BACKGROUND
red coralline algae  are found in coastal areas worldwide, encrusting rocks or growing as free-living individual thalli, which are known as maerl or rhodoliths  <cit> .
red coralline algae also play key roles in coastal ecosystems, providing nursery habitats for juvenile invertebrates, e.g.
<cit>  and significantly contributing to carbonate accretion  <cit> .
in tropical reef systems, red coralline algae act as settlement cues for coral larvae  <cit>  and help to stabilise and develop tropical reef structure  <cit> .
interest in red coralline algae is increasing because of their potential sensitivity to projected environmental changes such as ocean acidification, e.g.
<cit> , their use as a palaeoenvironmental proxy, e.g.
<cit> , and their fundamental role in maintaining ecosystem function  <cit> .
the success and development of coralline algae is, at least in part, driven by photosynthesis, yet comparatively little research has investigated their photosynthetic characteristics  <cit> .
it is generally considered that red coralline algal photosynthesis is optimally adapted to irradiance below that typically experienced in situ  <cit> , thus may be particularly susceptible to high-light induced stress  <cit> .
despite this, red coralline algae are found in a wide range of irradiances, from tropical coral reefs   <cit>  to the lower limit of the photic zone   <cit> .
however, under high light, thallus bleaching may occur in red coralline algae  <cit> ; high light and uv radiation has also been shown to damage the dna, photosynthetic apparatus and light harvesting pigments of non-coralline red macroalgae  <cit> .
light quality also has a significant effect on the photosynthetic capacity of red algae: blue light can stimulate pigment and protein production, whilst red light can promote growth  <cit> .
photosynthetic organisms often exhibit a strong diurnal cycle in photosynthetic efficiency or quantum yield .
such ‘dynamic photoinhibition’ reflects short-term photoacclimation mechanisms designed to minimise photo-damage during times of maximum irradiance, and to maximise photosynthesis during times of low irradiance.
this is typically observed as a decrease in fv/fm around noon, with maximum fv/fm values in early morning and late evening, e.g.
<cit> .
the extent of dynamic photoinhibition may be modified in response to the local environment, e.g.
tidal exposure  <cit> , water temperature  <cit>  or depth  <cit> .
photosynthetic parameters of tropical coralline have previously been determined, e.g.
<cit> .
however, these measurements were determined from specimens that had been maintained in a laboratory environment, which can impact the photosynthetic characteristics of red coralline algae  <cit> .
an alternative approach is to use in situ fluorescence techniques, which monitor the activity of photosystem ii, rather than providing a direct measurement of photosynthetic rate  <cit> .
pulse amplitude modulation  fluorescence provides a non-invasive method for assessing the photosynthetic characteristics of photosynthetic organisms, and has been successful applied in situ on red coralline algae  <cit> .
rapid light curves  have become well established in the fluorescence literature and may be preferable to traditional light curves because of their short run time  <cit> .
during a rlc, photosynthetic organisms are exposed to short periods of increasing levels of irradiance interspersed with short, saturating actinic pulses.
rlcs thus provide fluorescence information from limiting levels of irradiance through to saturating levels, yielding a proxy for electron transport rate  through photosystem ii, although the irradiance absorption of the organism and division between photosystems should be taken into account  <cit> .
photosynthesis-irradiance-type curves derived from rlc data permit the calculation of photosynthetic parameters including maximum  and effective  quantum yield of fluorescence and the light saturation coefficient .
however, unlike traditional light curves, a steady-state is not achieved during rlcs, thus results represent actual, rather than optimal, photosynthetic state, enabling relative changes in photosynthetic state across diurnal periods to be determined  <cit> .
dimethylsulphoniopropionate  is a sulphur compound produced by most marine algae for numerous cellular functions  <cit> , and is derived from methionine  <cit> , an indirect product of photosynthesis  <cit> .
dmsp is also the major precursor to dimethylsulphide , a biogenic gas which has been linked to local climate regulation through the formation of atmospheric aerosols and subsequent cloud development  <cit> .
red coralline algae are known to contain high concentrations of intracellular dmsp  <cit>  and, given that coralline algae may often be exposed to light saturating conditions, particularly in tropical regions, the proposed role of dmsp as an antioxidant  <cit>  may be important.
the diurnal regulation of intracellular dmsp concentrations in red coralline algae is currently unknown, but recent research shows that other tropical macroalgae may up-regulate intracellular dmsp concentrations in response to night-time reductions in carbonate saturation  <cit> .
