Inorganic carbon acquisition by Hormosira banksii (Phaeophyta: Fucales) and its epiphyte Notheia anomala (Phaeophyta: Fucales)

Abstract

AbstractMeasurements of parameters related to photosynthesis, with particular reference to the mechanism of inorganic C acquisition, were made on the obligate epiphyte Notheia anomala Harvey et Bailey (Phaeophyta: Fucales) and on its most usual basiphyte, Hormosira banksii (Turner) Decaisne (Phaeophyta: Fucales). The work on Hormosira banksii included a comparison of specimens which are normally emersed at low tide on rock platforms with specimens which are normally submersed in rockpools at low tide; the rockpool specimens are those which most commonly bear Notheia anomala. No significant differences were detected for any of the characteristics tested, i.e. photosynthetic rate as a function of incident 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 14C-inorganic C fixation; δ13C values of plant organic C. These data are consistent with there being no significant genotypic and, probably, phenotypic 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 the Fucales and of the Laminariales and Durvillaeales which have been investigated.Notheia anomala has a higher photosynthetic capacity on a fresh or dry weight basis than its basiphyte Hormosira banksii, possibly due to its larger surface area per unit mass and/or to a smaller allocation of resources to chemical defences in Notheia than in Hormosira. Overgrowth of Hormosira by Notheia may be a function of the delay of Notheia infestation until the basiphyte is reproductively active, and of more grazing of the Notheia than of the better defended Hormosira. The photosynthetic characteristics of Notheia were examined; photosynthesis resembles those of Fucales ‘emersed at low tide’ (including Hormosira) rather than those of brown algae ‘invariably submersed at low tide’. The high photosynthetic rates of Notheia and Hormosira on a tissue N basis may relate to their low N status as a result of living in a lower N-supply habitat than those of many other brown algae examined.