Photosynthesis, calcification, and photoadaptation, in reef-building crustose coralline algae on the Great Barrier Reef

Chisholm, John R.M. (1988) Photosynthesis, calcification, and photoadaptation, in reef-building crustose coralline algae on the Great Barrier Reef. PhD thesis, James Cook University.

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In situ rates of photosynthesis and calcification were determined for four species of reef-building crustose coralline algae on the windward crest and slope of a coral reef at Lizard Island, in the northern region of the Great Barrier Reef (GBR). The species studied were Porolithon onkodes (Heydrich) Foslie, Neogoniolithon fosliei (Heydrich) Setchell & Mason, Hydrolithon reinboldii (Weber-van Bosse & Foslie) Foslie, and Paragoniolithon conicum (Dawson) Adey, Townsend & Boykins. Rates were measured with an underwater respirometer incorporating oxygen, pH, light and temperature probes located within an ultra-violet transparent incubation chamber. Measurements of photosynthesis were also made in the laboratory using a 'Clark' type oxygen electrode and a specially constructed acrylic cell. The photosynthetic quotients (PQ) of the four species were determined from in situ measurements of oxygen, pH, and total alkalinity, with corrections for changes in seawater temperature over the period of incubation. Light-saturation curves for photosynthesis (P-I curves) and calcification (C-I curves) were constructed from each set of in situ and laboratory measurements using non-linear, least squares regression analysis. The curves were examined for evidence of photoadaptation and for estimation of the rates of maximal gross photosynthesis, absolute dark respiration, maximal light-enhanced calcification, and dark calcification.

Individuals from selected habitats at depths of between 0 and 18 m were studied both in situ and in the laboratory. The specimens received from 85 to 2% of surface irradiance as measured in units of photosynthetic photon flux density (PPFD) between 400 and 700 nm.

Light-saturation curves. In situ data were modelled by the hyperbolic tangent function. The less variable laboratory data revealed subtle differences in the shapes of the light-saturation curves of different species and were modelled by one of four related mathematical functions. The functions tested for accuracy of fit were the hyperbolic tangent, a simple exponential, a general exponential, and the right rectangular hyperbola. Specimens collected from 0 m exhibited slow rates of transition from light-limited to light-saturated photosynthesis. Many of these individuals were not entirely saturated at irradiance levels above those naturally occurring in the field. Rates of photosynthesis were generally lower in the laboratory than in situ. Specimens measured in situ at depths representing the lower limits of the species distributions were found to exhibit light-response curves which did not indicate saturated levels of photosynthesis or calcification.

Photoadaptation. Adaptive changes were observed in the photokinetic parameters describing the shapes of the light-saturation curves of all species under both laboratory and in situ conditions as the quantity of irradiance diminished. Compensation points ((Ic), 95% saturation levels (I₀₉₅), and intercepts between the initial slopes of the curves and the horizontal asymptotes ((Ik) decreased. Rates of respiration (R), maximal gross photosynthesis (Pgm), and light-enhanced calcification (C light) based on real surface area also tended to fall. The initial slopes of the curves (α) for photosynthesis (based on projected or real surface area) and calcification (based on total protein content) increased with decreasing irradiance. The ratios of gross photosynthesis to respiration (Pᵍml-R) increased marginally with decreasing irradiance. The rates of dark calcification (C dark) and light-enhanced calcification expressed on the basis of total protein content were variable and did not vary predictably with diminishing irradiance.

The natural logarithms (In) of Ic, Ik, and I₀₉₅ were directly proportional to the natural logarithms of the percentages of surface irradiance (In %SI) transmitted to the depths at which the algae were growing. Similar double logarithmic relationships were observed between Pᵍm, R,(C light)(based on real surface area) and In %SI for some species. The natural logarithms of for photosynthesis normalised on the basis of real surface area, and for calcification normalised on the basis of total protein content, were inversely proportional to In %SI for all species. Similar double logarithmic relationships were observed between Pᵍm l-R and In %SI for some species.

Primary production. Photosynthetic quotients (PQ) were determined for each species. Mean PQ values for P. onkodes, N. fosliel, and H. reinboldii ranged from 1.21 to 1.33. The mean PQ for P. COM. CUM was 1.07. Mean rates of maximal net organic carbon production per hour ranged from 0.083 to 0.168 g C m⁻² (real surface area) in situ, and from 0.068 to 0.148g C m⁻² in the laboratory. Mean rates of net carbon production integrated over the course of a 24 hour day ranged from 0.180 to 1.352 g C m⁻²(real surface area) in situ, and from 0.123 to 1.206 g C m⁻² in the laboratory. Rates of gross primary production or consumption per day were directly proportional to peak noon irradiance. The rate of carbon production per day could thus be estimated for any amount of irradiance on a cloudless day.

Calcification. Using an adaptation of the alkalinity anomaly technique, the precipitation or solution of calcium carbonate was estimated by subtracting the calculated change in pH resulting from photosynthesis and respiration from the measured change in pH. Mean maximal rates of in situ calcification per hour ranged from 0.156 to 0.923 g CaCO₃ m⁻² (real surface area). Mean rates of calcification integrated over a 24 hour day ranged from 0.87 to 9.86 g CaCO₃ m⁻² (real surface area). Rates of calcification per day were directly proportional to peak noon irradiance for all species except H. reinboldii.

The rate of calcification per unit of surface area decreased with increasing depth and decreasing irradiance. Calcification rates were always considerably higher in the light than in the dark. Dark rates of calcification were highly variable. In several cases solution of CaCO₃ was observed in the dark.

Relationships between calcification and irradiance, and photosynthesis and irradiance, were similar but not identical. In shallow water, greater irradiance was required for the saturation of calcification than for saturation of photosynthesis. These data suggest that calcification is largely controlled by photosynthesis but is probably influenced by other factors, among them tissue biomass.

Conclusions. Crustose coralline algae are highly significant producers of organic and inorganic carbon on coral reefs. Their rates of photosynthesis may have been underestimated by the use of semi-artificial procedures. Their rates of calcification are comparable with corals and in certain reef zones their great abundance may result in overall calcification rates which exceed that of 98-99% of the rest of the reef. As expected crustose coralline algae photoadapt and their ability to do so influences but does not necessarily control their distributions on the reef.

Item ID: 27501
Item Type: Thesis (PhD)
Keywords: crustose coralline algae, calcification; photoadaptation; photosynthesis; irradiance relationships
Date Deposited: 25 Jun 2013 04:52
FoR Codes: 06 BIOLOGICAL SCIENCES > 0607 Plant Biology > 060799 Plant Biology not elsewhere classified @ 51%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 49%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 49%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 51%
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