Photosynthesis of an endolithic Chlorococcum alga (Chlorophyta, Chlorococcaceae) from travertine calcium carbonate rocks of a tropical limestone Spring

Abstract

ABSTRACT A green alga, Chlorococcum sp. (Chlorococcales, Chlorophyceae), was grown from scrapings of the travertine rocks from the outlet stream of the Emerald Pool, Khlong Thom Nuea, Khlong Thom District Krabi province in Southern Thailand – a classic karstic stream habitat. It was a recalcitrant alga: organic solvents did not extract chlorophyll (Chl) from the algae very well but heated Dimethylsulphoxide (DMSO) was effective. Rapid light curve PAM experiments were used to quantify photosynthetic parameters: Eopt = 275 ± 9.7 µmol photons m–2 s–1, ETRmax = 277 ± 5.99 µmol e– g–1 Chl a s–1, photosynthetic efficiency (Alpha, α0) = 2.248 ± 0.112 e– g–1 Chl a. For Chlorococcum sp. deliberately grown in very low light, Eopt = 290 ± 4.89 µmol photons m–2 s–1 but ETRmax = 109 ± 0.97 µmol e– g–1 Chl a s–1, Alpha (α0) = 1.010 ± 0.192 e– g–1 Chl a. Overall, the amount of Chl b compared with Chl a was relatively low for most Chl a + b organisms (Chl b/a ≈ 0.21) but the ratio was not strongly affected by irradiance. The changes in ETRmax and α0 indicate that the Chlorococcum sp, responded to very low irradiance by increasing total Chl as Chl a, a marginal decrease in Chl b/a ratio but no substantial change in far-red absorption. The spectrum stripping technique confirmed Chlorococcum sp, is a Chl a + b organism albeit with a low Chl b content. Photosynthesis in this species was not very sensitive to pH in the range pH 5–9 but was inhibited at pH 10, indicating that it was very capable of using HCO3 – as an inorganic carbon source. Chlorococcum has a carbon concentrating mechanism (CCM), however based on its insensitivity to the depolarizing effects of high [K+], it is unlikely to be dependent on the membrane potential across the cell membrane. The alga is homiochlorophyllous and recovers rapidly from dessication without synthesis of new chlorophyll.