In situ measurements of photosynthetic irradiance responses of two Red Sea sponges growing under dim light conditions

Photosynthetic responses to irradiance by the photosymbionts of the two Red Sea sponges Theonella swinhoei (Gray) and Cliona vasti®ca (Hancock) growing under dim light conditions were measured in situ (in September 1997) using a newly developed underwater pulse amplitude modulated (PAM) ̄uorometer. Relative rates of photosynthetic electron transport (ETR) were calculated as the e€ective quantum yield of photosystem II (Y ) multiplied with the photosynthetic photon ̄ux (PPF). Photosynthesis versus irradiance (P-I ) curves, obtained within minutes, showed that individual specimens of both sponges, growing under very low light conditions, feature lower light saturation points as well as lower maximal ETRs than individuals growing under higher light. Evaluations of such curves using low irradiances of the actinic light source (20 to 130 lmol photons m s) showed a general decrease in Y, with a shoulder from the lowest irradiance applied till 20 to 30 lmol photons m s. Point measurements yielded ETRs close to what could be estimated from the P-I curves. These point measurements also revealed good correlations between the diurnally changing ambient irradiances (1 to 50 lmol photons m s) and average ETR values for both species. Further analysis showed that although Y values varied considerably between the di€erent point measurements, they did not decrease signi®cantly with light under these very low irradiances. Therefore, PPF rather than Y seems to determine the in situ diel photosynthetic performance at the low ambient irradiances experienced by these sponges. Introduction Although sponges are animals, many species feature symbiotic relationships with cyanobacteria or unicellular algae. These photosymbionts provide the sponge with organic photosynthate; this has been considered especially important for species inhabiting oligotrophic waters where planktonic foods are scarce (Wilkinson 1983). For example, it was argued that some species from the Great Barrier Reef were restricted to depths where irradiance levels were expected to be high enough to yield net diel O2 evolution (Cheshire and Wilkinson 1991). Similarly, it was found that substantial portions of the energy requirement of a freshwater sponge could be provided by the photosymbionts under sucient light conditions (Sand-Jensen and Foldager Pedersen 1994). While such evaluations are based on assessments of gas exchange as measured under controlled conditions, supportive photosynthetic measurements in situ have been precluded largely by the lack of suitable portable underwater instrumentation. Pulse amplitude modulated (PAM) ̄uorometry has recently been used in assessing photosynthetic properties of photosymbiont-containing invertebrates such as corals (Warner et al. 1996), and the development of a portable underwater PAM ̄uorometer (the DivingPAM, Walz Gmbh, Germany) has made in situ measurements possible as reported both for ascidians (Schreiber et al. 1997) and other organisms including several corals (Beer et al. 1998; Ralph et al. in preparation). While the advantage of using the Diving-PAM includes non-intrusive, fast and reliable in situ measurements (see preceding article of this issue), the method can only estimate rates of photosynthetic electron transport (ETR), ignoring any respirational components; thus gross rather than net rates of photosynthesis are measured. Therefore, it can not per se replace O2 or CO2 (including longer-incubation CO2) techniques in research where, e.g., gas exchange balances are sought. It could, however, be applied for Marine Biology (1998) 131: 613±617 Ó Springer-Verlag 1998 Communicated by O. Kinne, Oldendorf/Luhe S. Beer (&) Department of Plant Sciences, Tel Aviv University, Tel Aviv 69978, Israel M. Ilan Department of Zoology, Tel Aviv University, Tel Aviv 69978, Israel investigating the gross photosynthetic component in, for example, adaptations of photosymbiont-containing organisms to ambient light. In this work, we used PAM ̄uorometry in situ for measurements of photosynthetic parameters in two marine sponges growing under dim light conditions. Materials and methods The same Diving-PAM (Walz Gmbh, Germany) as in the preceding paper of this issue (Beer et al. 1998) was used for measuring chlorophyll ̄uorescence parameters of the two marine sponges Theonella swinhoei (Gray) and Cliona vasti®ca (Hancock). These species were chosen because of their photosynthetically active symbionts; T. swinhoei is known to contain cyanobacteria (Wilkinson 1979) while Cliona spp. have been reported to harbour zooxanthellae (Rosell and Uriz 1992). The investigated sponges grew at depths of 2 to 4 m in crevices of the coral reef near Eilat, Israel (northern Red Sea, 34°54¢E; 31°35¢N), and were accessed by