it is important to understand the natural variation in red coralline algal photosynthetic characteristics and their potential for minimising photo-damage.
such information is particularly informative when considering the contribution made by red coralline algae in carbonate reef accretion, ecosystem service provision and palaeoenvironmental reconstructions.
in that context, this study characterised the photosynthetic characteristics, pigment composition and intracellular dmsp concentrations of two tropical red coralline algae species across a diurnal period.
it was hypothesised that, where algae were exposed to diurnal changes in irradiance, photosynthetic and dmsp measurements would also respond with a diurnal pattern, indicating dynamic photoinhibition and supporting the putative antioxidant function for dmsp.
RESULTS
dark-acclimation
quantum yield was lowest in the light for lithophyllum kotschyanum  and porolithon sp.
.
after 10 s of ‘quasi’ dark-acclimation, photochemical quantum yield  increased in both l. kotschyanum  and porolithon sp.
.
no significant difference between quantum yield measurements from t +  <dig> mins  and t +  <dig> mins was observed .
photosynthetic characteristics
maximum quantum yield, fqˈ/fmˈmax
in both porolithon sp.
and the topside and underside of l. kotschyanum, fqˈ/fmˈmax was highest at dawn and dusk  and lowest at midday .
no significant difference between the three algal morphotypes was observed at 07 h <dig>  or 12 h <dig> .
minimum saturation intensity, ek
at 07 h <dig>  the ek of porolithon sp.
and the topside of l. kotschyanum was significantly higher than the underside of l. kotschyanum .
at 12 h <dig>  the ek of porolithon sp.
was significantly higher than both sides of l. kotschyanum, and the ek of the topside of l. kotschyanum was significantly higher than the underside .
the underside of l. kotschyanum exhibited no diurnal ek response; ek remained ~100 μmol photons m-2 s- <dig> throughout the day .
in contrast, the topside of l. kotschyanum was characterised by an increase in ek to ~400 μmol photons m-2 s- <dig> by 09 h <dig>  followed by a decline from 14 h <dig> to ~200 μmol photons m-2 s- <dig> .
porolithon sp.
was characterised by the largest diurnal pattern in ek: maximum ek was observed at 12 h <dig> , followed by an afternoon decline .
maximum retr, retrmax
no diurnal pattern in calculated retrmax on the underside of l. kotschyanum was observed, and was maintained below the topside of l. kotschyanum .
interestingly, contrasting diurnal patterns were observed for the topside of l. kotschyanum  and porolithon sp.
.
at 07 h <dig>  retrmax of the topside of l. kotschyanum was significantly higher than porolithon sp.
and the underside of l. kotschyanum .
in contrast, at 12 h <dig>  no significant difference between the algal morphotypes was observed .
pigment composition
peaks in absorbance  were observed at wavelengths expected for rhodophyta pigments according to hedley and mumby  <cit> : chl-a and α-carotenoids , α-carotenoids , phycoerythrin , phycocyanin  and allophycocyanin  .
pigment absorbance was pronounced from the underside of l. kotschyanum throughout the day, whilst spectra from the topside of l. kotschyanum spectra were flatter at 09 h <dig> and 12 h <dig> .
porolithon sp.
spectra exhibited the weakest absorbance, particularly at wavelengths indicative of phycoerythrin, phycocyanin and allophycocyanin .
the overall reflectance from porolithon sp.
and the underside of l. kotschyanum did not change throughout the day .
in contrast, the overall reflectance from the topside of l. kotschyanum exhibited a diurnal cycle: reflectance at dawn and dusk was similar to the thallus underside; reflectance progressively increased towards 12 h <dig> to a maximum of 60-80% .
intracellular dmsp
the underside of l. kotschyanum exhibited no diurnal pattern in intracellular dmsp concentrations .
the topside of l. kotschyanum was characterised by a modest increase in intracellular dmsp concentrations at 12 h <dig> .
intracellular dmsp concentrations in porolithon sp.
were comparable to l. kotschyanum at 07 h <dig> , but intracellular dmsp concentrations were significantly higher in porolithon sp.
at 12 h <dig> .
discussion
the ability of red coralline algae to colonise the shallow photic zone in tropical regions such as the red sea relies on efficient photosynthetic and photoprotective mechanisms that minimise photodamage, whilst maximising photosynthetic potential, from the naturally high irradiance levels.
