Anthony Cheshire

An in situ study of photosynthetic oxygen exchange and electron transport rate in the marine macroalga Ulva lactuca (Chlorophyta)

Direct comparisons between photosynthetic O2 evolution rate and electron transport rate (ETR) were made in situ over 24 h using the benthic macroalga Ulva lactuca (Chlorophyta), growing and measured at a depth of 1.8 m, where the midday irradiance rose to 400–600 μmol photons m−2 s−1. O2 exchange was measured with a 5-chamber data-logging apparatus and ETR with a submersible pulse amplitude modulated (PAM) fluorometer (Diving-PAM). Steady-state quantum yield ((Fm′−Ft)/Fm′) decreased from 0.7 during the morning to 0.45 at midday, followed by some recovery in the late afternoon. At low to medium irradiances (0–300 μmol photons m−2 s−1), there was a significant correlation between O2 evolution and ETR, but at higher irradiances, ETR continued to increase steadily, while O2 evolution tended towards an asymptote. However at high irradiance levels (600–1200 μmol photons m−2 s−1) ETR was significantly lowered. Two methods of measuring ETR, based on either diel ambient light levels and fluorescence yields or rapid light curves, gave similar results at low to moderate irradiance levels. Nutrient enrichment (increases in [NO3−], [NH4+] and [HPO42-] of 5- to 15-fold over ambient concentrations) resulted in an increase, within hours, in photosynthetic rates measured by both ETR and O2 evolution techniques. At low irradiances, approximately 6.5 to 8.2 electrons passed through PS II during the evolution of one molecule of O2, i.e., up to twice the theoretical minimum number of four. However, in nutrient-enriched treatments this ratio dropped to 5.1. The results indicate that PAM fluorescence can be used as a good indication of the photosynthetic rate only at low to medium irradiances.

Modelling the photosynthetic production by sponges on Davies Reef, Great Barrier Reef

Sponge populations on Australias Great Barrier Reef (GBR) may contain a mix of both phototrophic and heterotrophic species. The distribution of many of these sponges on reefs is assumed to be determined by light. A model was developed to investigate how the distribution of phototrophic sponges over depth is restricted by the availability of photosynthetically active radiation. Estimates of the balance between photosynthetic production and the total respiratory demand of entire sponge communities on Davies Reef (a middle-shelf reef of the Great Barrier Reef) are provided. These estimates are based upon published data for community composition and biomass, whilst photokinetic parameters have been determined for a variety of sponge species from oxygenexchange measurements. Phototrophic sponges on the fore-reef slope are predicted to exist at or above a state of net 24 h compensation (i.e., photosynthetic oxygen production by sponges balances or exceeds respiration over a 24 h period) to a depth of 30 m. It is proposed that phototrophic sponges are obligate phototrophs because the availability of light for photosynthesis corresponds with the lower depth limit of their distribution. Sponge communities (including both phototrophs and heterotrophs) from the fore-reef and lagoon exist close to a state of net 24 h compensation to a depth of 10 to 15 m. This balance shows diurnal variations, associated with the activity of phototrophs, such that instantaneous compensation of the community may occur to depths of 20 to 25 m when light is maximal.

Photosynthesis and respiration of phaeophycean-dominated macroalgal communities in summer and winter

Abstract This study presents the results of research on the photosynthesis and respiration of a fucoid dominated, macroalgal community in South Australia. We present quantitative measurements of photosynthesis, obtained in situ, using a deployable photorespirometry system. Maximal photosynthetic rates at saturating photon irradiances were the same between summer and winter (Pmgross = 30 μmol O2 g−1 wet weight h−1), but there were significant seasonal changes in the dark respiration rate from 4.5 μmol O2 g−1 wet weight h−1 in summer to 1.5 μmol O2 g−1 wet weight h−1 in winter. Both the compensation photon irradiance (Ic) and the subsaturating photon irradiance (Ik) were higher in summer (33.9 and 214 μmol photons m−2 s−1, respectively) than in winter (8.0 and 149 μmol photons m−2 s−1). Estimates of net 24-h production range from 73 μmol O2 g−1 wet weight day−1 in winter to 167 μmol O2 g−1 wet weight day−1 in summer. On an annual basis this represents a net biomass production of 20 kg m−2 from a standing biomass of 3 kg m−2 which results in an annual turnover of around six to seven times the standing biomass.

Growth Rate of Jamaican Coral Reef Sponges After Hurricane Allen

Growth rate estimates for five coral reef sponges on the Discovery Bay fore-reef are presented. These were determined from the size of individual sponges growing on the coral rubble that was deposited when Hurricane Allen struck the north coast of Jamaica in August 1980. Sponges collected in February 1986 were weighed and their growth rates determined using the MIX program, originally developed to analyze size-frequency data in fish populations. Sponge doubling times were between 232 and 304 days, with evidence that early exponential growth may be slowing down after four years. The fastest growing sponges were those with small populations of symbiotic cyanobacteria, indicating that there may be a selective advantage for those sponges with photosynthetic symbionts.