snorkelling. Three specimens of T. swinhoei (designated The 1 to 3) and two of C. vasti®ca (Cli 1 and 2), measuring 10 to 30 cm each, were identi®ed in habitats featuring variable, but low (1 to 50 lmol photons m s photosynthetic photon ̄ux, PPF), irradiances throughout the day, and all measurements were done on those individuals. The measurements were carried out during September 1997, at an ambient water temperature of 25 °C. Estimations of light absorption by the sponges were attempted by measuring the re ̄ectance of light by the tissue in the same way as for corals (see Beer et al. 1998). However, it was found that only about 1% of the incident light was re ̄ected by Theonella swinhoei, while much of the light reaching Cliona vasti®ca was visibly absorbed by a non-photosynthetic orange pigment. Because of these complications (as pointed out by a critical reviewer), it was decided not to include light absorption fractions in the calculations of ETR, and therefore only relative ETRs are given. Photosynthesis versus irradiance (P-I ) curves were generated by attaching the main light guide of the Diving-PAM to the surface of a sponge via the ``coral clip'' (Walz's terminology) with hooked rubber bands, and then initiating automatic light curve measurements using the internal light source of the instrument. In order to obtain lower than default actinic (photosynthesis-inducing) light levels, the Diving-PAM settings were changed to minimal values for the ``actinic light intensity'' and ``actinic light factor''. However, it was found that even lower irradiances were needed in order to obtain P-I curves with enough points below light saturation, and therefore the light curves were run with a circular, neutral density ®lter between the tip of the light guide and the sponge tissues. This ®lter, which reduced light by ca. 70%, was simply cut out as a circular disk from the negative of an exposed and developed ®lm, and was inserted into the ``coral clip''. Under such conditions, the ``measuring light intensity'' and ``gain'' had to be increased to their maximal settings in order to obtain an adequate ̄uorescence signal. The e€ective quantum yield of photosystem II (PSII) (Y ) was measured by the Diving-PAM as Y ˆ (Fm¢ ) F )/Fm¢ , where Fm¢ is the maximal ̄uorescence in light-adapted photosymbionts following the application of a 0.8 s for photosynthesis saturating light pulse (which reduces, or ``closes'', all reaction centres of PSII), and F is the ̄uorescence yield of the photosymbiont at a given irradiance. Relative ETRs were calculated as Y ́ PPF. Point measurements were carried out for di€erent areas of the three Theonella swinhoei and two Cliona vasti®ca sponges throughout the day in the same way as described earlier for corals (Beer et al. 1998). As in that work, the light guide cable of the PPF sensor (calibrated against the quantum sensor of a Li-Cor LI-18189 light meter) was ®xed to the ``leaf distance clip'' so as to measure ambient light perpendicular to the sponge tissue. Both Y and ETR were also plotted against PPF in order to evaluate which factor determined the photosynthetic rate at the low ambient irradiances at which these sponges grew. Results In experiments where di€erent light exposure times were compared (20 to 60 s), it was found that exposure of the sponges to the actinic light beam for 30 s at each irradiance level was enough to generate optimal P-I curves. Therefore, the ``actinic light width'' was set to 30 s for all subsequent light curve measurements. Responses of ETR to increasing PPF (P-I curves) of the three Theonella swinhoei and two Cliona vasti®ca sponges are depicted in Fig. 1. While T. swinhoei growing at the highest light level showed saturation of photosynthetic electron transport at 300 lmol photons m s, the two individuals growing in dimmer light showed saturation at only about half that irradiance. Also the photosynthetic rate at light saturation decreased gradually with decreasing ambient irradiances for the three sponges. Similarly, C. vasti®ca growing at very low light showed a considerably lower light saturation level, as well as a lower maximal photosynthetic rate, than did the specimen growing under the higher light level. At even lower actinic irradiances, obtained with the improvised neutral density ®lter (see ``Materials and methods''), Fig. 2 shows that Y decreased with PPF except for the ®rst three light levels following darkness, and that normal P-I curves could be obtained which Fig. 1 Theonella swinhoei, Cliona vasti®ca. Relative photosynthetic electron transport rates (rel. ETR) as functions of incident irradiance (I, measured as PPF) for the various a T. swinhoei (The 1±3) and b C. vasti®ca (Cli 1±2) individuals. Ambient incident irradiance (in lmol photons m s PPF) at the time of the light curve measurements of each sponge is indicated on the graphs 614