this study highlights inter- and intra-species specific differences in in situ photoacclimation, pigment composition, thallus reflectance and intracellular dmsp concentrations; factors that contribute to the survival, growth and development of coralline algae in high-irradiance habitats.
dynamic photoinhibition
varying degrees of dynamic photoinhibition were observed in this study.
significant diurnal patterns in photosynthetic characteristics, overall reflectance and intracellular dmsp concentrations were observed in porolithon sp.
and the topside of lithophyllum kotschyanum thalli, suggesting that these algal morphotypes exhibited a high level of dynamic photoinhibition.
rhodophyta pigments were also less clear in the spectra of porolithon sp., suggesting that the photosynthetic apparatus may be modified compared to l. kotschyanum to minimise photodamage.
these factors may have been adopted by porolithon sp.
because of the alga’s position on the reef platform.
the reef crest is shallow  and more exposed to wave action than the reef flat, which may cause localised irradiance enhancement  <cit> .
further, the reef crest may be periodically exposed to the air during spring tides .
such conditions necessitate efficient dynamic photoinhibition strategies that may be rapidly regulated in response to the highly variable diurnal light field, minimising photodamage, whilst optimising photosynthesis.
efficient dynamic photoinhibition strategies will allow red coralline algae to tolerate, rather than be inhibited by, the high irradiances found in the shallow waters of the tropics, enabling the successful development of coralline algae in tropical reef systems.
previous research, which involved prolonged periods of time in the laboratory  <cit> , may have underestimated the magnitude of dynamic photoinhibition in red coralline algae, because of coralline algal sensitivity to laboratory culture  <cit> .
it should also be noted that a reduction in quantum yield during periods of high irradiance does not imply a reduction in net photosynthesis  <cit> .
this, together with the presence of antioxidant compounds such as dmsp and carotenoids, may explain why coralline algae are found throughout the world’s photic zone, despite their apparent low-light adaptation  <cit> .
intra-species differences
the topside and underside of l. kotschyanum thalli were visually different in their pigmentation, and this was evident in their overall reflectance and ek.
interestingly, the overall reflectance of the topside of l. kotschyanum thalli varied throughout the day, becoming most reflective during times of highest irradiance, another potential dynamic photoinhibition strategy.
the shaded underside of l. kotschyanum was able to maintain pigmentation and did not exhibit a photosynthetic diurnal response in terms of ek, which remained low throughout the day.
this suggests that the underside of l. kotschyanum was lower-light acclimated, in a similar manner to self-shaded branch bases in the temperate coralline alga lithothamnion glaciale <cit> .
however, the underside of l. kotschyanum, whilst not exposed to full ambient par levels, may have received some light via seabed reflectance, and thus some modest diurnal irradiance patterns, perhaps explaining the observed diurnal patterns in fq'/fm'max and retrmax.
the carbonate sand of suleman reef is likely to be highly reflective given its coral source; coral skeletons  can reflect ultraviolet radiation as yellow light, maximising photosynthesis within coral tissues  <cit> .
additionally, the underside of l. kotschyanum may periodically receive ambient par via thallus rolling, although, given the stark differences in pigmentation, the rate of thallus rolling is likely to be low.
diurnal production of antioxidant compounds
porolithon sp., which exhibited the greatest photosynthetic diurnal changes, also exhibited a diurnal regulation of intracellular dmsp concentrations.
the highest concentrations were observed when irradiance was highest.
this is in contrast to other red sea macroalgae, which up-regulate intracellular dmsp concentrations in response to night-time reductions in carbonate saturation state  <cit> .
however, both high irradiance and low saturation state can induce oxidative stress, supporting the putative antioxidant function of dmsp and its breakdown products  <cit> .
given the apparently high requirement for dynamic photoinhibition strategies in porolithon sp., it may be supposed that any response to varying carbonate saturation is masked by the effect of large variations in day-time irradiance.
although not measured in this study, uv penetration is also high in the red sea  <cit>  and may have been elevated at the reef crest, further necessitating a requirement for intracellular antioxidants.
