PATTERNS OF REPRODUCTION,GENETIC DIVERSITY,AND GENETIC DIFFERENTIATION IN CALIFORNIA POPULATIONS OF THE GENICULATE CORALLINE ALGA LITHOTHRIX ASPERGILLUM (RHODOPHYTA)1

The reproductive composition and genetic diversity of populations of the red seaweed Lithothrix aspergillum Gray (O. Corallinales) were studied at three southern California sites (Shaw's Cove and Treasure Island, Laguna Beach; Indian Rock, Santa Catalina Island) and at a fourth site (Bodega Bay) located in northern California. Sexually reproducing populations were confined to southern California. Diploid individuals were numerically dominant over haploid (gametophytic) individuals at all sites. Intertidal and subtidal subpopulations from Shaw's Cove differed in their reproductive profiles. Most intertidal specimens found on emersed surfaces were densely branched, turf-forming, and bore tetrasporangial (68.6%), carposporangial (11.4%), or spermatangial (5.7%) conceptacles, reflecting a sexual life history; none produced asexual bispores. In contrast, 74.3% of the larger, loosely branched subtidal specimens bore bisporangial conceptacles indicative of asexual reproduction. Nearly 70% of the Indian Rock thalli showed no evidence of conceptacle formation. Only asexual, diploid bispore-producing thalli were obtained from the Bodega Bay site. Genetic diversity (mean number of alleles per locus, percent of polymorphic loci, and average expected heterozygosity) of diploid L. aspergillum populations varied with life-history characteristics and geographic location. A total of 30 alleles was inferred from zymograms of 16 loci examined by starch-gel electrophoresis; of these loci, 11 were polymorphic. The genetic diversity of sexual, diploid populations of L. aspergillum (alleles per locus [A/L] = 1.4-1.5; percent polymorphic loci [%P] = 37.5-50.0) was relatively high compared with other red seaweeds. Lowest diversity (A/L = 1.0; %P = 0.0) occurred in the exclusively asexual Bodega Bay population which consisted of genetic clones. All sexual L. aspergillum populations deviated significantly from Hardy-Wein-berg expectations due to lower than expected heterozygosity. Genetic differentiation (Wright's F_statistic [F_ST]; Nei's Genetic Distance [D]) among sexually reproducing southern California populations was low (F_ST= 0.030) on a local scale (ca. 5 km), suggesting high levels of gene flow, but high genetic differention (F_ST= 0.390 and 0.406) occurred among southern California populations separated by ca. 70 km. Very high genetic differentiation (F_ST= 0.583–0.683) was obtained between northern and southern California populations separated by 700–760 km. Our genetic and reproductive data suggest that the L. aspergillum population from Bodega Bay is sustained by perennation, vegetative propagation, or asexual reproduction by bispores and may represent an isolated remnant or a population established by a founder event.     

PARASITIC INTERACTIONS AND VEGETATIVE ULTRASTRUCTURE OF CHOREOAEMA THURETII (CORALLINALES,RHODOPHYTA)1

The monotypic coralline red alga, Choreonema thuretii (Bornet) Schmitz (Choreonematoideae), grows endophytically within three geniculate genera of the Corallinoideae. Although the thallus of Choreonema is reduced, lacks differentiated plastids, and is endophytic except for its conceptacles, its status as a parasite has been questioned because cellular connections to the host had not been ob served. Transmission electron microscopy, however, disclosed a previously undescribed type of parasitic interaction in which Choreonema interacts with its host through specialized cells known as lenticular cells. These small, lens-shaped cells are produced from the single file of host-penetrating vegetative cells. Pit plug morphology between vegetative and lenticular cells is polarized. Plug caps facing the vegetative cell have normal coralline morphology, while those facing the lenticular cell are composed of three layers. Regions of lenticular cells near host cells protrude toward the host cell; upon encountering the host cell wall, the prolrusion produces numerous finger-like fimbriate processes that make cellular connections with the host cell. Lenticular cells may extend several protrusions toward a host cell or penetrate more than one host cell; two or more lenticular cells may also penetrate the same host cell. The lack of secondary pit connections, cell fusions, and passage of parasitic nuclei suggest that this parasitic relationship may be evolutionarily older than previously reported cases of parasitism in red algae.     

Effects of elevated pCO2 on epilithic and endolithic metabolism of reef carbonates

We investigated effects of elevated partial pressure of CO₂ (pCO₂) on the metabolism of epilithic and endolithic phototrophic communities that colonized experimental coral blocks. Blocks of the massive coral Porites lobata were exposed to colonization by epilithic and endolithic organisms at an oceanic site in Kaneohe Bay (Hawaii) for 6 months, and then were transported to laboratory tanks. A bubbling system was used to maintain two treatments for 3 months, one at ambient pCO₂ (400 ppm) and the second at elevated pCO₂ (750 ppm). Net photosynthetic rates of epilithic communities in the high pCO₂ treatment, dominated by encrusting coralline algae, decreased by 35% while respiration rates remained constant. In contrast, metabolism of endolithic phototrophs, comprised of cyanobacteria and algae, was not significantly affected by the elevated pCO₂ even though endoliths contributed about 63% to block production.     

