Several macroalgae from the Swedish West Coast (11 green, 5 red
and 11 brown algal species) and two Ulva spp. from the Mediterranean
were tested for mechanisms of HCO3−
utilization. In all except one (Palmaria palmata), photosynthesis
supported by inorganic carbon (Ci)
at pH 8.7 was rapidly and substantially (>50%) inhibited by acetazolamide
(AZ), an inhibitor of extracellular carbonic anhydrase activity.
This suggests that extracellular dehydration followed by uptake of the
CO2 formed is an important component of the HCO3−
utilization
mechanism(s) of these algae. In representatives of most of the green algal
genera, e.g. Enteromorpha, Ulva, Chaetomorpha
and Monostroma,
a specific inhibitor response could be induced. This response, a sensitivity
to the anion exchange inhibitor
4,4′-diisothiocyanatostilbene-2,2′-disulphonate (DIDS) in the
presence of AZ, is
characteristic of an additional mechanism of HCO3−
utilization, a direct import of HCO3−
across the plasma membrane, which has been previously described only for
some Ulva spp. and Enteromorpha intestinalis. The
DIDS sensitivity could be induced in several ways, all involving a potential
shortage of the supply of CO2 for photosynthesis: through
photosynthesis at high pH, through growth in strong light and nutrient-rich
seawater, or through photosynthesis in the presence of AZ.
Induction of photosynthesis at high pH (which is a specific feature of
green macroalgae) was not always accompanied by a development
of this specific DIDS sensitivity; it occurred in Chaetomorpha melagonium
but not in C. linum. For Ulva lactuca cultivated in natural
seawater (in continuous moderate irradiance), induction of DIDS sensitivity
by high pH treatment was successful all year round, except
during a period in late August. The results thus suggest that green macroalgae
capable of developing photosynthesis at high pH are also
capable of developing a DIDS-sensitive mechanism for HCO3−
uptake, but that some additional conditions (other than low CO2
availability) must be fulfilled. The ability to dehydrate extracellular
HCO3−
still remained after induction of the direct HCO3−
uptake, and
the two mechanisms for HCO3−
utilization may operate independently of one another. In fact the response
to DIDS was more
pronounced after the addition of AZ. The possibility that the results are
related to a plasticity of the HCO3−-utilizing
mechanisms of
green macroalgae is discussed. Neither red nor brown macroalgae showed
any sensitivity to DIDS in the presence of AZ.