CONCLUSIONS
this study highlights the ability of red coralline algae to tolerate high levels of irradiance through dynamic photoinhibition strategies that may have been previously underestimated.
although high irradiance is not the only factor that may affect the success of coralline algae , the growth and survival of coralline algae is dependent on photosynthesis.
importantly for conservation and reef management, significant diurnal variations may be observed and the colour of the algae does not necessarily reflect the algae’s photosynthetic or photoprotective capacity .
nutrients are generally limiting in the red sea  <cit> , which may mean that sulphur-containing metabolites such as dmsp are favoured over other metabolites , allowing nitrogen to be used elsewhere in the cells, e.g.
in protein synthesis  <cit> .
thus, in the red sea, dmsp may play a more important metabolic and ecological role than in other regions; this and other studies  <cit>  suggest dmsp provides protection against irradiance- and carbonate saturation-induced oxidative stress.
this has implications for the future success of coralline algae in tropical reef systems, as carbonate saturations states are projected to decline  <cit>  and uv irradiation is projected to increase  <cit> .
the methods used in this study, particularly the spectral reflectance and pam fluorometry are simple to conduct, non-destructive and, in the case of pam fluorometry, may be conducted in situ and thus may be suitable for tropical reef management and conservation studies.
this research highlights the importance of understanding natural variability in the photosynthetic and biochemical characteristics of coralline algae when assessing potential for reef accretion, ecosystem service provision and palaeoenvironmental reconstructions by coralline algae.

 red coralline algae are found in coastal areas worldwide, encrusting rocks or growing as free-living individual thalli, which are known as maerl or rhodoliths. red coralline algae also play key roles in coastal ecosystems, providing nursery habitats for juvenile invertebrates, e.g. and significantly contributing to carbonate accretion. in tropical reef systems, red coralline algae act as settlement cues for coral larvae and help to stabilise and develop tropical reef structure. interest in red coralline algae is increasing because of their potential sensitivity to projected environmental changes such as ocean acidification, e.g., their use as a palaeoenvironmental proxy, e.g., and their fundamental role in maintaining ecosystem function. the success and development of coralline algae is, at least in part, driven by photosynthesis, yet comparatively little research has investigated their photosynthetic characteristics. it is generally considered that red coralline algal photosynthesis is optimally adapted to irradiance below that typically experienced in situ, thus may be particularly susceptible to high-light induced stress. despite this, red coralline algae are found in a wide range of irradiances, from tropical coral reefs to the lower limit of the photic zone. however, under high light, thallus bleaching may occur in red coralline algae; high light and uv radiation has also been shown to damage the dna, photosynthetic apparatus and light harvesting pigments of non-coralline red macroalgae. light quality also has a significant effect on the photosynthetic capacity of red algae: blue light can stimulate pigment and protein production, whilst red light can promote growth. photosynthetic organisms often exhibit a strong diurnal cycle in photosynthetic efficiency or quantum yield. such ‘dynamic photoinhibition’ reflects short-term photoacclimation mechanisms designed to minimise photo-damage during times of maximum irradiance, and to maximise photosynthesis during times of low irradiance. this is typically observed as a decrease in fv/fm around noon, with maximum fv/fm values in early morning and late evening, e.g.. the extent of dynamic photoinhibition may be modified in response to the local environment, e.g. tidal exposure, water temperature or depth. photosynthetic parameters of tropical coralline have previously been determined, e.g.. however, these measurements were determined from specimens that had been maintained in a laboratory environment, which can impact the photosynthetic characteristics of red coralline algae. an alternative approach is to use in situ fluorescence techniques, which monitor the activity of photosystem ii, rather than providing a direct measurement of photosynthetic rate. pulse amplitude modulation fluorescence provides a non-invasive method for assessing the photosynthetic characteristics of photosynthetic organisms, and has been successful applied in situ on red coralline algae. rapid light curves have become well established in the fluorescence literature and may be preferable to traditional light curves because of their short run time. during a rlc, photosynthetic organisms are exposed to short periods of increasing levels of irradiance interspersed with short, saturating actinic pulses. rlcs thus provide fluorescence information from limiting levels of irradiance through to saturating levels, yielding a proxy for electron transport rate through photosystem ii, although the irradiance absorption of the organism and division between photosystems should be taken into account. photosynthesis-irradiance-type curves derived from rlc data permit the calculation of photosynthetic parameters including maximum and effective quantum yield of fluorescence and the light saturation coefficient. however, unlike traditional light curves, a steady-state is not achieved during rlcs, thus results represent actual, rather than optimal, photosynthetic state, enabling relative changes in photosynthetic state across diurnal periods to be determined. dimethylsulphoniopropionate