Coralline algal skeletal mineralogy affects grazer impacts

In macroalgal‐dominated systems, herbivory is a major driver in controlling ecosystem structure. However, the role of altered plant–herbivore interactions and effects of changes to trophic control under global change are poorly understood. This is because both macroalgae and grazers themselves may be affected by global change, making changes in plant–herbivore interactions hard to predict. Coralline algae lay down a calcium carbonate skeleton, which serves as protection from grazing and is preserved in archival samples. Here, we compare grazing damage and intensity to coralline algae in situ over 4 decades characterized by changing seawater acidity. While grazing intensity, herbivore abundance and identity remained constant over time, grazing wound width increased together with Mg content of the skeleton and variability in its mineral organization. In one species, decreases in skeletal organization were found concurrent with deeper skeletal damage by grazers over time since the 1980s. Thus, in a future characterized by acidification, we suggest coralline algae may be more prone to grazing damage, mediated by effects of variability between individuals and species.     

Coralline algal structure is more sensitive to rate,rather than the magnitude,of ocean acidification

Marine pCO₂ enrichment via ocean acidification (OA), upwelling and release from carbon capture and storage (CCS) facilities is projected to have devastating impacts on marine biomineralisers and the services they provide. However, empirical studies using stable endpoint pCO₂ concentrations find species exhibit variable biological and geochemical responses rather than the expected negative patterns. In addition, the carbonate chemistry of many marine systems is now being observed to be more variable than previously thought. To underpin more robust projections of future OA impacts on marine biomineralisers and their role in ecosystem service provision, we investigate coralline algal responses to realistically variable scenarios of marine pCO₂ enrichment. Coralline algae are important in ecosystem function; providing habitats and nursery areas, hosting high biodiversity, stabilizing reef structures and contributing to the carbon cycle. Red coralline marine algae were exposed for 80 days to one of three pH treatments: (i) current pH (control); (ii) low pH (7.7) representing OA change; and (iii) an abrupt drop to low pH (7.7) representing the higher rates of pH change observed at natural vent systems, in areas of upwelling and during CCS releases. We demonstrate that red coralline algae respond differently to the rate and the magnitude of pH change induced by pCO₂ enrichment. At low pH, coralline algae survived by increasing their calcification rates. However, when the change to low pH occurred at a fast rate we detected, using Raman spectroscopy, weaknesses in the calcite skeleton, with evidence of dissolution and molecular positional disorder. This suggests that, while coralline algae will continue to calcify, they may be structurally weakened, putting at risk the ecosystem services they provide. Notwithstanding evolutionary adaptation, the ability of coralline algae to cope with OA may thus be determined primarily by the rate, rather than magnitude, at which pCO₂ enrichment occurs.     

Phenotypic plasticity of coralline algae in a High CO2 world

It is important to understand how marine calcifying organisms may acclimatize to ocean acidification to assess their survival over the coming century. We cultured the cold water coralline algae, Lithothamnion glaciale, under elevated pCO₂ (408, 566, 770, and 1024 μatm) for 10 months. The results show that the cell (inter and intra) wall thickness is maintained, but there is a reduction in growth rate (linear extension) at all elevated pCO₂. Furthermore a decrease in Mg content at the two highest CO₂ treatments was observed. Comparison between our data and that at 3 months from the same long‐term experiment shows that the acclimation differs over time since at 3 months, the samples cultured under high pCO₂ showed a reduction in the cell (inter and intra) wall thickness but a maintained growth rate. This suggests a reallocation of the energy budget between 3 and 10 months and highlights the high degree plasticity that is present. This might provide a selective advantage in future high CO₂ world.     

Trichocytes in Lithophyllum kotschyanum and Lithophyllum spp. (Corallinales,Rhodophyta) from the NW Indian Ocean

The current diagnosis of the genus Lithophyllum includes absent or rare trichocyte occurrence. After examining holotype material, single trichocytes have been revealed to occur abundantly in Lithophyllum kotschyanum Unger, and in freshly collected specimens of Lithophyllum spp. from the Red Sea, Gulf of Aden and Socotra Island (Yemen). Trichocyte occurrence is not considered a diagnostic character at specific or supraspecific levels in the Lithophylloideae, and the ecological significance of trichocyte formation is discussed. The generitype species, L. incrustans Philippi, does not show trichocytes nor do many other Lithophyllum species from diverse geographic localities, but the presence of abundant trichocytes in other congeneric taxa requires emendation of the genus diagnosis. Therefore, the diagnosis of Lithophyllum is here emended by eliminating the adjective “rare” in the sentence concerning trichocyte occurrence, as follows: “Trichocytes present or absent, if present occurring singly.”