Going with the flow: detection of drift in response to hypo-saline stress by the estuarine benthic diatom Cylindrotheca closterium.

Avoidance response is a well-known mechanism for escaping environmental stress. For organisms with reduced active movement, such as benthic microalgae, drifting could be a specifically selected mean of avoiding less favorable environments. To test this hypothesis, a system was developed to assess if hypo-saline stress triggers drift in the estuarine benthic diatom Cylindrotheca closterium. Concurrently, the effects of salinity on growth inhibition were also investigated in order to compare the sensitivity of this endpoint with the drift response, and to estimate the immediate population decline caused by both drift and population growth responses. It was verified that the salinity value that inhibited the algal population growth by 50% (IGS50) was 19, while the salinity value that triggered the drift response by 50% of the population (TDS50) was 15. These results indicate that drift is an identifiable response triggered to escape stressful environments. The combination of the two responses (population growth and drift) showed that population decline based exclusively on the inhibition of population growth may result in an underestimation of the risk, compared with the decline when drifting to avoid stress is also taken into account.

CELL-WALL DEVELOPMENT AND BIPOLAR GROWTH IN THE DESMID PENIUM MARGARITACEUM (ZYGNEMATOPHYCEAE, STREPTOPHYTA). ASYMMETRY IN A SYMMETRIC WORLD(1).

Cell-wall (CW) development in the desmid Penium margaritaceum (Ehrenb.) Bréb. was studied using immunofluorescence labeling of living cells with the monoclonal antibodies (mAbs) JIM5 and JIM7, which recognize unesterified and methyl-esterified homogalacturonan (HG), respectively. During cell expansion, HG was secreted in a high-esterified form at a narrow band, called the HG secretion band or HGSB, at the isthmus or the polar tip of a daughter semicell. As newly secreted HG is displaced outward on the cell surface, deesterification and subsequent calcium (Ca(2+) )-complexing occurred to yield a rigid covering. HG secretion and CW/cell expansion were reversibly inhibited by dark, brefeldin A (BFA), and incubation in 0.24-0.36 M sucrose but were not altered by treatment with actin/microfilament drugs. The HGSB was detected near the nucleus during most cell-cycle events. Centrifugation displaced the nucleus away from the HGSB, but HG synthesis was not affected. HGSB activity was preceded by, and coordinated with, Calcofluor labeling, which suggests that cellulose production in CW/cell-expansion sites was critical to expansion control. In many first-cell-division products, asymmetric patterning of HG was noted in the CW. These asymmetric patterns most likely were a result of timing mechanisms and displacement of the nucleus-HGSB during the cell cycle.