is a sulphur compound produced by most marine algae for numerous cellular functions, and is derived from methionine, an indirect product of photosynthesis. dmsp is also the major precursor to dimethylsulphide, a biogenic gas which has been linked to local climate regulation through the formation of atmospheric aerosols and subsequent cloud development. red coralline algae are known to contain high concentrations of intracellular dmsp and, given that coralline algae may often be exposed to light saturating conditions, particularly in tropical regions, the proposed role of dmsp as an antioxidant may be important. the diurnal regulation of intracellular dmsp concentrations in red coralline algae is currently unknown, but recent research shows that other tropical macroalgae may up-regulate intracellular dmsp concentrations in response to night-time reductions in carbonate saturation. it is important to understand the natural variation in red coralline algal photosynthetic characteristics and their potential for minimising photo-damage. such information is particularly informative when considering the contribution made by red coralline algae in carbonate reef accretion, ecosystem service provision and palaeoenvironmental reconstructions. in that context, this study characterised the photosynthetic characteristics, pigment composition and intracellular dmsp concentrations of two tropical red coralline algae species across a diurnal period. it was hypothesised that, where algae were exposed to diurnal changes in irradiance, photosynthetic and dmsp measurements would also respond with a diurnal pattern, indicating dynamic photoinhibition and supporting the putative antioxidant function for dmsp.  dark-acclimation quantum yield was lowest in the light for lithophyllum kotschyanum and porolithon sp.. after 10 s of ‘quasi’ dark-acclimation, photochemical quantum yield increased in both l. kotschyanum and porolithon sp.. no significant difference between quantum yield measurements from t +  mins and t +  mins was observed. photosynthetic characteristics maximum quantum yield, fqˈ/fmˈmax in both porolithon sp. and the topside and underside of l. kotschyanum, fqˈ/fmˈmax was highest at dawn and dusk and lowest at midday. no significant difference between the three algal morphotypes was observed at 07 h or 12 h. minimum saturation intensity, ek at 07 h the ek of porolithon sp. and the topside of l. kotschyanum was significantly higher than the underside of l. kotschyanum. at 12 h the ek of porolithon sp. was significantly higher than both sides of l. kotschyanum, and the ek of the topside of l. kotschyanum was significantly higher than the underside. the underside of l. kotschyanum exhibited no diurnal ek response; ek remained ~100 μmol photons m-2 s- throughout the day. in contrast, the topside of l. kotschyanum was characterised by an increase in ek to ~400 μmol photons m-2 s- by 09 h followed by a decline from 14 h to ~200 μmol photons m-2 s-. porolithon sp. was characterised by the largest diurnal pattern in ek: maximum ek was observed at 12 h, followed by an afternoon decline. maximum retr, retrmax no diurnal pattern in calculated retrmax on the underside of l. kotschyanum was observed, and was maintained below the topside of l. kotschyanum. interestingly, contrasting diurnal patterns were observed for the topside of l. kotschyanum and porolithon sp.. at 07 h retrmax of the topside of l. kotschyanum was significantly higher than porolithon sp. and the underside of l. kotschyanum. in contrast, at 12 h no significant difference between the algal morphotypes was observed. pigment composition peaks in absorbance were observed at wavelengths expected for rhodophyta pigments according to hedley and mumby: chl-a and α-carotenoids, α-carotenoids, phycoerythrin, phycocyanin and allophycocyanin. pigment absorbance was pronounced from the underside of l. kotschyanum throughout the day, whilst spectra from the topside of l. kotschyanum spectra were flatter at 09 h and 12 h. porolithon sp. spectra exhibited the weakest absorbance, particularly at wavelengths indicative of phycoerythrin, phycocyanin and allophycocyanin. the overall reflectance from porolithon sp. and the underside of l. kotschyanum did not change throughout the day. in contrast, the overall reflectance from the topside of l. kotschyanum exhibited a diurnal cycle: reflectance at dawn and dusk was similar to the thallus underside; reflectance progressively increased towards 12 h to a maximum of 60-80%. intracellular dmsp the underside of l. kotschyanum exhibited no diurnal pattern in intracellular dmsp concentrations. the topside of l. kotschyanum was characterised by a modest increase in intracellular dmsp concentrations at 12 h. intracellular dmsp concentrations in porolithon sp. were comparable to l. kotschyanum at 07 h, but intracellular dmsp concentrations were significantly higher in porolithon sp. at 12 h. discussion the ability of red coralline algae to colonise the shallow photic zone in tropical regions such as the red sea relies on efficient photosynthetic and photoprotective mechanisms that minimise photodamage, whilst maximising photosynthetic potential, from the naturally high irradiance levels. this study highlights inter- and intra-species specific differences in in situ photoacclimation, pigment composition, thallus reflectance and intracellular dmsp concentrations; factors that contribute to the survival, growth and development of coralline algae in high-irradiance habitats. dynamic photoinhibition varying degrees of dynamic photoinhibition were observed in this study. significant diurnal patterns in photosynthetic characteristics, overall reflectance and intracellular dmsp concentrations were observed in porolithon sp. and the topside of lithophyllum kotschyanum thalli, suggesting that these algal