Effects of nutrient treatments on photoacclimation of the hermatypic coral
Stylophora pistillata (Esper) were studied. Studies on photoacclimation of colonies from different light regimes in the field were evaluated and
used to design laboratory experiments. Coral colonies were collected in the Gulf of Eilat (Israel) from January to March 1993.
Exterior branches of colonies from different depths (1 to 40 m) displayed different trends in production characteristics at
reduced and very low levels of illumination. From 24 ± 3% to 12 ± 2% of incident surface photosynthetic active radiation (PAR
o), zooxanthella population density and chlorophyll
a+
c per 10
6 zooxanthellae increased, a trend seen in the range of light levels optimal for coral growth (90 to 30% PAR
o). The
P
max of CO
2 per 10
6 zooxanthellae decreased, while
P
max of CO
2 per 10
3 polyps increased, indicating an increase in zooxanthella population density at low light levels. Proliferous zooxanthella
frequency (PZF, a measure of zooxanthella division) declined significantly at light levels <18 ± 3% PAR
o. At the lowest levels of illumination (<5% PAR
o), zooxanthella population density decreased, as did the PZF; chl
a+
c per 10
6 zooxanthellae was unchanged. In 28-d experiments, exterior coral branches from the upper surfaces of colonies from 3 m depth
(65 ± 4% PAR
o) were incubated in aquaria under bright (80 to 90% PAR
o), reduced (20 to 30% PAR
o), and extremely low (2 to 4% PAR
o) light intensities. At each light intensity, the corals were maintained in three feeding treatments: sea water (SW); ammonium
enriched SW (SW + N); SW with
Artemia salina nauplii (SW + A). An increase in
P
max of CO
2 per 10
3 polyps was found in corals acclimated to reduced light (20 to 30% PAR
o) in nutrient-enriched SW, while in SW, where the increase in zooxanthella population density was smaller, it did not occur.
Nutrient enrichments (SW + N at 2 to 4% PAR
o and SW + A at 20 to 30% PAR
o) increased zooxanthella population density, but had no effect on chl
a+
c per 10
6 zooxanthellae. Acclimation for 14 d to reduced (10 to 20% PAR
o) and extremely low (1 to 3% PAR
o) light intensities shifted
14C photoassimilation into glycerol and other compounds (probably glycerides), rather than sugars. Both ammonium addition and
feeding with
Artemia salina nauplii resulted in an increase in photosynthetic assimilation of
14C into amino acids. We conclude that acclimation to reduced light consists of two processes: an increase in photosynthetic
pigments and in zooxanthella population density. Both processes require nitrogen, the increase in zooxanthella population
density needing more; this adaptation is therefore limited in nitrogen-poor sea water.
Received: 19 June 1998 / Accepted: 13 June 2000
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