The prevalence and production of turf-forming algae on a temperate subtidal coast

M. Copertino, S.D. Connell and A. Cheshire. 2005. The prevalence and production of turf-forming algae on a temperate subtidal coast. Phycologia 44: 241–248. This study shows that canopy-forming algae, composed mainly of fucoids and the kelp Ecklonia radiata (both Phaeophycea), dominated space on South Australian coasts relative to turf-forming algae. However, where canopy-forming algae are absent turf-forming algae are the primary occupiers of space (~ 70%). On some reefs where canopy-forming algae are restricted in spatial extent, turfs can occupy as much as 40% of reefs. Turf-forming algae are an abundant component of algal assemblages, but relatively little is known about their contribution to the primary productivity on temperate reefs, relative to canopy-forming species. This study reveals that net productivity rates of turfs at one South Australian location were very high across depths (1.3–2.9 g C m−2 day−1 or 23.2–88.0 mg C g ash-free dry weight−1 day−1), comparable to the values discovered on tropical reefs. Although turf-forming algae are much more productive than canopy-forming algae on a biomass basis, the annual net production per area is two to seven times lower for turfs than for canopy-forming algae. However, if negligible exudation rates are assumed for turfs (up to 1%), the biomass of carbon produced by turf algae represents 44–71% of the carbon incorporated into biomass of kelps. Taken together, these results suggest that while canopy-forming algae can be correctly assumed to be the major source of total carbon produced on temperate reefs, the contribution of turf-forming algae may be substantial to the biomass production and turnover on South Australian reefs.

PHOTOINHIBITION AND PHOTOACCLIMATION OF TURF ALGAL COMMUNITIES ON A TEMPERATE REEF, AFTER IN SITU TRANSPLANTATION EXPERIMENTS1

The photophysiology of turf algal communities was studied in situ on a temperate reef off the coast of South Australia. Algal communities were grown on artificial substrate at depths of 2, 4, and 10 m. To investigate the response of the algal communities to changing light environments in both the short and long term, reciprocal transplantation experiments were conducted among these depths on a seasonal basis. The extent of photoinhibition was assessed every 3 h for the first 2 days following transplantation and then on a daily basis for 16 days after transplantation. Photosynthetic acclimation was assessed using photosynthesis–light curves obtained from transplanted and non‐transplanted turfs after the acclimation period. Transplanted turfs responded very quickly to the light shift. Algae acclimated to low light (10 m depth) were highly susceptible to photoinhibition and photodamage, having greater decreases in maximum and effective quantum yields than turfs from shallower depths. Yield recovery and acclimation usually occurred very rapidly in algae from all depths (3–5 days), but were faster in spring and summer compared with winter. Changes in photosynthetic capacity (across seasons, depths, and after transplantation to a different depth) were accompanied by changes in respiration, so that the ratio of net to gross photosynthetic capacity (Pmnet : Pmgross) remained high and constant over the whole range of light levels. We discuss the possible acclimation strategies of turfs, taking into account the balance between photoacclimation, production, and growth strategy.

A CLADISTIC ANALYSIS OF THE EVOLUTION AND BIOGEOGRAPHY OF DURVILLAEA (PHAEOPHYTA)1

The genus Durvillaea currently has four recognized species found along many exposed, rocky coastlines of the temperate to sub‐Antarctic regions in the Southern Hemisphere. We propose that the current species distributions are related primarily to vicariance events and subsequent speciation associated with the breakup of Gondwana between 40 and 100 Ma. From an ancestral species, a stipitate species developed in the Tasman basin, with separation and speciation resulting in the D. potatorum/ D. willana complex in southeastern Australia and New Zealand. A second line of evolution led to D. chathamensis and D. antarctica characterized by a honeycombed medulla. The extensive distribution of D. antarctica throughout the Southern Hemisphere is related to both vicariance and dispersal events. The status of D. chathamensis as a species distinct from D. antarctica is questioned. The affinities of an as yet undescribed taxon from the Antipodes Islands are thought to be with the D. potatorum complex but require further study before they can be defined more precisely.

PHOTOPHYSIOLOGY OF A TURF ALGAL COMMUNITY: INTEGRATED RESPONSES TO AMBIENT LIGHT AND STANDING BIOMASS1

This study investigated the variation in the relationship between photosynthesis and ambient light (P‐E curves) for turf algal communities on a temperate reef off the coast of South Australia, analyzing the integrated effects of ambient light and standing biomass. The photophysiology of turfs was studied in situ on a seasonal basis, examining algal communities growing on artificial substrate (plates) at depths of 4 m and 10 m. P‐E curves and estimates for the photokinetic parameters (Pm, Rd, α, Ek, and Ec) were obtained through oxygen evolution methods, using an automated underwater respirometer. Photoacclimation responses to changes in ambient light were strongly affected by the biomass of the community. Pm showed an inverse relationship to standing biomass, irrespective of depth and season, which was considered to be a response to self‐shading and boundary layer effects. Biomass effects imposed a high variance on estimates for all photosynthetic parameters, overshadowing differences observed for season and depth. Biomass also affected photoinhibition on turf communities, where significant afternoon depression of photosynthesis was observed in sparse turf patches when compared to denser patches. High areal productivity rates were maintained across all seasons with a significant decrease only being observed during winter.