morphotypes exhibited a high level of dynamic photoinhibition. rhodophyta pigments were also less clear in the spectra of porolithon sp., suggesting that the photosynthetic apparatus may be modified compared to l. kotschyanum to minimise photodamage. these factors may have been adopted by porolithon sp. because of the alga’s position on the reef platform. the reef crest is shallow and more exposed to wave action than the reef flat, which may cause localised irradiance enhancement. further, the reef crest may be periodically exposed to the air during spring tides. such conditions necessitate efficient dynamic photoinhibition strategies that may be rapidly regulated in response to the highly variable diurnal light field, minimising photodamage, whilst optimising photosynthesis. efficient dynamic photoinhibition strategies will allow red coralline algae to tolerate, rather than be inhibited by, the high irradiances found in the shallow waters of the tropics, enabling the successful development of coralline algae in tropical reef systems. previous research, which involved prolonged periods of time in the laboratory, may have underestimated the magnitude of dynamic photoinhibition in red coralline algae, because of coralline algal sensitivity to laboratory culture. it should also be noted that a reduction in quantum yield during periods of high irradiance does not imply a reduction in net photosynthesis. this, together with the presence of antioxidant compounds such as dmsp and carotenoids, may explain why coralline algae are found throughout the world’s photic zone, despite their apparent low-light adaptation. intra-species differences the topside and underside of l. kotschyanum thalli were visually different in their pigmentation, and this was evident in their overall reflectance and ek. interestingly, the overall reflectance of the topside of l. kotschyanum thalli varied throughout the day, becoming most reflective during times of highest irradiance, another potential dynamic photoinhibition strategy. the shaded underside of l. kotschyanum was able to maintain pigmentation and did not exhibit a photosynthetic diurnal response in terms of ek, which remained low throughout the day. this suggests that the underside of l. kotschyanum was lower-light acclimated, in a similar manner to self-shaded branch bases in the temperate coralline alga lithothamnion glaciale. however, the underside of l. kotschyanum, whilst not exposed to full ambient par levels, may have received some light via seabed reflectance, and thus some modest diurnal irradiance patterns, perhaps explaining the observed diurnal patterns in fq'/fm'max and retrmax. the carbonate sand of suleman reef is likely to be highly reflective given its coral source; coral skeletons can reflect ultraviolet radiation as yellow light, maximising photosynthesis within coral tissues. additionally, the underside of l. kotschyanum may periodically receive ambient par via thallus rolling, although, given the stark differences in pigmentation, the rate of thallus rolling is likely to be low. diurnal production of antioxidant compounds porolithon sp., which exhibited the greatest photosynthetic diurnal changes, also exhibited a diurnal regulation of intracellular dmsp concentrations. the highest concentrations were observed when irradiance was highest. this is in contrast to other red sea macroalgae, which up-regulate intracellular dmsp concentrations in response to night-time reductions in carbonate saturation state. however, both high irradiance and low saturation state can induce oxidative stress, supporting the putative antioxidant function of dmsp and its breakdown products. given the apparently high requirement for dynamic photoinhibition strategies in porolithon sp., it may be supposed that any response to varying carbonate saturation is masked by the effect of large variations in day-time irradiance. although not measured in this study, uv penetration is also high in the red sea and may have been elevated at the reef crest, further necessitating a requirement for intracellular antioxidants.  this study highlights the ability of red coralline algae to tolerate high levels of irradiance through dynamic photoinhibition strategies that may have been previously underestimated. although high irradiance is not the only factor that may affect the success of coralline algae, the growth and survival of coralline algae is dependent on photosynthesis. importantly for conservation and reef management, significant diurnal variations may be observed and the colour of the algae does not necessarily reflect the algae’s photosynthetic or photoprotective capacity. nutrients are generally limiting in the red sea, which may mean that sulphur-containing metabolites such as dmsp are favoured over other metabolites, allowing nitrogen to be used elsewhere in the cells, e.g. in protein synthesis. thus, in the red sea, dmsp may play a more important metabolic and ecological role than in other regions; this and other studies suggest dmsp provides protection against irradiance- and carbonate saturation-induced oxidative stress. this has implications for the future success of coralline algae in tropical reef systems, as carbonate saturations states are projected to decline and uv irradiation is projected to increase. the methods used in this study, particularly the spectral reflectance and pam fluorometry are simple to conduct, non-destructive and, in the case of pam fluorometry, may be conducted in situ and thus may be suitable for tropical reef management and conservation studies. this research highlights the importance of understanding natural variability in the photosynthetic and biochemical characteristics of coralline algae when assessing potential for reef accretion, ecosystem service provision and palaeoenvironmental reconstructions by coralline algae. 
