Stress responses of the marine diatom Skeletonema spp. to polyunsaturated aldehydes (PUAS)

Abstract

Diatom-derived polyunsaturated aldehydes (PUAs) are secondary metabolites acting as teratogens against grazers, allelochemicals by inhibiting the growth of closeby phytoplankton species, and also as signal molecules to determine the cell fate of diatom populations. The work presented here focuses on Nitric Oxide (NO) and Reactive Oxygen Species (ROS) production in response to PUAs in Skeletonema marinoi, a cosmopolitan, bloom forming and PUA-producing diatom species. In the first part of the thesis I addressed the problem of whether PUA-producing species might have evolved different stress response mechanisms with respect to non-PUA producing ones and if this may r underlie their different ecological success. S. marinoi was exposed to different PUAs: DECA, which is not produced by S. marinoi, OCTA and HEPTA, which are PUAs commonly produced by this diatom and a mixture of these last two (MIX). A reduction in NO production was observed in response to all PUAs tested, probably due to consumption of physiological levels of NO, possibly indicating that this messanger acts as a growth regulator under optimal growth conditions. In the second part, a comparison with the non-PUA producing diatom Phaeodactylum tricornutum revealed different reactions to the same PUAs (i.e. DECA), with an increase in NO production in DECA-exposed P. tricornutum, whereas in response to OCTA a reduction in NO production was evident. Thus, NO production in response to PUAs appears to be both PUA-specific and species-specific. Additionally, S. marinoi cells exposed to the photoinhibitor DCMU presented an increase in NO production, indicating that NO production in S. marinoi is likely to be also stress specific. In terms of ROS production, S. marinoi showed a sharp increase in ROS production upon exposure to all PUAs except for DECA that is not produced by this diatom. In addition, an enhanced synthesis of xanthophylls in OCT A-exposed S. marinoi cells was observed, likely to act as antioxidants against ROS production thereby assuring the maintenance of the photosynthetic performance at intermediate concentrations of PUAs. Additionally, in the congeneric species Skeletonema tropicum, PUAs induced an overexpression of a gene coding for a death specific protein (ScDSP), thought to be involved in an autocatalytic-type of cell death, also linked to a concentration-dependent increase in ROS production. This points to a key role of ROSs in mediating the response to PUAs leading to resistance or cell death, as for instance during the final stages of blooms. Altogether, the data show a differential reaction of cells to PUAs, depending on the diatom species, the PUA type and concentration and the stress factor applied. This suggests a role of PUAs as signal molecules at the cellular and population levels. This is likely to underlie the ecological differences and evolutionary success of PUA and nonPUA producing species.