FACTORS AFFECTING THE RATE OF CALCIFICATION IN HALIMEDA OPUNTIA (L.) LAMOUROUX AND HALIMEDA DISCOIDEA DECAISNE1

Halimeda is a prominent part of the calcifying algae in the coral-reef lagoon ecosystems in the Caribbean. Experiments were performed on the Cayo Enrique Reef off Puerto Rico and in the laboratories of the University of Maryland to study factors influencing the calcification processes. Halimeda opuntia has a higher percentage of calcium carbonate than does Halimeda discoidea and a faster rate of incorporation. Halimeda opuntia and Halimeda discoidea show a stimulation of incorporation by light as well as a diurnal rhythm under identical conditions of illumination. Both phenomena parallel the rhythm of chloroplast migration within the plant. Calcification is also stimulated by the addition of carbon dioxide. Such evidence clearly indicated a light-linked mechanism which could involve photo-synthesis. However other metabolic processes, such as respiration, are also implicated. Aeration alone accelerates calcium incorporation. Nitrogen sources inhibit the incorporation of calcium during the day, indicating that cellular ammonia production is probably not responsible for precipitation. The differential wash-out rates of calcium absorbed during the day compared to those at night support the concept of a 2-step mechanism for calcification.     

HALIMEDA HUMMII SP. NOV., HALIMEDA CRYPTICA V. ACERIFOLIA VAR. NOV. (CAULERPALES,CHLOROPHYTA), AND ADDITIONAL RECORDS OF HALIMEDA SPECIES FROM: PUERTO RICO1

A new species, Halimeda hummii, and a new variety of Halimeda cryptica Colinvaux and Graham both originally collected from the edge of the continental shelf on the southwest coast of Puerto Rico are newly described. The new species is irregular in its segment morphology and is the smallest species of Halimeda presently known. Halimeda cryptica var. acerifolia from deep water possesses distinctive segments resembling maple leaves. Halimeda copiosa Goreau and Graham and typical H. cryptica are also collected in deep water and are newly recorded from Puerto Rico.     

NEW SPECIES OF HALIMEDA: A TAXONOMIC REAPPRAISAL12

A study of new Halimeda material available from the International Indian Ocean Expedition, and of type and other specimens not previously available to me, further indicated that Halimeda opuntia is a composite of species rather than a single species. The characteristics of some of its infraspecific taxa differ sufficiently from typical H. opuntia that I have elevated them to species. Extended diagnoses of three, H. hederacea, H. minima, and H. distorta, are given in this paper. Some characteristics new in Halimeda taxonomy are introduced, and distinguishing macroscopic and microscopic characteristics for the new species are discussed. Available ecological data are included.     

CALCIFICATION IN THE GREEN ALGA HALIMEDA. I. AN ULTRASTRUCTURE STUDY OF THALLUS DEVELOPMENT1

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.     

MOLECULAR AND MORPHOMETRIC DATA PINPOINT SPECIES BOUNDARIES IN HALIMEDA SECTION RHIPSALIS (BRYOPSIDALES,CHLOROPHYTA)1

Molecular systematic studies have changed the face of algal taxonomy. Particularly at the species level, molecular phylogenetic research has revealed the inaccuracy of morphology‐based taxonomy: Cryptic and pseudo‐cryptic species were shown to exist within many morphologically conceived species. This study focused on section Rhipsalis of the green algal genus Halimeda. This section was known to contain cryptic diversity and to comprise species with overlapping morphological boundaries. In the present study, species diversity within the section and identity of individual specimens were assessed using ITS1–5.8S–ITS2 (nrDNA) and rps3 (cpDNA) sequence data. The sequences grouped in a number of clear‐cut genotypic clusters that were considered species. The same specimens were subjected to morphometric analysis of external morphological and anatomical structures. Morphological differences between the genotypic cluster species were assessed using discriminant analysis. It was shown that significant morphological differences exist between genetically delineated species and that allocation of specimens to species on the basis of morphometric variables is nearly perfect. Anatomical characters yielded better results than external morphological characters. Two approaches were offered to allow future morphological identifications: a probabilistic approach based on classification functions of discriminant analyses and the classical approach of an identification key.     

VARIABILITY IN THE ECOPHYSIOLOGY OF HALIMEDA SPP. (CHLOROPHYTA,BRYOPSIDALES) ON CONCH REEF,FLORIDA KEYS,USA1

The photosynthetic performance, pigmentation, and growth of a Halimeda community were studied over a depth gradient on Conch Reef, Florida Keys, USA during summer–fall periods of 5 consecutive years. The physiology and growth of H. tuna (Ellis & Solander) Lamouroux and H. opuntia (L.) Lamouroux on this algal dominated reef were highly variable. Maximum rate of net photosynthesis (P_max), respiration rate, and quantum efficiency (α) did not differ between populations of either species at 7 versus 21 m, even though the 21‐m site received a 66% lower photon flux density (PFD). Physiological parameters, as well as levels of photosynthetic pigments, varied temporally. P_max, saturation irradiance, compensation irradiance, and growth were greatest in summer months, whereas α, chl a, chl b, and carotenoid concentrations were elevated each fall. Halimeda tuna growth rates were higher at 7 m compared with 21 m for only two of five growth trials. This may have arisen from variability in light and nutrient availability. Individuals growing at 7 m received a 29% greater PFD in August 2001 than in 1999. In August 1999 and 2001 seawater temperatures were uniform over the 14‐m gradient, whereas in August 2000 cold water regularly intruded upon the 21‐m but not the 7‐m site. These results illustrate the potentially dynamic relationship between nutrients, irradiance, and algal productivity. This suggests the necessity of long‐term monitoring over spatial and temporal gradients to accurately characterize factors that impact productivity.     

SPATIAL AND TEMPORAL VARIABILITY IN BIOMASS AND PRODUCTION OF PSAMMOPHYTIC HALIMEDA INCRASSATA (BRYOPSIDALES,CHLOROPHYTA) IN A CARIBBEAN REEF LAGOON1

Estimates for the production of calcium carbonate by Halimeda spp. have been based on limited measures in small areas or over short periods, subsequently extrapolated over larger temporal and spatial scales. The accuracy of these extrapolations depends on the variations in time and space of the parameters used for the derivations of the production, which were evaluated in the present study for Halimeda incrassata (J. Ellis) J. V. Lamour. in the Puerto Morelos reef lagoon, Mexican Caribbean. Growth, biomass, and CaCO₃ content of the branches of the thalli were determined at 1–3 monthly intervals from November 1997 until June 1999, using the marking technique with the coloring agent Alizarin Red‐S. Biomass exhibited great variability (V=25.4%), and fluctuations were closely related to changes in thallus density. Growth and CaCO₃‐content changes were related to the solar cycle, having coefficients of 15.4% and 2.5%, respectively. Additionally, calcified biomasses of the thalli were determined over a large spatial (31 stations in ～7 km² area) and time (6 y) scale. Spatial variability in calcified biomass was 59%, and the coefficient of variation attained its highest value (69%) for the samples collected over a 6 y period, from 1990 to 1996 (except 1995). Based on overall average values, branches of H. incrassata in the study area had a mean turnover of 30 d, with an annual production of 815 g CaCO₃ per square meter.