however, natural variability in the photosynthetic characteristics of red coralline algae is poorly understood. measured parameters included: photosynthetic characteristics, pigment composition, thallus reflectance and intracellular concentrations of dimethylsulphoniopropionate, an algal antioxidant that is derived from methionine, an indirect product of photosynthesis. were characterised by significant diurnal patterns in response to the high-light regime. red coralline algae are critical components of tropical reef systems, and their success and development is, at least in part, dependent on photosynthesis. this study highlights a number of dynamic photoinhibition strategies adopted by coralline algae, enabling them to tolerate, rather than be inhibited by, the naturally high irradiance of tropical reef systems; a factor that may become more important in the future under global change projections. minimum saturation intensity and intracellular dmsp concentrations in porolithon sp. a smaller diurnal pattern in minimum saturation intensity in the topside of l. kotschyanum was also evident. l. kotschyanum thalli were characterised by a bleached topside and a pigmented underside. the overall reflectance of the topside of l. kotschyanum also exhibited a diurnal pattern, becoming increasingly reflective with increasing ambient irradiance. the underside of l. kotschyanum, which is shaded from ambient light exposure, exhibited a much smaller diurnal variability. in this context, this research has significant implications for tropical reef management planning and conservation monitoring, which, if natural variability is not taken into account, may become flawed. the information provided by this research may be used to inform future investigations into the contribution of coralline algae to reef accretion, ecosystem service provision and palaeoenvironmental reconstruction. dimethylsulphoniopropionate pam fluorometrymaerlrhodolithcoral reefcrustose coralline algae photosynthesisphotosynthetic pigment this study investigated diurnal variability in encrusting porolithon sp.
 red coralline algae are critical components of tropical reef systems, and their success and development is, at least in part, dependent on photosynthesis. however, natural variability in the photosynthetic characteristics of red coralline algae is poorly understood. this study investigated diurnal variability in encrusting porolithon sp. and free-living lithophyllum kotschyanum. measured parameters included: photosynthetic characteristics, pigment composition, thallus reflectance and intracellular concentrations of dimethylsulphoniopropionate, an algal antioxidant that is derived from methionine, an indirect product of photosynthesis. l. kotschyanum thalli were characterised by a bleached topside and a pigmented underside.  minimum saturation intensity and intracellular dmsp concentrations in porolithon sp. were characterised by significant diurnal patterns in response to the high-light regime. a smaller diurnal pattern in minimum saturation intensity in the topside of l. kotschyanum was also evident. the overall reflectance of the topside of l. kotschyanum also exhibited a diurnal pattern, becoming increasingly reflective with increasing ambient irradiance. the underside of l. kotschyanum, which is shaded from ambient light exposure, exhibited a much smaller diurnal variability.  this study highlights a number of dynamic photoinhibition strategies adopted by coralline algae, enabling them to tolerate, rather than be inhibited by, the naturally high irradiance of tropical reef systems; a factor that may become more important in the future under global change projections. in this context, this research has significant implications for tropical reef management planning and conservation monitoring, which, if natural variability is not taken into account, may become flawed. the information provided by this research may be used to inform future investigations into the contribution of coralline algae to reef accretion, ecosystem service provision and palaeoenvironmental reconstruction. dimethylsulphoniopropionate pam fluorometrymaerlrhodolithcoral reefcrustose coralline algae photosynthesisphotosynthetic pigment 