INORGANIC CARBON ACQUISITION BY HORMOSIRA-BANKSII (PHAEOPHYTA, FUCALES) AND ITS EPIPHYTE NOTHEIA-ANOMALA (PHAEOPHYTA, FUCALES)

Measurements of parameters related to photosynthesis, with particular reference to the mechanism of inorganic C acquisition, were made on th e obligate epiphyte Notheia anomala Harvey et Bailey (Phaeophyta: Fuca les) and on its most usual basiphyte, Hormosira banksii (Turner) Decai sne (Phaeophyta: Fucales). The work on Hormosira banksii included a co mparison of specimens which are normally emersed at low tide on rock p latforms with specimens which are normally submersed in rockpools at l ow tide; the rockpool specimens are those which most commonly bear Not heia anomala. No significant differences were detected for any of the characteristics tested, i.e. photosynthetic rate as a function of inci dent photon flux density, external inorganic C concentration and plant N content; capacity to use HCO3-; diel changes in titratable acidity of cell contents ('CAM-like' behaviour); rates of dark C-14-inorganic C fixation; delta(13)C values of plant organic C. These data are consi stent with there being no significant genotypic and, probably, phenoty pic differences in inorganic C acquisition between the two populations . The inorganic C acquisition properties of Hormosira banksii resemble more closely those of other Fucaceae than those of other members of t he Fucales and of the Laminariales and Durvillaeales which have been i nvestigated. Notheia anomala has a higher photosynthetic capacity on a fresh or dry weight basis than its basiphyte Hormosira banksii, possi bly due to its larger surface area per unit mass and/or to a smaller a llocation of resources to chemical defences in Notheia than in Hormosi ra. Overgrowth of Hormosira by Notheia may be a function of the delay of Notheia infestation until the basiphyte is reproductively active, a nd of more grazing of the Notheia than of the better defended Hormosir a. The photosynthetic characteristics of Notheia were examined; photos ynthesis resembles those of Fucales 'emersed at low tide' (including H ormosira) rather than those of brown algae 'invariably submersed at lo w tide'. The high photosynthetic rates of Notheia and Hormosira on a t issue N basis may relate to their low N status as a result of living i n a lower N-supply habitat than those of many other brown algae examin ed.