Photolithotrophy, photoheterotrophy, and chemoheterotrophy: Patterns of resource utilization on an annual and a diurnal basis within a pelagic microbial community

Abstract

An annual investigation of rates of photolithotrophy, photoheterotrophy, and chemoheterotrophy utilizing glucose and bicarbonate was made within the pelagic zone of a small, hardwater, southwestern Michigan lake. Sampling proceeded on a monthly, diurnal, and depth-wise basis. Annual mean photoheterotrophic uptake was estimated at 2.6μg C m(-3)h(-1). Two periods of relatively high activity were observed: one during spring overturn and the second during the late summer period. In general, greatest contributions to overall carbon cycling occurred during morning to midday incubation periods and at intermediate depths within the water column. Rates of chemoheterotrophy averaged 6.9μg C m(-3)h(-1) and were relatively uniform throughout the annual period. Greatest overall chemoheterotrophic activity was associated with periods of overturn. In general, this activity increased throughout the day and with increasing depth within the water column. The annual mean for photolithotrophic fixation was 1.33 mg C m(-3)h(-1). Greatest contributions to rates of photosynthesis were associated with epilimnetic waters during early morning and midday incubations. Relatively minor contributions to inorganic fixation were made by waters below the 6-meter contour. Spring overturn and late summer represented periods of particularly great photolithotrophic activity. Quantitative comparisons among carbon pathways indicate that rates of pelagic heterotrophy, both photo- and chemoheterotrophy combined, contribute small quantities of carbon to overall carbon metabolism in this oligotrophic system. Qualitative comparisons among pathways indicate strong spatial and temporal separation. The late summer period showed greatest seasonal separation of the three pathways. Spring values represented a period of relatively high activity for all three pathways. On a depth-wise basis, photolithotrophic activity was greatest near the surface and chemolithotrophic activity greatest near the bottom. Photoheterotrophy took an intermediate position between the two. Diurnally, photoheterotrophy and photolithotrophy showed greatest activity during midday and early morning periods, whereas chemoheterotrophy increased throughout the daylight period and reached maximal values in sunset incubations.

Photolithotrophy, photoheterotrophy and chemoheterotrophy during spring phytoplankton development (Lake Pavin)

“…The values for chemoheterotrophic assimilation were slightly higher, representing 0.03 to 3.75% of fixation of inorganic carbon by photosynthesis. These results are similar to those reported in other studies carried out on cultures [6,32] or in the natural environment [13,23]. McKinley and Wetzel [23], however, did point out that photoheterotrophic phenomena can represent up to 20% of inorganic carbon fixation in certain oligotrophic lakes at low light intensities.…”

“…These results are similar to those reported in other studies carried out on cultures [6,32] or in the natural environment [13,23]. McKinley and Wetzel [23], however, did point out that photoheterotrophic phenomena can represent up to 20% of inorganic carbon fixation in certain oligotrophic lakes at low light intensities. In a study on an oligotrophic lake, Vincent and Goldman [41] found that heterotrophy represented 1.5 to 15% of the carbon fixed by photosynthetic activity in the euphotic zone, whereas in the deeper layers carbon assimilated from acetate represented 25 to 247% of the carbon assimilated by the photosynthetic route by certain phytoplankton species (Monoraphidium contortum, Friedmannia sp.…”

“…The maximum value for photoheterotrophic activity was situated at a greater depth (15 m) than that of photosynthetic activity (5 m). These results are identical to the observations made by McKinley and Wetzel [23], and are probably related to the fact that photoheterotrophic processes can take place at lower light intensities than those required for photosynthetic activities [18,27]. In contrast, the vertical profile of chemoheterotrophic activity is different from the standard profile proposed by McKinley and Wetzel [23], to the extent that the greatest chemoheterotrophic activity recorded in this study was in the upper layers of the lake.…”

“…The radioactivity retained on the filters was counted using a liquid scintillation counter. According to the works of McKinley [22] and McKinley and Wetzel [23], the values for photoheterotrophic assimilation were calculated from the difference between the activity measured in the light bottle and that measured in the dark bottle, on the basis that: (a) activity in the light bottle = potential photoheterotrophic activity + potential chemoheterotrophic activity + background and (b) activity in the dark bottle = potential chemoheterotrophic activity + background.…”

“…Photolithotrophy and photo-and chemoheterotrophy are the main pathways of carbon flux within lacustrine pelagic communities [11,23]. Phytoplankton is frequently considered as being a strictly autotrophic community in terms of its carbon requirements, but it is nevertheless capable of assimilating organic carbon directly, either in dissolved form by osmotrophy [10,26], or in particulate form by Send offprint requests to: C. Amblard.…”