The physiology of mesophotic Scleractinia varies with depth in response to environmental change. Previous research has documented trends in heterotrophy and photosynthesis with depth, but has not addressed between-site variation for a single species. Environmental differences between sites at a local scale and heterogeneous microhabitats, because of irradiance and food availability, are likely important factors when explaining the occurrence and physiology of Scleractinia. Here, 108 colonies of Agaricia lamarcki were sampled from two locations off the coast of Utila, Honduras, distributed evenly down the observed 50 m depth range of the species. We found that depth alone was not sufficient to fully explain physiological variation. Pulse Amplitude-Modulation fluorometry and stable isotope analyses revealed that trends in photochemical and heterotrophic activity with depth varied markedly between sites. Our isotope analyses do not support an obligate link between photosynthetic activity and heterotrophic subsidy with increasing depth. We found that A. lamarcki colonies at the bottom of the species depth range can be physiologically similar to those nearer the surface. As a potential explanation, we hypothesize sites with high topographical complexity, and therefore varied microhabitats, may provide more physiological niches distributed across a larger depth range. Varied microhabitats with depth may reduce the dominance of depth as a physiological determinant. Thus, A. lamarcki may ‘avoid’ changes in environment with depth, by instead existing in a subset of favourable niches. Our observations correlate with site-specific depth ranges, advocating for linking physiology and abiotic profiles when defining the distribution of mesophotic taxa.

Successful remobilization of seed reserves is the driving force behind seedling establishment for maximum final crop outcomes. The remobilization of stored maize seed phosphorus (P), and its allocation towards growing seedlings is critical for P-autotrophy during early ontogeny. We aimed to (1) evaluate the time frame of the origin of P utilized by maize seedlings, including the heterotrophic, transitional and autotrophic phases; and (2) compare P and carbon (C) dynamics in both seed and seedling compartments during the same phases. Using isotopic signatures (32P), we identified different P fluxes (P-heterotrophy, P-transition and P-autotrophy) and determined the proportion of P fluxes from heterotrophic seed P and external P uptake during 23 d of early ontogeny. The P-heterotrophic growth phase lasted from the first to the fourth day after sowing, when seedlings were entirely made up of heterotrophic P originating from remobilized seed P pool. In our experimental conditions, the P-transitional phase, when growing seedlings were supported by both heterotrophic and autotrophic P, lasted from the fifth to the fifteenth day after sowing. Thereafter, seed P reserves were exhausted and seedlings depended entirely on external P uptake, indicating the P-autotrophic stage. Although seed P reserves were remobilized earlier than C reserves, the length of the three growth phases for P and C was similar in the maize seedlings.

Volcanic areas with highly acidic solfatara soils and temperatures of up to 56 °C are inhabited by the red algal genus Galdieria. We examined three highly acidic but non-volcanic habitats in the western part of the Czech Republic for the occurrence of this red alga. In soil samples from the National Nature Reserve of Soos we found, together with Euglena mutabilis, Pseudococcomyxa simplex and species of Chlorella, a new strain of Galdieria. In contrast to all other Galdieria strains described so far, the strain from Soos exhibited a low temperature optimum for growth of about 30 °C. Other properties, such as the substrate spectrum for heterotrophic growth, ultrastructure, fatty acid composition, thermostability of enzymes and the nitrogen source, showed no obvious differences from other strains of Galdieria. Within a phylogenetic tree based on 18S rRNA sequence data, the strain from Soos occupied a position at the base of the ‘Galdieria’-branch. Our findings indicate that the genus Galdieria is not restricted to volcanic and mining areas and that strains of Galdieria are able to compete successfully with green algae in habitats like Soos.

Seedlings of the myco-heterotrophic orchid Corallorhiza trifida which had been germinated in the field in mesh bags developed hyphal links and mycorrhizas with Betula pendula and Salix repens, but not with Pinus sylvestris, when transplanted into soil microcosms. The fungus connecting the myco-heterotroph to Betula and Salix formed endomycorrhiza in the orchid with typical pelotons, but formed ectomycorrhizas with the autotrophs. The orchid plants, when linked to Betula and Salix by fungal hyphae, gained 6–14% in weight over 25–28 wk. In microcosms supporting P. sylvestris, and in control microcosms which lacked autotrophs, the Corallorhiza plants lost 13% of their weight over the same period. In the course of the 28-wk experimental period new Corallorhiza seedlings, in addition to those added as part of the experiment, appeared in the microcosms containing Salix and Betula but not in the Pinus microcosms. Shoots of Betula and Salix plants grown in association with Corallorhiza were fed with 14CO2, and the movement of the isotope was subsequently traced by a combination of digital autoradiography and tissue oxidation. Direct transfer of C from both autotrophs to the myco-heterotroph occurred in all cases where the associates had become connected by a shared fungal symbiont. Orchid seedlings lacking these hyphal connections, introduced to the microcosms as controls immediately before isotope feeding, failed to assimilate significant amounts of C. The results provide the first experimental confirmation that growth of Corallorhiza trifida can be sustained by supply of C received directly from an autotrophic partner through linked fungal mycelia.

The unicellular, acido- and thermophilic red alga Galdieria sulphuraria is unique in its ability to grow heterotrophically on at least 27 different sugars and polyols. The enzymatic machinery necessary for metabolizing this variety of compounds is constitutively expressed in the alga. The uptake system, however, has to be induced. From in vivo studies we conclude that the uptake system consists of several transporters with a partly overlapping substrate specificity. These transporters can be grouped into hexose, polyol and pentose transporters. The mode of induction depends on the substrate supplied for heterotrophic growth. When autotrophic cells were supplied with hexoses, hexose and polyol transporters were induced from the onset of heterotrophic conditions. Depletion of substrate in the medium triggered the induction of other transporters. Dulcitol, as well as other polyols and pentoses, led to a co-induction of all transporters as judged from induction and competition experiments. The heterotrophic capabilities of Galdieria are intriguing, because the natural habitat of the alga contains only traces of organic carbon. The substrates for heterotrophic growth apparently originate from decaying cells releasing hydrolysed cell material. Trace amounts of glucose were sufficient for the induction of sugar uptake in Galdieria. The co-induction of transporters obviously enables the cells in endolithic mats to import a wide spectrum of organic carbon in order to survive periods of light limitation.

The habitat of the acido- and thermophilic red algae Galdieria sulphuraria and Cyanidium caldarium was examined in acidic hot sulphur springs in the vicinity of Naples (Italy). These species grew on soil and rocks, but a large part of the populations was cryptoendolithic. The endolithic algal layer (1–3 mm in thickness) was covered by amorphous silica (1–2 mm in thickness) containing traces of hydrotroilite (FeS.nH2O) and elemental sulphur. Organotrophic bacteria and fungi were not found in the algal layer. Light penetration measurements showed that 0·1–1% of the sunlight reached the upper part of the algal layer. Thus, low-light-adapted algae should be able to perform some photosynthesis in this endolithic habitat. Under conditions where light is even more limited, e.g. in winter or after darkening of the covering layer, many of the cells might not survive. Aqueous extracts of these algae are excellent growth substrates for Galdieria sulphuraria. Therefore, we propose that compounds released from dead cells in the endolithic layer are used by the surrounding Galdieria cells for heterotrophic metabolism. This would increase their chance of surviving prolonged periods under detrimental conditions.