Chapter 07: Vulnerability of macroalgae of the Great Barrier Reef to climate change

Assessing the vulnerability of benthic macroalgae is complicated by the fact that the taxon ‘algae’ is an unnatural (and, some suggest, outdated) grouping that encompasses several distinct and diverse evolutionary lines. Adl et al.3 suggest that ‘algae’ remains a useful functional term, denoting photosynthetic protists and their multicellular derivatives which are not embryophytes (higher plants), as well as cyanobacteria. However, they also show that ‘algae’, like ‘protists’, is not a formal taxon (and therefore should not be capitalised), nor a single, homogeneous group.This is Chapter 7 of Climate change and the Great Barrier Reef: a vulnerability assessment. The entire book can be found at http://hdl.handle.net/11017/13

A deep dive into the ecology of Gamay (Botany Bay, Australia): current knowledge and future priorities for this highly modified coastal waterway

Context: Gamay is a coastal waterway of immense social, cultural and ecological value. Since European settlement, it has become a hub for industrialisation and human modification. There is growing desire for ecosystem-level management of urban waterways, but such efforts are often challenged by a lack of integrated knowledge. Aim and methods: We systematically reviewed published literature and traditional ecological knowledge (TEK), and consulted scientists to produce a review of Gamay that synthesises published knowledge of Gamay’s aquatic ecosystem to identify knowledge gaps and future research opportunities. Key results: We found 577 published resources on Gamay, of which over 70% focused on ecology. Intertidal rocky shores were the most studied habitat, focusing on invertebrate communities. Few studies considered multiple habitats or taxa. Studies investigating cumulative human impacts, long-term trends and habitat connectivity are lacking, and the broader ecological role of artificial substrate as habitat in Gamay is poorly understood. TEK of Gamay remains a significant knowledge gap. Habitat restoration has shown promising results and could provide opportunities to improve affected habitats in the future. Conclusion and implications: This review highlights the extensive amount of knowledge that exists for Gamay, but also identifies key gaps that need to be filled for effective management

Mangroves and seagrasses

Sandwiched between two of the world’s iconic tropical ecosystems of coral reefs and rainforests, are two important coastal communities: mangroves and seagrasses. While corals flourish in shallow warm seas, and rainforests cover wetter upland regions, all are dependent on this unique association. Mangroves inhabit the sheltered intertidal margin part barely above mean sea level. Seagrasses occupy depths from intertidal to deeper habitats, depending on the clarity of the water column. Like coral reefs, each of these biota-structured ecosystems play an important role in coastal processes with highly developed linkages and connectivity between and among them. These relationships are vital to the survival of each. For example, while sediment-loving mangroves depend on shorelines sheltered by coral reef structures, they in turn protect sediment-sensitive corals from receiving unwanted materials flushed downstream from surrounding land catchments

Calcification in the green alga Halimeda: II. THE exchange OF ca2+ AND the OCCURRENCE of AGE gradients IN calcification AND photosynthesis

In Halimeda cylindracea and H. tuna segments, the concentration of CaCO3, MgCO3, protein, and chlorophyll, as well as segment volume and wet and dry weight, increase with 'age' i.e. from the apex of a branch downwards. Photosynthetic and calcification rates decrease with age as does the degree of light stimulation of calcification.Studies of the exchange of 45Ca between the Halimeda thallus and the sea water under various conditions showed that most of the Ca exchange is between the cell walls, the aragonite crystals, and the intercellular space. The cell wall has two distinguishable phases with half-times (t0·5) of 200 and 35 min while the CaCO3 has a rapidly exchanging phase with a t0·5 of approximately 6 min. The t0·5 of the exchange of Ca between the intercellular space and the external medium is estimated at about 6 min, on the basis of uptake studies. If the integrity of the barrier between the intercellular space and the external sea water, created by the adpressed peripheral utricles is destroyed the t0·5 is smaller (≪3 min).These kinetic studies as well as comparative measurements of calcification rates by both isotopic and chemical methods show that the 45Ca method for measuring calcification rates overestimates the calcification rate, due to binding of 45Ca in the cell walls and retention of 45Ca in the intercellular space. The 14C method gives more accurate results and has the further advantage of allowing simultaneous measurement of the photosynthetic and calcification rate on the same segment

Calcification in algae: Mechanisms and the role of metabolism

The deposition of CaCO3 by algae (calcification) is a widespread phenomenon and the deposits of either aragonite or calcite may be extra‐, inter‐, or intracellular. This variability in location and crystal isomorph suggests that different calcification mechanisms operate in different algal groups. Despite this difference, all algal calcification systems have some common features. These include the need for a suitable CaCO3‐crystal nucleation mechanism and the stimulation of calcification by photosynthesis. The physiology and biochemistry of algal calcification are discussed in relation to the above processes and compared to noncalcareous algae

