenzymesynthesis,microorganismscanavoidthewastefulproductionofinducibleenzymes,whicharenotusefulwhentheirgrowthisnotlimitedbyUDOM(3,19,24,35).B.InhibitionofActivityItisimportanttoconsiderthattherepression/derepressionofanectoenzymenotbeequatedtothereversibleinhibitionofactivity.Evenifanectoenzymeissynthesized,itsactivitymaybeinhibitedbytheaccumulationoftheendproductorbyhighconcentrationsofthesubstrate(19).Twogeneraltypesofreversibleinhibitionareknown:competitiveandnoncompetitiveinhibition.Competitiveinhibitionoccurswhenaninhibitingcompoundisstructurallysimilartothenaturalsubstrateand,bymimicry,bindstotheenzyme.Indoingso,itcompeteswithanenzyme’snaturalsubstratefortheactivesubstrate-bindingsite.Thehallmarkofcompetitiveinhibitionofmanyectoenzymes(e.g.,alkalinephosphatase,β-glucosidase,aminopeptidase)isthatitdecreasestheaffinityofanectoenzyme(anincreaseoftheapparentMichaelisconstantisobserved)forthesubstrateand,therefore,inhibitstheinitialvelocityofthereaction(Fig.3)(13,26,37).Competitiveinhibitionisreversibleandcanbeovercomebyincreasedsubstrateconcentration,andthereforethemaximumvelocity(Vmax)ofthereactionisunchanged(Fig.3A).Noncompetitive ... the cyto-plasmic membrane, where they hydrolyze macromolecules in close vicinity to the cell. The resulting low-molecular-weight products are then transported across the cell mem-brane and utilized...