Biological nitrogen fixation (acetylene reduction) associated with blue-green algal (cyanobacterial) communities in the Beachwood Mangrove Nature Reserve. I. The effect of environmental factors on acetylene reduction activity

Nitrogen fixation of blue-green algae (cyanobacteria) associated with Avicennia marina (Forssk.) Vierh. pneumatophores and wet and dry surface sediments was investigated in the Beachwood Mangrove Nature Reserve by means of the acetylene reduction technique. Studies revealed percentage moisture and temperature to be the prime factors influencing ARA (acetylene reduction activity) in these habitats and rates were highest under submerged conditions and at 22°C. High concentrations of inorganic nitrogen (between 1 and 5 mg l−1) significantly depressed ARA in all habitats. Increases in ARA occurred with increase in light intensity up to 40 μE m−2 s−1, with negligible dark rates being recorded in association with the wet and dry surface sediments. Significant dark rates of ARA and stimulation of ARA by sucrose in association with the pneumatophores indicated that bacteria may also be contributing to ARA in this habitat. No organic carbon stimulation was noted in the other sites. Salinity had little effect on ARA over the range generally experienced in each habitat. Stikstofvaslegging deur blougroenalge (cyanobacteria) wat met Avicennia marina (Forssk.) Vierh.-pneumatofore en nat en droë sedimente geassosieer is, is in die Beachwood-Mangelietnatuurreservaat, deur middel van die asetileenreduksietegniek ondersoek. Ondersoeke het aangetoon dat die persentasie vog en temperatuur die belangrikste faktore is wat ARA (asetileenreduksieaktiwiteit) in hierdie habitatte beïnvloed en die waardes was die hoogste by onderwatertoestande en 22°C. Hoë konsentrasies van anorganiese stikstof (tussen 1 en 5 mg l− 1) het ARA in alle habitatte aansienlik onderdruk. Toename in ARA het voorgekom by ligintensiteite tot by 40 μE m− 2 s− 1, met weglaatbare waardes wat gedurende die donker in assosiasie met nat en droë oppervlakke van sedimente geregistreer is. Beduidende donker-ARA-waardes en die stimulering van ARA deur sukrose in assosiasie met die pneumatofore het aangedui dat bakterieë ook ’n bydraende rol tot ARA in hierdie habitat kan speel. Geen stimulering deur die teenwoordigheid van koolwaterstowwe is in die ander opname-gebiede aangetoon nie. Daar is bevind dat die soutgehalte ’n klein invloed op ARA, in elke habitat gehad het.