Calcification in the green Alga Halimeda. I. An ultrastructure study of thallus development

The ultrastructure of 4 species of the calcareous, siphonaceous alga Halimeda (H. cylindracea Decaisne, H. discoidea Decaisne, H. macroloba Decaisne and H. tuna (Ellis & Solander) Lamour) has been studied, and the observed changes during growth and development are related to changes in the degree of calcification. A distinct gradient in the types and quantities of cell organelles exists in a growing apical filament. As these filaments grow, branch, and eventually develop into a mature segment, changes in the organization of organelles such as mitochondria and chloroplasts are observed. Calcification begins when the chloroplasts reach structural maturity and when the peripheral utricles adhere (fuse). This adhesion of the peripheral utricles isolates the intercellular space (ICS) in which calcification occurs from the external seawater. Calcification begins in the outermost (pilose) cell wall layer of the walls facing into the ICS. The cell walls at the thallus exterior undergo extensive changes after utricular fusion; the pilose layer is lost, the cuticles of adjacent utricles fuse forming a ridge at their junction, and multiple cuticles are formed. The aragonite (CaCO3) crystals which are initially precipitated within the pilose wall layer, rapidly increase in size and number, eventually filling much of the ICS. Only the initial nucleation of aragonite is associated with the pilose wall layer, the later precipitation of aragonite is totally independent of the pilose layer. In older segments secondary deposition of CaCO3 also occurs around existing aragonite needles

Chloroplast development in the caulerpalean alga Halimeda

The development of the chloroplast in the caulerpalean algaeHalimeda cylindracea, H. discoidea andH. macroloba has been studied by electron microscopy. The chloroplast develops from a proplastid via a starch containing young plastid. This young plastid may also develop into an amyloplast. The plastids contain a unique concentric lamellar system at one end. This has been called the thylakoid organising body (TOB) as it is at the base of this body that thylakoid formation is initiated. The TOB persists throughout the life of the chloroplast

Calcification in the green alga Halimeda: III. The sources of inorganic carbon for photosynthesis and calcification and a model of the mechanism of calcification

Calcification and photosynthetic rates in Halimeda tuna were measured by the 14C method under conditions of differing pH and total inorganic carbon (ΣCO2) concentrations. The effects of pH and ΣCO2 on photosynthesis and respiration were also monitored with a polarographic O2 electrode. The results obtained indicate that the intercellular pH and ΣCO2 differ from those of the external medium. Experiments carried out over a range of pH values show that Halimeda can use HCO−3HCO3- for photosynthesis. Photosynthesis appears to stimulate calcification by removing CO2 from the intercellular spaces. As these spaces are isolated from the external sea water by the layer of cell wall of the adpressed peripheral utricles, the removal of CO2 results in a rise in [CO2−3CO32-] and a rise in pH. This results in an increased rate of CaCO3 precipitation. Respiratory CO2 evolution has an inhibitory effect on calcification by decreasing the pH and [CO2−3CO32-]. A model for calcification in Halimeda is proposed based on the results of this and previous papers. Calcification in Halimeda is seen to be a result of the anatomy of the thallus in which the sites of calcification are within a semi-isolated chamber where removal or addition of CO2 due to photosynthesis or respiration can effectively change [COCO2−3CO32-] thereby resulting in precipitation of CaCO3. In the Appendix to this paper theoretical calculations illustrate the effects of CO2, HCO−3HCO3-⁠, and CO2−3CO32- removal or addition in a closed system on the relative concentrations of the other inorganic carbon species

Calcification in the green alga Halimeda: IV. The action of metabolic inhibitors on photosynthesis and calcification

The effects of a number of metabolic inhibitors on calcification and photosynthesis in Halimeda tuna, H. discoidea, and H. macroloba are described. The inhibitors used are CCCP, DNP, DCMU, azide, cyanide, chloramphenicol, cycloheximide, and Diamox. The effects of these inhibitors, although complex, are consistent with our model of calcification in Halimeda. Inhibition of photosynthetic CO2 uptake inhibits calcification as does stimulation of respiratory CO2 evolution (i.e. uncoupling). There is also indirect evidence for the presence of a possible light stimulated H+ efflux which inhibits calcification. The observed calcification rate is therefore the result of a number of factors which affect the concentration of CO⅔−and the pH in the intercellular space of the Halimeda thallus. The results obtained with the carbonic anhydrase inhibitor Diamox provide further evidence for the effective separation of the intercellular space from the external medium by the appressed peripheral utricles